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A1199Are We Alone?
The Search for Life in the UniverseSummer 2019
Instructor: Shami Chatterjee
Web Page: http://www.astro.cornell.edu/academics/courses/astro1199/
Think about projects! HW 3 is posted – due Wednesday
Now: Habitable Zones and Life
Habitable ZonesRequirements:
– Liquid water sustained over billions of years.• Need “low” incidence rate of high-mass impacts.• Places conditions on stability of a planet’s orbit.• Stability of host star’s luminosity and low incidence of
stellar flares.– Need stable overall environment:
• No cosmic “bad days” ( local gamma-ray bursts, supernovae, etc.)
Earth’s Habitable Zone• We know that the HZ is smaller than the distance between
orbits of Venus and Mars [0.72, 1.5AU].
• If we ignore the atmosphere: HZ = [0.56,1.05AU].
• If we account for reflection of 31% of the Sun’s radiation off cloud tops: HZ = [0.47,0.87AU].
• If we take into account the greenhouse effect): HZ = [0.6,1.11AU].
• Feedback cycles (carbon-silicate cycle) yield Kasting et al’sHZ = [0.95, 1.37AU].
Habitable Zones Around Stars and in the Milky Way Galaxy
Continuously Habitable Zone around main-sequence stars Galactic Habitable Zone
GALAXY-SCALE CATASTROPHESGRBs, SGRs, etc.
Most Distant Star Burst DetectedBy Larry O'Hanlon, Discovery News
March 8, 2006 — The discovery of the most distance and ancient stellar explosion has now been confirmed and pushed back another 100 million light-years to 12.8 billion light-years away. Since cosmic time and distance are both measured by the speed of light, the explosion known as GRB050904 took place 12.8 billion years ago, when the universe was a relatively youthful 900 million years old.
Ordovician: 488-444 Myr ago bracketed by a minor and a major extinction event
Gamma-ray burst energeticsGamma-Ray Bursts:
Energy Release with Human Experience------------------------------------1 erg = energy to make a mosquito jump 10^3 ergs = ball drop 10^10 ergs = hit by truck 10^15 ergs = smart bomb 10^20 ergs = H bomb 10^26 ergs = killer asteroid 10^40 ergs = Death Star
Energy Release with Astronomical Experience -------------------------------------------10^33 ergs/s = Sun 10^39 ergs/s = nova 10^41 ergs/s = SN10^45 ergs/s = galaxy 10^52 ergs/s = GRB
Soft gamma repeater burst of 2004 DecemberSGRs = neutron stars with magnetic fields strong enough to crack their crusts
ISSN 1063-7737, Astronomy Letters, 2007, Vol. 33, No. 1, pp. 1–18. c⃝ Pleiades Publishing, Inc., 2007.Original Russian Text c⃝ D.D. Frederiks, S.V. Golenetskii, V.D. Palshin, R.L. Aptekar, V.N. Ilyinskii, F.P. Oleinik, E.P. Mazets, T.L. Cline, 2007, published in Pis’ma vAstronomicheskiı Zhurnal, 2007, Vol. 33, No. 1, pp. 3–21.
Giant Flare in SGR 1806–20and Its Compton Reflection from the Moon
D. D. Frederiks1, S. V. Golenetskii1, V. D. Palshin1, R. L. Aptekar1*,V. N. Ilyinskii1, F. P. Oleinik1, E. P. Mazets1, and T. L. Cline2
1Ioffe Physical–Technical Institute, Russian Academy of Sciences,ul. Politekhnicheskaya 26, St. Petersburg, 194021 Russia
2Goddard Space Flight Center, NASA, Greenbelt, MD 20771, USAReceived August 17, 2006
Abstract—We analyze the data obtained when the Konus–Wind gamma-ray spectrometer detected agiant flare in SGR 1806–20 on December 27, 2004. The flare is similar in appearance to the two knownflares in SGR 0526–66 and SGR 1900+14 while exceeding them significantly in intensity. The enormousX-ray and gamma-ray flux in the narrow initial pulse of the flare leads to almost instantaneous deepsaturation of the gamma-ray detectors, ruling out the possibility of directly measuring the intensity, timeprofile, and energy spectrum of the initial pulse. In this situation, the detection of an attenuated signalof inverse Compton scattering of the initial pulse emission by the Moon with the Helicon gamma-rayspectrometer onboard the Coronas-F satellite was an extremely favorable circumstance. Analysis of thissignal has yielded the most reliable temporal, energy, and spectral characteristics of the pulse. The temporaland spectral characteristics of the pulsating flare tail have been determined from Konus–Wind data. Itssoft spectra have been found to contain also a hard power-law component extending to 10 MeV. A weakafterglow of SGR 1806–20 decaying over several hours is traceable up to 1 MeV. We also consider theoverall picture of activity of SGR 1806–20 in the emission of recurrent bursts before and after the giantflare.
PACS numbers : 97.60.Jd; 98.70.Rz; 95.85.Pw; 95.30.JxDOI:10.1134/S106377370701001X
Key words: neutron stars, flares, gamma rays, Compton scattering.
INTRODUCTION
The first two soft gamma repeaters, SGR 0526–66(Mazets et al. 1979a; Golenetskii et al. 1984) andSGR 1900+14 (Mazets et al. 1979b), were dis-covered and localized in March 1979. The thirdSGR 1806–20 was discovered in 1983 (Atteia et al.1987; Laros et al. 1987). And only in 1998 was thefourth SGR 1627–41 discovered (Woods et al. 1999).The situation with the possible fifth SGR 1801–23(Cline et al. 2000) arouses scepticism, since only twosoft bursts separated by an interval of several hourshave been detected from this source.
The emission of recurrent bursts by the gamma re-peaters is highly nonuniform in time. The gamma re-peaters are predominantly in quiescence. This phasecan last for years, being interrupted by reactivationperiods that can be very intense.
The temporal and spectral characteristics for all ofthe above gamma repeaters that have been observed
*E-mail: [email protected]
over several years in the Konus–Wind experimentare summarized in a unified catalog of SGR activity(Aptekar et al. 2001).
Giant flares, very rare events comparable in peakemission power in the source (∼1045–1047 erg s−1)to the luminosity of quasars, are the second, incom-parably more impressive type of SGR activity.
The giant flare of March 5, 1979, had remained aunique event for more than 19 years. On August 27,1998, a giant flare came from SGR 1900+14. All themain features of the flare in SGR 0526–66 manifestedthemselves in this flare: a narrow, very intense initialemission peak with a hard energy spectrum accom-panied by a relatively weaker, spectrally soft tail thatdecayed for several minutes while pulsating (Mazetset al. 1999a; Hurley et al. 1999; Feroci et al. 1999).The third similar, but even more intense flare thatcame from SGR 1806–20 on December 27, 2004,was observed on many spacecraft equipped with X-ray and gamma-ray detectors: INTEGRAL, MarsOdyssey, Wind, Swift, RXTE, RHESSI, and others
1
Habitable and HazardousHabitable zones - Ingredients for life:
• Liquid water, Organics.• Stability, Free energy, Time (Gyr).
Hazards: changes in environment, catastrophes.• Geophysical:
Volcanism, methane clathrates.• Solar system:
Solar flares, impacts, orbital instabilities.• Astrophysical:
Supernovae, gamma-ray bursts, magnetars.
LIFE ON EARTHLearning from a sample of one
• What is life?• What is the place of life in the overall natural world?• Is life an inevitable outcome of physics & chemistry?• What were the conditions on early earth?• What were the necessary conditions for terrestrial life?• Why is it difficult to establish the details of the
synthesis of life on the early earth?• Are the conditions for life common or rare on other
planets?• What are we evolving into?
The Origin and Evolution of Life
A definition for life?NASA effort to create a ‘‘working definition’’ for their Exobiology and Astrobiology research programs:
Life is ‘‘a self-sustaining chemical system capable of Darwinian evolution.’’ (Joyce, 1994)
àConcise!à “Self sustaining” and “evolving”.àToo specific? Un-observable?
What is life?1. A self-organized non-equilibrium system
such that
2. its processes are governed by a stored symbolic program
and
3. it can reproduce itself, including the program.
From: Smolin, The Lives of the Cosmos, p. 156
26 MAY 2006 VOL 312 SCIENCE www.sciencemag.org1140
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BOOKS ET AL.
The scientific field devoted to the originof life on Earth is very young, havingtaken its first experimental steps in the
1950s. Though the question has captivatedhuman imagination since the dawn of history,its scientific pursuit has depended on severalcrucial conceptual developments during the20th century. First, the emergence of life had tobe conceived of as an integral part of the gen-eral process of evolution, leading from the geo-chemistry of the barren Earth to the universalcommon ancestor, which later diversified intothe Darwinian tree of life. Following the rise ofmolecular biology in the 1950s and 1960s, theorigin-of-life question could be formulated inbiochemical and genetic terms, making it asubject of experimental investigation.
Early on, most scientists engaged in thisresearch were chemists who attempted to for-mulate plausible scenarios for the prebioticsynthesis of organic building blocks, biologi-cally relevant polymers, and the first metaboli-cally or genetically functional chemical struc-tures. In the late 1970s, however, geologistsalso became increasingly involved in the field.Their participation was associated with the riseof a new paradigm positing that the synthesis oforganic building blocks and the emergence oflife itself took place not in the“primordial soup” of the tradi-tional hypotheses but in thevicinity of undersea hydrother-mal vents, at high temperatureand under extreme pressure.Supporters of this new concep-tion claim that origin-of-lifetheories can now be subjectedto more rigorous constraintsposed by specific primordialphysical settings (1). On theother hand, the “soup people”—in particular,Stanley Miller, renowned pioneer of the 1953prebiotic simulation experiments, and his col-leagues—reject the alternative paradigm asempirically untenable (2).
In Genesis, Robert Hazen tells the story ofthese debates over the origin of life. There is noone better suited to examine recent develop-ments in the experimental study of the topic.Trained in mineralogy and crystallography, hehas been personally involved in the major lines
of research through which Earthscientists have come to shape thefield. Describing these contribu-tions, he vividly portrays numer-ous experiments and observations.Hazen’s academic home, the Geo-physical Laboratory at the Car-negie Institution of Washington,which specializes in investiga-tions of chemical reactions underextreme conditions, serves as anideal setting for his experiments onthe effects of high pressure andtemperature on organic synthesisand particularly on the possiblerole of minerals abundant in hydro-thermal vents in such synthesis.Describing the scientific status ofthis lab, its remarkable members,and their close professional andpersonal relationships, Hazen weavesthe scientific and the personal intoan engaging, sometimes dramatictale. He highlights the excitement involved inresearch, the many setbacks and disappoint-ments, and the inevitable internal politicswithin the origin-of-life community. In addi-tion, his research team’s membership in the
NASA Astrobiology Instituteallows him to comment on therole of geologists in the study ofpossible conditions for life onMars and other extraterrestrialsites within the context of thenew “deep-origin” paradigm.
An underlying theme of thebook is Hazen’s conception ofthe origin of life as part of awider “theory of emergence”(3), a perspective based mainly
on the ideas of theoretical biologist HaroldMorowitz, a colleague of Hazen’s at GeorgeMason University. According to this ambitioustheory, the growth of organization and complex-ity in physical, chemical, biological, and socialsystems follows a general, though as-yet-unknown, principle on a par with the universallaws of nature. Considering the origin of life asa quintessential process of emergence, Hazensuggests that uncovering “the missing law”should advance origin-of-life research. How-ever, although various complex systems doshare common features, the “new science ofemergence” is in danger of downplaying theunique features of living systems as well asthe distinction between physical and chemicalselection on the one hand and natural selection
on the other. Moreover, as Hazen acknowledges,the basic concepts underlying this grand scheme(e.g., complexity) are far from clear. Since theorigin-of-life field itself lacks firm, unequivocalconclusions, it is doubtful whether such additionalconceptual baggage offers much scientific value.
Among the many issues dividing the origin-of-life community, none is more crucial thanthe controversy between “RNA-first” and“metabolism-first” scenarios. This divisionstems from the difficulty of deciding whichemerged earlier, genetic polymers or metaboliccycles. Because nucleic acids and proteinenzymes are tightly interdependent in extantliving cells, an adequate theory must establishhow either could have originally functioned onits own. After describing the rival positionseven-handedly, noting the pros and cons ofboth, Hazen commendably feels that he has toplace his bets on the table. He comes down onthe side of metabolism-first, probably in theform of a molecular layer on a surface of arock. Interestingly, he bases his choice on the“theory of emergence” and the hypothesis thatlife emerged through stages of increasing com-plexity. But wouldn’t a primitive genetic system,made of RNA or a simpler genetic polymer,also have to emerge through such stages?
The chemical requirements for the establish-ment of a self-replicating genetic system underprebiotic conditions are clearly extremely com-plex. Nonetheless, the support for the RNA-firstnotion, despite its difficulties, reflects the dou-ble realization that the emergence of life’s com-
Search for Life’s BeginningsIris Fry
ORIGIN OF LIFE
Genesis
The Scientific Quest forLife’s Origin
by Robert M. Hazen
Joseph Henry Press,Washington, DC, 2005. 359pp. $27.95, C$37.95. ISBN 0-309-09432-1.
Original Eden? The discovery of hydrothermal vent communitiesled to the proposal that hydrothermal systems provided a site for therapid emergence of life through a sequence of abiotic syntheses.
The reviewer is at the Cohn Institute for the History andPhilosophy of Science and Ideas, Tel Aviv University, andthe Department of Humanities and Arts, Technion–IsraelInstitute of Technology, Haifa 32000, Israel. E-mail:[email protected]
Published by AAAS
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Terrestrial Life: Origins and Evolution
Simple molecules ß
organic moleculesß
reaction chainsß
the first cells ß
prokaryotes ß
eukaryotesß
multicellular life
Nitrogen, CO2,methane
atmosphere
ß
trace O2
ß
Nitrogen, O2trace CO2
~ 2-4 Gyr
Cambrianexplosion~ 4 Gyr
Late heavy bombardment
Snowball Earth?
References and Notes1. J. F. Nye, R. Soc. London Proc. Ser. A 219, 477 (1953);
R. LeB Hooke, Rev. Geophys. Space Phys. 19, 664(1981); C. J. van der Veen and I. M. Whillans, J.Glaciol. 36, 324 (1990).
2. Examples include velocity variations occurring overmonths to weeks [R. LeB Hooke, P. Calla, et al., J.Glaciol. 35, 235 (1989)] and days to hours [A. Ikenand R. A. Bindschadler, ibid. 32, 101 (1986)].
3. S. M. Hodge, ibid. 13, 349 (1974); B. Kamb et al., J.Geophys. Res. 99, 15231 (1994); J. Harbor et al.,Geology 25, 739 (1997).
4. A dense array of radio-echo sounding measurementswere processed with three-dimensional migration tech-niques. Comparisons of these measurements with bore-hole observations suggest that the radar is accurate towithin about 8.5 m (B. C. Welch, W. T. Pfeffer, J. T.Harper, N. F. Humphrey, J. Glaciol., in press).
5. J. T. Harper and N. F. Humphrey, Geology 23, 901(1995).
6. M. F. Meier, U.S. Geol. Surv. Prof. Pap. 351 (1960);
W. S. B. Paterson and J. C. Savage, J. Geophys. Res. 68,4537 (1963); C. F. Raymond, J. Glaciol. 10, 55 (1971);R. LeB. Hooke, P. Holmlund, N. R Iverson, ibid. 33, 72(1987) were all forced to smooth inclinometry databecause of high levels of noise.
7. The instrument was constructed by Slope IndicatorCanada, Ltd. (Vancouver, BC). Measurement errorsassociated with a prototype of this instrument arediscussed by E. W. Blake and G. K. C. Clarke [ J.Glaciol. 38, 113 (1992)]. However, analysis of ac-tual data from the instrument used suggests thatinstrument errors are slightly improved from man-ufacturer specifications [ J. T. Harper, thesis, Uni-versity of Wyoming (1997); S. V. Huzurbazar, un-published material]. Additionally, the uniformity ofthe borehole walls enabled a high degree of repeat-ability for the measurements.
8. We follow the method of C. F Raymond, J. Glaciol.10, 39 (1971).
9. We use a cubic spline function with an iterativescheme designed to minimize the curvature of thefunction between data points [I. C. Briggs, Geophysics
1974, 39 (1974)]. This interpolation was tested ex-tensively with synthetic data.
10. J. T. Harper, N. F. Humphrey, W. T. Pfeffer, B. C.Welch, U.S. Army Cold Reg. Res. Eng. Lab. Spec. Rep.96-27 (1996), p. 41.
11. This measurement was made within the samereach and time of year as the deformation exper-iments, but during a subsequent year. Sliding andsurface velocities were determined by continuousfilming of the base of a borehole with concurrentsurveying of velocity at the surface.
12. Funded by grants from NSF (OPP-9122966 to N.F.H.and OPP-9122916 to W.T.P.). Additional funding forcomputer visualization was provided by NSF’sEPSCoR (Experimental Program to Stimulate Com-petitive Research) program (EPS9550477), throughthe University of Wyoming’s Spatial Data and Visu-alization Center project. D. Bahr, B. Welch, and B.Raup all made significant contributions to portions ofthe work presented here.
16 April 1998; accepted 28 July 1998
A Neoproterozoic SnowballEarth
Paul F. Hoffman,* Alan J. Kaufman, Galen P. Halverson,Daniel P. Schrag
Negative carbon isotope anomalies in carbonate rocks bracketing Neoprotero-zoic glacial deposits in Namibia, combined with estimates of thermal subsi-dence history, suggest that biological productivity in the surface ocean col-lapsed for millions of years. This collapse can be explained by a global glaciation(that is, a snowball Earth), which ended abruptly when subaerial volcanicoutgassing raised atmospheric carbon dioxide to about 350 times the modernlevel. The rapid termination would have resulted in a warming of the snowballEarth to extreme greenhouse conditions. The transfer of atmospheric carbondioxide to the ocean would result in the rapid precipitation of calcium carbonatein warm surface waters, producing the cap carbonate rocks observed globally.
During the 200 million years (My) precedingthe appearance of macroscopic metazoans,!750 to 550 million years ago (Ma) (1), thefragmentation of a long-lived supercontinent(2) was accompanied by intermittent, but wide-spread, glaciation (3–5). Many of the glacialdeposits contain carbonate debris or are directlyoverlain by carbonate rocks (6, 7 ), includinginorganic sea-floor precipitates, which are nor-mally limited to warm-water settings (8). Post-glacial carbonate rocks (cap carbonates) occureven in terrigenous-dominated sections (6, 7 ).Certain glacial units contain large sedimentaryiron formations (9), which reappear after a1-billion-year hiatus in the stratigraphic record.The glacial intervals are spanned by decreasesof as much as 14 per mil in the "13C value ofthe surface ocean (10, 11). These isotopic ex-cursions are enormous in comparison with any
excursions in the preceding 1.2 billion years(12) or in the Phanerozoic eon (13).
Paleomagnetic evidence suggests that theice line reached sea level close to the equatorduring at least two Neoproterozoic glacial epi-sodes (14). The origin of these extreme glacia-tions has been controversial (1, 15, 16 ). Kirsch-vink (17 ) proposed a snowball Earth, createdby a runaway albedo feedback, in which theworld ocean was virtually covered by sea icebut continental ice cover was thin and patchybecause of the virtual elimination of the hydro-logic cycle. Kirschvink applied this hypothesisto explain the low-paleolatitude glacial depositsas well as the occurrence of banded iron for-mations, suggesting that an ocean sealed by seaice would quickly become anoxic and rich indissolved ferrous iron (17 ). Here, we presentnew data on the amplitude, timing, and durationof inorganic "13C variations in Neoproterozoicrocks of northern Namibia and the relationbetween these variations and glaciation.We show that the snowball Earth hypothe-sis best explains the geological and geo-chemical observations, including the "13Cexcursions and the existence of carbonatesimmediately following glaciations.
We studied the Otavi Group (Fig. 1), acarbonate platform covering the southern prom-ontory of the Congo Craton in northern Nami-bia (15, 18, 19). In the late Neoproterozoic, theCongo Craton was a Bahama-type sea-levelplatform that was about the size of the conter-minous United States. Paleomagnetic data fromthe eastern part of the craton (20) imply that theOtavi Group was at !12°S paleolatitude at743 # 30 Ma and at !39°S at 547 # 4 Ma. TheOtavi Group contains two discrete glacial units(Chuos and Ghaub formations) of Sturtian(!760 to 700 Ma) age (15, 19). Both units areunderlain by thick carbonate successions withhigh "13C values, and both units are overlain bydistinctive cap carbonates, recording negative"13C excursions (10, 11).
The younger of the two glacial units (theGhaub Formation) is represented by unstratifieddiamictons, debris flows, and, at the top, varve-like detrital couplets crowded with ice-rafteddropstones (15). Both the onset and the termi-nation of glaciogenic sedimentation wereabrupt. The glacial deposits are composed pre-dominantly of dolomite and limestone debrisderived from the underlying Ombaatjie plat-form (Fig. 1). Clast and matrix lithologic com-positions covary; thus, we interpreted the ma-trix as being detrital in origin and not as aseawater proxy. Glacial deposits on the plat-form are thin and highly discontinuous (not dueto subsequent erosion). Alternately groundedand floating sea ice caused large horizontalplates to be detached from the directly under-lying bedrock. The subglacial erosion surfacehas remarkably little relief on the platform(!50 m relative to underlying strata over adistance of 150 km), suggesting that any fall inrelative sea level was limited or short-lived.Comparatively thick sections ($ 180 m) of dia-mictons and debris flows occur on the conti-nental slope, suggesting that the ice groundingline remained close to the platform edge (Fig.1). These observations are consistent with anabrupt development and a subsequent dissipa-tion of grounded sea ice on a tropical or sub-
P. F. Hoffman, G. P. Halverson, D. P. Schrag, Depart-ment of Earth and Planetary Sciences, Harvard Uni-versity, Cambridge, MA 02138, USA. A. J. Kaufman,Department of Geology, University of Maryland, Col-lege Park, MD 20742, USA.
*To whom correspondence should be addressed. E-mail: [email protected]
R E P O R T S
28 AUGUST 1998 VOL 281 SCIENCE www.sciencemag.org1342
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References and Notes1. J. F. Nye, R. Soc. London Proc. Ser. A 219, 477 (1953);
R. LeB Hooke, Rev. Geophys. Space Phys. 19, 664(1981); C. J. van der Veen and I. M. Whillans, J.Glaciol. 36, 324 (1990).
2. Examples include velocity variations occurring overmonths to weeks [R. LeB Hooke, P. Calla, et al., J.Glaciol. 35, 235 (1989)] and days to hours [A. Ikenand R. A. Bindschadler, ibid. 32, 101 (1986)].
3. S. M. Hodge, ibid. 13, 349 (1974); B. Kamb et al., J.Geophys. Res. 99, 15231 (1994); J. Harbor et al.,Geology 25, 739 (1997).
4. A dense array of radio-echo sounding measurementswere processed with three-dimensional migration tech-niques. Comparisons of these measurements with bore-hole observations suggest that the radar is accurate towithin about 8.5 m (B. C. Welch, W. T. Pfeffer, J. T.Harper, N. F. Humphrey, J. Glaciol., in press).
5. J. T. Harper and N. F. Humphrey, Geology 23, 901(1995).
6. M. F. Meier, U.S. Geol. Surv. Prof. Pap. 351 (1960);
W. S. B. Paterson and J. C. Savage, J. Geophys. Res. 68,4537 (1963); C. F. Raymond, J. Glaciol. 10, 55 (1971);R. LeB. Hooke, P. Holmlund, N. R Iverson, ibid. 33, 72(1987) were all forced to smooth inclinometry databecause of high levels of noise.
7. The instrument was constructed by Slope IndicatorCanada, Ltd. (Vancouver, BC). Measurement errorsassociated with a prototype of this instrument arediscussed by E. W. Blake and G. K. C. Clarke [ J.Glaciol. 38, 113 (1992)]. However, analysis of ac-tual data from the instrument used suggests thatinstrument errors are slightly improved from man-ufacturer specifications [ J. T. Harper, thesis, Uni-versity of Wyoming (1997); S. V. Huzurbazar, un-published material]. Additionally, the uniformity ofthe borehole walls enabled a high degree of repeat-ability for the measurements.
8. We follow the method of C. F Raymond, J. Glaciol.10, 39 (1971).
9. We use a cubic spline function with an iterativescheme designed to minimize the curvature of thefunction between data points [I. C. Briggs, Geophysics
1974, 39 (1974)]. This interpolation was tested ex-tensively with synthetic data.
10. J. T. Harper, N. F. Humphrey, W. T. Pfeffer, B. C.Welch, U.S. Army Cold Reg. Res. Eng. Lab. Spec. Rep.96-27 (1996), p. 41.
11. This measurement was made within the samereach and time of year as the deformation exper-iments, but during a subsequent year. Sliding andsurface velocities were determined by continuousfilming of the base of a borehole with concurrentsurveying of velocity at the surface.
12. Funded by grants from NSF (OPP-9122966 to N.F.H.and OPP-9122916 to W.T.P.). Additional funding forcomputer visualization was provided by NSF’sEPSCoR (Experimental Program to Stimulate Com-petitive Research) program (EPS9550477), throughthe University of Wyoming’s Spatial Data and Visu-alization Center project. D. Bahr, B. Welch, and B.Raup all made significant contributions to portions ofthe work presented here.
16 April 1998; accepted 28 July 1998
A Neoproterozoic SnowballEarth
Paul F. Hoffman,* Alan J. Kaufman, Galen P. Halverson,Daniel P. Schrag
Negative carbon isotope anomalies in carbonate rocks bracketing Neoprotero-zoic glacial deposits in Namibia, combined with estimates of thermal subsi-dence history, suggest that biological productivity in the surface ocean col-lapsed for millions of years. This collapse can be explained by a global glaciation(that is, a snowball Earth), which ended abruptly when subaerial volcanicoutgassing raised atmospheric carbon dioxide to about 350 times the modernlevel. The rapid termination would have resulted in a warming of the snowballEarth to extreme greenhouse conditions. The transfer of atmospheric carbondioxide to the ocean would result in the rapid precipitation of calcium carbonatein warm surface waters, producing the cap carbonate rocks observed globally.
During the 200 million years (My) precedingthe appearance of macroscopic metazoans,!750 to 550 million years ago (Ma) (1), thefragmentation of a long-lived supercontinent(2) was accompanied by intermittent, but wide-spread, glaciation (3–5). Many of the glacialdeposits contain carbonate debris or are directlyoverlain by carbonate rocks (6, 7 ), includinginorganic sea-floor precipitates, which are nor-mally limited to warm-water settings (8). Post-glacial carbonate rocks (cap carbonates) occureven in terrigenous-dominated sections (6, 7 ).Certain glacial units contain large sedimentaryiron formations (9), which reappear after a1-billion-year hiatus in the stratigraphic record.The glacial intervals are spanned by decreasesof as much as 14 per mil in the "13C value ofthe surface ocean (10, 11). These isotopic ex-cursions are enormous in comparison with any
excursions in the preceding 1.2 billion years(12) or in the Phanerozoic eon (13).
Paleomagnetic evidence suggests that theice line reached sea level close to the equatorduring at least two Neoproterozoic glacial epi-sodes (14). The origin of these extreme glacia-tions has been controversial (1, 15, 16 ). Kirsch-vink (17 ) proposed a snowball Earth, createdby a runaway albedo feedback, in which theworld ocean was virtually covered by sea icebut continental ice cover was thin and patchybecause of the virtual elimination of the hydro-logic cycle. Kirschvink applied this hypothesisto explain the low-paleolatitude glacial depositsas well as the occurrence of banded iron for-mations, suggesting that an ocean sealed by seaice would quickly become anoxic and rich indissolved ferrous iron (17 ). Here, we presentnew data on the amplitude, timing, and durationof inorganic "13C variations in Neoproterozoicrocks of northern Namibia and the relationbetween these variations and glaciation.We show that the snowball Earth hypothe-sis best explains the geological and geo-chemical observations, including the "13Cexcursions and the existence of carbonatesimmediately following glaciations.
We studied the Otavi Group (Fig. 1), acarbonate platform covering the southern prom-ontory of the Congo Craton in northern Nami-bia (15, 18, 19). In the late Neoproterozoic, theCongo Craton was a Bahama-type sea-levelplatform that was about the size of the conter-minous United States. Paleomagnetic data fromthe eastern part of the craton (20) imply that theOtavi Group was at !12°S paleolatitude at743 # 30 Ma and at !39°S at 547 # 4 Ma. TheOtavi Group contains two discrete glacial units(Chuos and Ghaub formations) of Sturtian(!760 to 700 Ma) age (15, 19). Both units areunderlain by thick carbonate successions withhigh "13C values, and both units are overlain bydistinctive cap carbonates, recording negative"13C excursions (10, 11).
The younger of the two glacial units (theGhaub Formation) is represented by unstratifieddiamictons, debris flows, and, at the top, varve-like detrital couplets crowded with ice-rafteddropstones (15). Both the onset and the termi-nation of glaciogenic sedimentation wereabrupt. The glacial deposits are composed pre-dominantly of dolomite and limestone debrisderived from the underlying Ombaatjie plat-form (Fig. 1). Clast and matrix lithologic com-positions covary; thus, we interpreted the ma-trix as being detrital in origin and not as aseawater proxy. Glacial deposits on the plat-form are thin and highly discontinuous (not dueto subsequent erosion). Alternately groundedand floating sea ice caused large horizontalplates to be detached from the directly under-lying bedrock. The subglacial erosion surfacehas remarkably little relief on the platform(!50 m relative to underlying strata over adistance of 150 km), suggesting that any fall inrelative sea level was limited or short-lived.Comparatively thick sections ($ 180 m) of dia-mictons and debris flows occur on the conti-nental slope, suggesting that the ice groundingline remained close to the platform edge (Fig.1). These observations are consistent with anabrupt development and a subsequent dissipa-tion of grounded sea ice on a tropical or sub-
P. F. Hoffman, G. P. Halverson, D. P. Schrag, Depart-ment of Earth and Planetary Sciences, Harvard Uni-versity, Cambridge, MA 02138, USA. A. J. Kaufman,Department of Geology, University of Maryland, Col-lege Park, MD 20742, USA.
*To whom correspondence should be addressed. E-mail: [email protected]
R E P O R T S
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NEWS OF THE WEEK
In 1998, a handful of geoscientists at Harvard
University breathed new life into a daring idea:
that Earth froze over from pole to pole more
than a half-billion years ago, threatening life
with extinction but perhaps prodding it to
greater evolutionary heights (Science,
28 August 1998, p. 1342). On page 1241 of
this issue, geoscientists report evidence
that the tropics also hosted glaciers more than
100 million years before that sup-
posed global freeze. Such low-
latitude glaciation is a hallmark of
so-called hard snowball Earth sce-
narios, in which a kilometer of ice
sealed off the world ocean.
But despite the new work, the
much-studied hypothesis has
fallen on hard times. “In many
people’s minds, the hard snow-
ball is dead,” says geochemist
Michael Arthur of Pennsylvania
State University (PSU), Univer-
sity Park, who was not involved in
the new work. Earth was pro-
foundly cold in those geologi-
cally weird days, many agree—a
“slushball” of a planet, perhaps.
But sealed in ice? Unlikely.
The new contribution to the
snowball debate comes from Har-
vard geologist Francis Macdonald
and colleagues at Harvard and
elsewhere. They dated volcanic
ash layered within deposits of the
so-called Sturtian glacial era to an
age of 716.5 million years. That’s
the same age as rocks whose pale-
omagnetic record places them
and the Sturtian glaciers in the tropics.
Researchers speculated about possible
ancient tropical glaciers for several decades
before geobiologist Joseph Kirschvink of
the California Institute of Technology in
Pasadena coined the term “snowball Earth”
in 1992. But the hard-snowball concept
gained ground only after geologist Paul
Hoffman—then at Harvard University and
now retired—and three colleagues boosted
it in the 1998 Science paper. Drawing on
simple climate modeling, the authors con-
cluded that any ice that reached tropical lat-
itudes during the Marinoan glaciation,
about 650 million years ago, would not
have s topped there. Instead, once the
highly reflective ice covered enough area, a
climatic feedback would inevitably drive
the ice to the equator and create global
glaciation: a hard snowball.
Some more-recent paleoclimate modeling,
however, suggests that the leap from low-
latitude glaciation to a hard snowball may be
difficult or even impossible. “We can get ice
on land,” says climate modeler Mark Chandler
of the Goddard Institute for Space Studies in
New York City. “It’s the ocean we can’t freeze
over.” Model oceans can hold lots of heat and
move it around in cur-
rents, frustrating a
complete freeze-over,
Chandler says. A few years ago, “the pattern
was that the more sophisticated the model, the
less likely you’d get a hard snowball result,” he
says. Discouraged, Chandler and others
moved on to other projects.
Atmospheric physicist James Kasting of
PSU now favors a slightly more modest
“thin ice” snowball. He and climate mod-
eler David Pollard of PSU have considered
how a continent poleward of an inland sea
might hold off thick ice intruding from
higher latitudes and preserve small areas of
thin ocean ice, thin enough to let sunlight
through for marine plants. “We think the
thin-ice solution satisfies all the constraints
better than the other models.”
But almost all geologists now reject any
worldwide freeze, says geologist Philip Allen
of Imperial College London. “When the
snowball came up, the [geological] commu-
nity was very open to it,” he says. Now, “it’s
my impression that 90% of the geological
community is quite hostile to the idea.”
Allen and other geologists went to the field
to study glacial deposits from about the time
of the proposed Marinoan hard snowball.
Instead of stagnation, the sediments recorded
signs of water and ice in motion: ice moving,
ocean currents flowing, and waves moving on
an open sea. “We do not have a hard snowball
Earth,” says Allen. Hoffman hasn’t disputed
such interpretations, but he has argued that
they could reflect conditions either just before
or after a hard snowball.
Most geochemists aren’t sold on a hard
snowball either. Key to the Harvard group’s
argument was the contention that a bizarre
chemical deposit found on the top of glacial
deposits—the cap carbonate formation—
could have formed after a glacial period only
if the world ocean had been sealed off from
the atmosphere for millions of years. Only
rare cracks in the ice or open water maintained
by volcanic hot spots kept the biota going, the
group maintained. Geologist Alan Jay
Kaufman of the University of
Maryland, College Park, a
co-author of the 1998 Science
paper, has shifted his stance.
After studying the isotopic
records of carbon, strontium,
and sulfur, he now supports the
slushball view. The sulfur iso-
topes in particular, he says,
suggest “that there was more
than cracks in the ice.”
Hoffman is unperturbed.
Resistance to the hard snowball “is really typi-
cal of scientific controversy,” he says. “The
problem is the experts reach a quick judgment
and dig themselves into a position.” The idea of
a recent ice age, he notes, took 40 years and a
new generation of scientists to win acceptance
in the 19th century. In his view, “the evidence
[for a hard snowball] is getting stronger and
stronger.” He cites oxygen isotope findings
published last year supporting the existence of
extremely high atmospheric carbon dioxide
concentrations predicted by the hard snowball.
Still, Hoffman says, “I don’t expect to live to
see the conclusion on Snowball Earth, though I
think I know how it will turn out.”
–RICHARD A. KERR
Snowball Earth Has Melted Back To a Profound Wintry MixPALEOCLIMATOLOGY
Definitely chilly. Clear signs of glacia-tion, such as rocks dropped to the seafloor from icebergs (inset), show up intropical deposits (dark peak).
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www.sciencemag.org SCIENCE VOL 327 5 FEBRUARY 2010 655
PERSPECTIVES
mechanism consisting of photosyn-thetic fi xation of CO
2 in sunlit surface
waters and subsequent transfer of fi xed carbon to the ocean’s depths through the settling of plankton and other par-ticulate organic material (see the fi g-ure). The settling particles are eventu-ally respired back to CO
2 by bacteria,
increasing the CO2 concentration in
the ocean’s interior by a factor of 2 to 3 over that in surface waters. In large high-latitude regions, especially the Southern Ocean, phytoplankton fi xa-tion of carbon, and hence the carbon pump, is restricted by inadequate sup-plies of iron, an hypothesis fi rst pro-posed by the late John Martin ( 7– 9). Iron concentrations in seawater are extremely low because of the low sol-ubility of Fe(III), the thermodynami-cally stable redox form of iron. Iron concentrations in the Southern Ocean are especially low ( 10) because of low inputs of iron-rich wind-borne dust from arid land regions, the major path-way of iron input to the ocean ( 11).
In a related process, iron can further regulate the ocean’s carbon pump by limiting nitrogen fixation by marine photosynthetic bacteria, a process that controls the ocean’s biologically avail-able chemical forms (primarily nitrate) of nitrogen ( 12). Photosynthesis in much of the ocean (such as the mid-ocean gyres) is not limited directly by iron but by the availabil-ity of nitrogen ( 13). By limiting nitrogen fi x-ation, iron indirectly infl uences the ocean’s biological pump by infl uencing the supply of nitrogen ( 12).
Thus, any process that reduces the avail-ability of iron to phytoplankton should restrict the ocean’s biological carbon pump, thereby increasing atmospheric CO
2 concen-
trations and global warming. Shi et al. found that an increase in CO
2 and concomitant
decrease in pH within the range expected to occur in surface seawater by 2100 decreased iron uptake by a diatom species by 10 to 20% but had no effect on the relation between specifi c growth rate and cellular iron con-centration. By itself, this fi nding suggests that a lowering of the ocean water pH from increasing CO
2 may decrease iron availabil-
ity to phytoplankton, thereby restricting the biological carbon pump.
But when we consider other complicat-ing factors, the implications are less clear. The decrease in biological iron uptake with decreasing pH is believed to be largely linked to a pH-driven increase in the binding of iron by dissolved organic molecules (called
ligands), which reduces the concentration of highly biologically available dissolved inor-ganic Fe(III) species. Independent measure-ments indeed indicate that lower pH increases iron binding to organic ligands in seawater ( 14). But increased organic binding should also decrease the tendency of iron to precipi-tate as Fe(III) oxides and to adsorb on parti-cle surfaces—two key mechanisms by which iron is removed from seawater—and could thus increase iron retention rates and dis-solved iron concentrations.
Moreover, rates of Fe(III) reduction to soluble Fe(II) also generally increase [and rates of reoxidation to Fe(III) decrease] with decreasing seawater pH ( 3). These changes may increase concentrations of biologi-cally available Fe(II). In a recent mesocosm experiment with coastal seawater, a tripling of the CO
2 partial pressure over current levels
increased concentrations of ferrous iron by a factor of 2, but effects on biological uptake were not assessed ( 15).
Finally, inputs of iron from wind-borne desert dust ( 11) will likely change with chang-ing regional climate patterns, and inputs of highly soluble, iron-rich soot particles from the burning of oil and other fossil fuels may increase ( 16).
Thus, the availability of iron to phyto-plankton in a future CO
2-enriched Earth will
be controlled by a number of complex and
poorly resolved factors, whose combined effect is uncertain. By providing new infor-mation on iron availability at low pH, Shi et
al. help to reduce that uncertainty and pro-vide new insights needed to refi ne ocean car-bon cycling models. Their fi ndings also high-light the important issue of potential future changes in iron regulation of ocean produc-tivity and climate with increasing anthropo-genic CO
2 emissions.
References 1. R. A. Feely, S. C. Doney, S. R. Cooley, Oceanogr. 22, 36
(2009). 2. D. Shi et al., Science 327, 676 (2010). 3. F. J. Millero, R. Woosley, B. Ditrolio, J. Waters, Oceanogr.
22, 72 (2009). 4. W. Stumm, J. J. Morgan, Aquatic Chemistry (Wiley, New
York, NY, 1981). 5. J. A. Kleypas, K. K. Yates, Oceanogr. 22, 108 (2009). 6. F.-X. Fu et al., Limnol. Oceanogr. 53, 2472 (2008). 7. J. H. Martin, S. E. Fitzwater, Nature 331, 341 (1988). 8. J. H. Martin, Paleooceanogr. 5, 1 (1990). 9. K. H. Coale et al., Science 304, 408 (2004). 10. J. H. Martin, R. M. Gordon, S. E. Fitzwater, Nature 345,
156 (1990). 11. T. D. Jickells et al., Science 308, 67 (2005). 12. P. G. Falkowski, Nature 387, 272 (1997). 13. J. Moore, S. Doney, K. Lindsay, Global Biogeochem.
Cycles 18, GB4028 (2004). 14. M. Gledhill, C. M. G. van den Berg, R. F. Nolting, K. R.
Timmermans, Mar. Chem. 59, 283 (1998). 15. E. Breitbarth et al., Biogeosciences Discuss. 6, 6781
(2009). 16. E. R. Sholkovitz, P. N. Sedwick, T. M. Church, Geochim.
Cosmochim. Acta 73, 3981 (2009).
10.1126/science.1186151
Algal cellCO
2
CO2
CO2, HCO3
–
CO2
H2O
H2CO3
HCO3–
CO3
2–
CO3
2–
H+
, H+
SURFACE OCEAN
DEEP OCEAN
Advection
and mixing
Settlingorganicparticles
Iron chemistry
Fe
N, P, Si
Major nutrients(N, P, Si)
The carbon pump. Anthropogenic CO2 emissions increase surface ocean concentrations of CO
2 and hydrogen ions,
which in turn affects iron chemistry and iron-limited photosynthetic fi xation of CO2 by algal cells. This CO
2 fi xation
regulates the ocean’s ability to absorb CO2 by the settling of algal-derived particulate organic matter to the deep
ocean. CO2 is released into the deep ocean by bacterial respiration of the settling particles; here, it undergoes the
same chemical reactions as shown for surface ocean waters. In deep ocean waters (below ~800 m depth), CO2 has a
residence time of ~1000 years.
Published by AAAS
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Geologic ErasEra Period Epoch Approx. number of years
(millions)Cenozoic Quaternary Holocene (Recent)
Pleistocene
65
225
570
Tertiary PlioceneMioceneOligoceneEocenePaleocene
Mesozoic CretaceousJurassicTriassic
Paleozoic PermianCarboniferous(Pennsylvanian and Mississippian)DevonianSilurianOrdovicianCambrian
Precambrian
Burgess Shale (BC)
Fossil-bearing shale in Canadian Rockies: exceptional record of ocean life during the Cambrian explosion, 0.5 Gyr ago.
Burgess Shale (BC)
Anomalocaris fossil
Ottoia worm fossil
Walcott’s quarry
• Darwinism (as in “The Origin of Species”)
• Neo-Darwinism
è Evolution by natural selection (survival of and reproduction by the fittest), with possible contribution from inheritance of acquired traits (Lamarckism).
è Modern synthesis:Evolution by natural selection + Genetics + Ecology + Molecular biology – Inheritance of acquired traits.
• Evolution:• Change is central• Mass extinctions = creative force (“creative destruction”)
• Diversity of Life: • Why is there so much? Is it all necessary? (to whom or what?)• What is the context for homo sapiens?• What is the human impact on diversity?
• Historical concepts of evolution:– Gradualism vs. catastrophism.– Western religious thought: largely catastrophist.– Post-Newton, enlightenment: gradualism.
• Extinctions = measure of fitness (bad genes).– “All nature is at war, one organism with another”.
– 20th century paleontology, Earth science, astrophysics:• Reassertion of catastrophism: extinctions = lottery (bad luck)
The Biological Case• Organic building blocks are ubiquitous:
Interstellar clouds, molecules, dust, comets, etc.
• Early Earth: Life formed rapidly after the late heavy bombardment
• RNA world ® DNA world ?– RNA = autocatalytic.– A matter of inevitable chemistry? – If so, expect that bacteria = chemistry. Þ Ubiquitous life.
• Are there alternative chemistries?
Alternative Biochemistries?• Alternative chirality molecules:
Terrestrial: L amino acids, D sugars
• Non-carbon based biochemistry?“Carbon chauvinism”. Silicon biochemistry (‘organosilicon’)?
à Si less versatile than C; cannot form double bonds (so no analog to carbonyl group compounds).
à SiO2 = analog to CO2 but does not dissolve in water.
à Carbon cosmically more abundant 10:1, but less abundant in Earth’s crust. Yet life is carbon based.
à But … c.f. silicate skeletons of diatoms. http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry
Silane = analog to methane
No Si analog
Alternative Biochemistries?Other exotic element bases:à Chlorine as an alternative to oxygen (electron
receptor).à Arsenic as an alternative to phosphorus: some
microbes metabolize As.
Non-water solvents?à Ammonia: not as versatile as water in ability to
form both acids and bases (bonding properties); temperature range: 195 to 240 K.
à Methane CH4: Titan? (91 to 112 K.)à Hydrogen fluoride (HF).
http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry
Silane = analog to methane
No Si analog
ASTROBIOLOGYVolume 9, Number 2, 2009© Mary Ann Liebert, Inc.DOI: 10.1089/ast.2008.0251
Hypothesis Article
Signatures of a Shadow Biosphere
Paul C.W. Davies,1 Steven A. Benner,2 Carol E. Cleland,3 Charles H. Lineweaver,4Christopher P. McKay,5 and Felisa Wolfe-Simon6
Abstract
Astrobiologists are aware that extraterrestrial life might differ from known life, and considerable thought hasbeen given to possible signatures associated with weird forms of life on other planets. So far, however, verylittle attention has been paid to the possibility that our own planet might also host communities of weird life.If life arises readily in Earth-like conditions, as many astrobiologists contend, then it may well have formedmany times on Earth itself, which raises the question whether one or more shadow biospheres have existed inthe past or still exist today. In this paper, we discuss possible signatures of weird life and outline some simplestrategies for seeking evidence of a shadow biosphere. Key Words: Weird life—Multiple origins of life—Bio-genesis—Biomarkers—Extremophiles—Alternative biochemistry. Astrobiology 9, 241–249.
241
1. Background
THE HISTORY OF OUR DEVELOPING UNDERSTANDING of life onEarth has been characterized by repeated discovery, dri-
ven largely by improvements in techniques to explore theEarth’s biosphere. The age of enlightenment brought explo-ration technologies that led to the discovery of new biota inthe Americas, Australia, and Africa. The invention of the mi-croscope uncovered an unexpected microbial world. RNAsequencing in the 1960s and 1970s revealed that the prokary-otic biosphere itself consists of two domains that are as dif-ferent from each other as they are from eukaryotes. Together,these discoveries revolutionized our understanding of thehistory of life on Earth over the past three billion years.
Today, it is believed that microbes constitute the vast ma-jority of terrestrial species. Nevertheless, the microbial realmremains poorly explored and characterized. Less than 1% ofmicrobes has been cultured and described (Amann et al.,1995; Pace, 1997; Hugenholtz et al., 2006). Because microbialmorphology is very limited, it is in most cases difficult, if notimpossible, to deduce much about the nature of microbial
life by simply looking at it. Gene sequencing has so farproven to be the only reliable method to determine the re-lationship of a given microbial species to other known life.This extensive ignorance raises the intriguing issue of howsure we can be that all microbial types have been identified.Might it be the case that the exploration of the biosphere isnot complete, and deep additional branches of the tree of lifehave so far been overlooked? Is it even possible that micro-bial life exists that does not share a common descent withfamiliar organisms and, therefore, constitutes a different treealtogether, deriving from an independent genesis?
It is relatively uncontroversial that at least one very dif-ferent kind of life existed on early Earth. It had no encodedproteins but rather used RNA as the sole genetically encodedcomponent of biocatalysts. This conjecture is supported bythe catalytic properties of RNA and the detailed structure ofthe ribosome, a complex structure built from both proteinand RNA, but where the RNA is clearly responsible for theprotein synthesis (Moore and Steitz, 2002). It is not clear thatexisting life-detection strategies, which mainly target the ri-bosomal machinery, would register any surviving RNA or-
1BEYOND: Center for Fundamental Concepts in Science, Arizona State University, Tempe, Arizona.2Foundation for Applied Molecular Evolution, Gainesville, Florida.3Department of Philosophy and the Center for Astrobiology, University of Colorado, Boulder, Colorado.4Planetary Science Institute, Research School of Astronomy and Astrophysics & Research School of Earth Sciences, Australian National
University, Canberra, Australia.5Space Science Division, NASA Ames Research Center, Moffett Field, California.6Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts.
Perspective
The universal nature of biochemistryNorman R. Pace*
University of Colorado, Boulder, CO 80309-0347
People have long speculated about thepossibility of life in settings other than
Earth. Only in the past few centuries,however, have we been able to conceive ofthe specific nature of such settings: otherplanets around our own sun and solarsystems similar to our own elsewhere inthe physical universe. Speculation on thenature of life elsewhere often has paidlittle heed to constraints imposed by thenature of biochemistry, however. A cen-tury of fanciful science fiction has resultednot only in social enthusiasm for the questfor extraterrestrial life, but also in fancifulnotions of the chemical and physical formsthat life can take, what the nature of lifecan be. Since the time of the Vikingmissions to Mars, in the mid-1970s, ourview of life’s diversity on Earth has ex-panded significantly, and we have a betterunderstanding of the extreme conditionsthat limit life. Consequently, our searchfor extant life elsewhere in the solar sys-tem can now be conducted with broaderperspective than before.
How can life be detected regardless of itsnature and origin? Considering the recentspectacular advances in observational as-tronomy, it seems likely that the first sign oflife elsewhere will be the spectroscopic de-tection of co-occurring nonequilibriumgases, for instance oxygen and methane, inthe atmosphere of a planet around somedistant star. Co-occurrence of such gaseswould indicate that they are replenished,perhaps most readily explained by the in-fluence of life (1). By observation of oxygenand methane, Earth could possibly be seenas a home for life even from distant galaxies.Other potential habitats for life in this solarsystem, such as Mars and Europa, however,are not so obvious. The search for life onthose bodies will be conducted at the level ofanalytical chemistry. As we undertake thedetection of extraterrestrial life, it is instruc-tive to try to put constraints on what thenature of life can be. These constraints, therequirements for life, tell us where and howto look for life, and the forms that it cantake.
What Is Life?An early question that needs to be con-fronted, indeed a question that in the lastanalysis requires definition, is: What is life?
Most biologists would agree that self-replication, genetic continuity, is a funda-mental trait of the life process. Systems thatgenerally would be deemed nonbiologicalcan exhibit a sort of self-replication, how-ever (2). Examples would be the growth ofa crystal lattice or a propagating clay struc-ture. Crystals and clays propagate, unques-tionably, but life they are not. There is nolocus of genetic continuity, no organism.Such systems do not evolve, do not changein genetic ways to meet new challenges.Consequently, the definition of life shouldinclude the capacity for evolution as well asself-replication. Indeed, the mechanism ofevolution—natural selection—is a conse-quence of the necessarily competing drivesfor self-replication that are manifest in allorganisms. The definition based on thoseprocesses, then, would be that life is anyself-replicating, evolving system.
The processes of self-replication and evo-lution are not reliably detectable, even in theterrestrial setting. Consequently, in thepractical search for life elsewhere we needto incorporate information on the nature ofthe chemistries that can provide the basis forself-replication and evolution. Consideringthe properties of molecules likely to beneeded to replicate and evolve, it is predict-able that life that we encounter anywhere inthe universe will be composed of organicchemicals that follow the same general prin-ciples as our own organic-based terrestriallife. The operational definition of life thenbecomes: Life is a self-replicating, evolvingsystem expected to be based on organicchemistry.
Why Organic Chemistry?The basic drive of life is to make more ofitself. The chemical reactions required forthe faithful propagation of a free-living or-ganism necessarily require high degrees ofspecificity in the interactions of the mole-cules that carry out the propagation. Suchspecificity requires information, in the formof complex molecular structure—large mol-ecules. The molecules that serve terrestrialorganisms typically are very large, proteinsand RNAs with molecular weights of thou-sands to millions of daltons, or even larger asin the case of genetic DNA. It is predictablethat life, wherever we encounter it, will becomposed of macromolecules.
Only two of the natural atoms, carbonand silicon, are known to serve as the back-bones of molecules sufficiently large to carrybiological information. Thought on thechemistry of life generally has focused oncarbon as unique (3). As the structural basisfor life, one of carbon’s important features isthat unlike silicon it can readily engage inthe formation of chemical bonds with manyother atoms, thereby allowing for the chem-ical versatility required to conduct the reac-tions of biological metabolism and propa-gation. The various organic functionalgroups, composed of hydrogen, oxygen, ni-trogen, phosphorus, sulfur, and a host ofmetals, such as iron, magnesium, and zinc,provide the enormous diversity of chemicalreactions necessarily catalyzed by a livingorganism. Silicon, in contrast, interacts withonly a few other atoms, and the large siliconmolecules are monotonous compared withthe combinatorial universe of organicmacromolecules.
Life also must capture energy and trans-form that energy into the chemistry of rep-lication. The electronic properties of car-bon, unlike silicon, readily allow theformation of double or even triple bondswith other atoms. These chemical bondsallow the capture and delocalization of elec-tronic energy. Some carbon-containingcompounds, therefore, can be highly polar-ized and thereby capture ‘‘resonance ener-gy’’ and transform this chemical energy todo work or to produce new chemicals in acatalytic manner. The potential polarizabil-ity of organic compounds also contributes tothe specificity of intermolecular interac-tions, because ionic and van der Waalscomplementarities can shift to mesh with orto repulse one another. Finally, it is criticalthat organic reactions, in contrast to silicon-based reactions, are broadly amenable toaqueous conditions. Several of its propertiesindicate that water is likely to be the milieufor life anywhere in the universe (2).
The likelihood that life throughout theuniverse is probably carbon-based is en-couraged by the fact that carbon is one ofthe most abundant of the higher elements.Astronomical studies find complex or-ganic compounds strewn throughout in-
*E-mail: [email protected].
PNAS ! January 30, 2001 ! vol. 98 ! no. 3 ! 805–808
PERS
PECT
IVE
SPEC
IAL
FEA
TURE
• What is life?• Why organic
chemistry (C vs Si)• The universal
nature of biochemistry
• HZ: chemical, physical, pressure-volume settings.
• Genetic signature of terrestrial life
• Bets on transport between planets
“So, if we go to Mars or Europa and find living creatures there, and read their rRNA genes, we should not be surprised if the sequences fall into our own relatedness group, as articulated in the tree of life.”
A Bacterium That Can Grow by UsingArsenic Instead of PhosphorusFelisa Wolfe-Simon,1,2* Jodi Switzer Blum,2 Thomas R. Kulp,2 Gwyneth W. Gordon,3
Shelley E. Hoeft,2 Jennifer Pett-Ridge,4 John F. Stolz,5 Samuel M. Webb,6 Peter K. Weber,4
Paul C. W. Davies,1,7 Ariel D. Anbar,1,3,8 Ronald S. Oremland2
Life is mostly composed of the elements carbon, hydrogen, nitrogen, oxygen, sulfur, andphosphorus. Although these six elements make up nucleic acids, proteins, and lipids and thusthe bulk of living matter, it is theoretically possible that some other elements in the periodictable could serve the same functions. Here, we describe a bacterium, strain GFAJ-1 of theHalomonadaceae, isolated from Mono Lake, California, that is able to substitute arsenic forphosphorus to sustain its growth. Our data show evidence for arsenate in macromolecules thatnormally contain phosphate, most notably nucleic acids and proteins. Exchange of one of themajor bio-elements may have profound evolutionary and geochemical importance.
Biological dependence on the six majornutrient elements carbon, hydrogen, nitro-gen, oxygen, sulfur, and phosphorus (P)
is complemented by a selected array of other ele-ments, usually metals or metalloids present intrace quantities that serve critical cellular func-tions, such as enzyme co-factors (1). There aremany cases of these trace elements substitutingfor one another. A few examples include the sub-stitution of tungsten for molybdenum and cad-mium for zinc in some enzyme families (2, 3) andcopper for iron as an oxygen-carrier in some ar-thropods andmollusks (4). In these examples andothers, the trace elements that interchange sharechemical similarities that facilitate the swap. How-ever, there are no prior reports of substitutionsfor any of the six major elements essential forlife. Here, we present evidence that arsenic cansubstitute for phosphorus in the biomolecules ofa naturally occurring bacterium.
Arsenic (As) is a chemical analog of P, whichlies directly below P on the periodic table. Arsenicpossesses a similar atomic radius, as well as nearidentical electronegativity to P (5). The most com-mon form of P in biology is phosphate (PO4
3–),which behaves similarly to arsenate (AsO4
3–) overthe range of biologically relevant pH and redoxgradients (6). The physicochemical similarity be-tween AsO4
3– and PO43– contributes to the bio-
logical toxicity of AsO43– because metabolic
pathways intended for PO43– cannot distinguish
between the two molecules (7) and AsO43– may
be incorporated into some early steps in the path-ways [(6) and references therein]. However, it isthought that downstream metabolic processes aregenerally not compatible with As-incorporatingmolecules because of differences in the reactiv-ities of P and As compounds (8). These down-
stream biochemical pathways may require themore chemically stable P-based metabolites; thelifetimes of more easily hydrolyzed As-bearinganalogs are thought to be too short. However,given the similarities of As and P—and by anal-ogywith trace element substitutions—we hypoth-esized that AsO4
3– could specifically substitutefor PO4
3– in an organism possessing mechanismsto cope with the inherent instability of AsO4
3–
compounds (6). Here, we experimentally testedthis hypothesis by using AsO4
3–, combined withno added PO4
3–, to select for and isolate a mi-crobe capable of accomplishing this substitution.
Geomicrobiology of GFAJ-1. Mono Lake,located in eastern California, is a hypersaline andalkaline water body with high dissolved arsenicconcentrations [200 mMon average (9)].We usedlake sediments as inocula into an aerobic definedartificial medium at pH 9.8 (10, 11) containing10 mM glucose, vitamins, and trace metals but noadded PO4
3– or any additional complex organicsupplements (such as yeast extract or peptone),with a regimen of increasing AsO4
3– additionsinitially spanning the range from 100 mMto 5mM.These enrichments were taken through manydecimal-dilution transfers, greatly reducing anypotential carryover of autochthonous phosphorus
1NASA Astrobiology Institute, USA. 2U.S. Geological Survey,Menlo Park, CA 94025, USA. 3School of Earth and SpaceExploration, Arizona State University, Tempe, AZ 85287, USA.4Lawrence Livermore National Laboratory, Livermore, CA 94551,USA. 5Department of Biological Sciences, Duquesne University,Pittsburgh, PA 15282, USA. 6Stanford Synchrotron RadiationLightsource, Menlo Park, CA 94025, USA. 7BEYOND: Centerfor Fundamental Concepts in Science, Arizona State University,Tempe, AZ 85287, USA. 8Department of Chemistry and Bio-chemistry, Arizona State University, Tempe, AZ 85287, USA.
*To whom correspondence should be addressed. E-mail:[email protected]
5µm
5µm
1µm
C
D
E
A
B
5 x 105
5 x 106
5 x 107
5 x 108
cells
ml-1
0 120 240 360 480Time (h)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
OD
680
Fig. 1. Growth and electron microscopy of strain GFAJ-1. (A and B) Growth curves of GFAJ-1 grown onthe defined synthetic medium amended with either 1.5 mM PO4
3– (solid circles), 40 mM AsO43– (solid
squares), or neither PO43– nor AsO4
3– (open triangles). Cell growth was monitored both by an increase in(A) optical density and (B) cell numbers of the cultures. Symbols represent the mean T SD of (A) n = 6experimental and n = 2 controls and (B) n = 3 experimental and n = 1 control. This was a singleexperiment with six replicates; however, material was conserved to extend the duration of the experimentto allow material for cell-counting samples. (C and D) Scanning electron micrographs of strain GFAJ-1under two conditions, (C) +As/–P and (D) –As/+P. (E) Transmission electron micrography of +As/–P GFAJ-1showed internal vacuole-like structures. Scale bars are as indicated in the figure (11).
RESEARCHARTICLE
www.sciencemag.org SCIENCE VOL 332 3 JUNE 2011 1163
A Bacterium That Can Grow by UsingArsenic Instead of PhosphorusFelisa Wolfe-Simon,1,2* Jodi Switzer Blum,2 Thomas R. Kulp,2 Gwyneth W. Gordon,3
Shelley E. Hoeft,2 Jennifer Pett-Ridge,4 John F. Stolz,5 Samuel M. Webb,6 Peter K. Weber,4
Paul C. W. Davies,1,7 Ariel D. Anbar,1,3,8 Ronald S. Oremland2
Life is mostly composed of the elements carbon, hydrogen, nitrogen, oxygen, sulfur, andphosphorus. Although these six elements make up nucleic acids, proteins, and lipids and thusthe bulk of living matter, it is theoretically possible that some other elements in the periodictable could serve the same functions. Here, we describe a bacterium, strain GFAJ-1 of theHalomonadaceae, isolated from Mono Lake, California, that is able to substitute arsenic forphosphorus to sustain its growth. Our data show evidence for arsenate in macromolecules thatnormally contain phosphate, most notably nucleic acids and proteins. Exchange of one of themajor bio-elements may have profound evolutionary and geochemical importance.
Biological dependence on the six majornutrient elements carbon, hydrogen, nitro-gen, oxygen, sulfur, and phosphorus (P)
is complemented by a selected array of other ele-ments, usually metals or metalloids present intrace quantities that serve critical cellular func-tions, such as enzyme co-factors (1). There aremany cases of these trace elements substitutingfor one another. A few examples include the sub-stitution of tungsten for molybdenum and cad-mium for zinc in some enzyme families (2, 3) andcopper for iron as an oxygen-carrier in some ar-thropods andmollusks (4). In these examples andothers, the trace elements that interchange sharechemical similarities that facilitate the swap. How-ever, there are no prior reports of substitutionsfor any of the six major elements essential forlife. Here, we present evidence that arsenic cansubstitute for phosphorus in the biomolecules ofa naturally occurring bacterium.
Arsenic (As) is a chemical analog of P, whichlies directly below P on the periodic table. Arsenicpossesses a similar atomic radius, as well as nearidentical electronegativity to P (5). The most com-mon form of P in biology is phosphate (PO4
3–),which behaves similarly to arsenate (AsO4
3–) overthe range of biologically relevant pH and redoxgradients (6). The physicochemical similarity be-tween AsO4
3– and PO43– contributes to the bio-
logical toxicity of AsO43– because metabolic
pathways intended for PO43– cannot distinguish
between the two molecules (7) and AsO43– may
be incorporated into some early steps in the path-ways [(6) and references therein]. However, it isthought that downstream metabolic processes aregenerally not compatible with As-incorporatingmolecules because of differences in the reactiv-ities of P and As compounds (8). These down-
stream biochemical pathways may require themore chemically stable P-based metabolites; thelifetimes of more easily hydrolyzed As-bearinganalogs are thought to be too short. However,given the similarities of As and P—and by anal-ogywith trace element substitutions—we hypoth-esized that AsO4
3– could specifically substitutefor PO4
3– in an organism possessing mechanismsto cope with the inherent instability of AsO4
3–
compounds (6). Here, we experimentally testedthis hypothesis by using AsO4
3–, combined withno added PO4
3–, to select for and isolate a mi-crobe capable of accomplishing this substitution.
Geomicrobiology of GFAJ-1. Mono Lake,located in eastern California, is a hypersaline andalkaline water body with high dissolved arsenicconcentrations [200 mMon average (9)].We usedlake sediments as inocula into an aerobic definedartificial medium at pH 9.8 (10, 11) containing10 mM glucose, vitamins, and trace metals but noadded PO4
3– or any additional complex organicsupplements (such as yeast extract or peptone),with a regimen of increasing AsO4
3– additionsinitially spanning the range from 100 mMto 5mM.These enrichments were taken through manydecimal-dilution transfers, greatly reducing anypotential carryover of autochthonous phosphorus
1NASA Astrobiology Institute, USA. 2U.S. Geological Survey,Menlo Park, CA 94025, USA. 3School of Earth and SpaceExploration, Arizona State University, Tempe, AZ 85287, USA.4Lawrence Livermore National Laboratory, Livermore, CA 94551,USA. 5Department of Biological Sciences, Duquesne University,Pittsburgh, PA 15282, USA. 6Stanford Synchrotron RadiationLightsource, Menlo Park, CA 94025, USA. 7BEYOND: Centerfor Fundamental Concepts in Science, Arizona State University,Tempe, AZ 85287, USA. 8Department of Chemistry and Bio-chemistry, Arizona State University, Tempe, AZ 85287, USA.
*To whom correspondence should be addressed. E-mail:[email protected]
5µm
5µm
1µm
C
D
E
A
B
5 x 105
5 x 106
5 x 107
5 x 108
cells
ml-1
0 120 240 360 480Time (h)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
OD
680
Fig. 1. Growth and electron microscopy of strain GFAJ-1. (A and B) Growth curves of GFAJ-1 grown onthe defined synthetic medium amended with either 1.5 mM PO4
3– (solid circles), 40 mM AsO43– (solid
squares), or neither PO43– nor AsO4
3– (open triangles). Cell growth was monitored both by an increase in(A) optical density and (B) cell numbers of the cultures. Symbols represent the mean T SD of (A) n = 6experimental and n = 2 controls and (B) n = 3 experimental and n = 1 control. This was a singleexperiment with six replicates; however, material was conserved to extend the duration of the experimentto allow material for cell-counting samples. (C and D) Scanning electron micrographs of strain GFAJ-1under two conditions, (C) +As/–P and (D) –As/+P. (E) Transmission electron micrography of +As/–P GFAJ-1showed internal vacuole-like structures. Scale bars are as indicated in the figure (11).
RESEARCHARTICLE
www.sciencemag.org SCIENCE VOL 332 3 JUNE 2011 1163
è Does not meet the “Extraordinary Evidence” threshold. Currently, the jury is still out…
The Biological Case• Complex life:
• Sculpted by catastrophes (6 major extinctions).• Impacts are good: evacuate niches.
» No dinosaurs at top of food chain» Þ mammals ascendent.
• Impacts are bad: e.g. the next one.• Is there an optimal impact rate for promoting
complexity?• Is intelligence/technology inevitable?
• Hominids/Earth » 5 Myr / 4.6 Gyr » 0.1%• Is H. Sapiens late or early compared to
other solar systems?
ContextPrior to 1850:
Spontaneous generation.Pasteur:
Life begets life.20th century view:
Life begets life now but spontaneous generation occurred in the early oceans.Alternative:
“Panspermia:” life was transported to Earth in cosmic dust from comets, etc.(Arrhenius 1859-1927)
What is life?• Life = matter + elan vital (vitalism)• Life = matter + physics + chemistry (mechanistic)• “Life” also includes artificial life?
Is there a sharp transition between living and non-living things?Are the conditions in the early Earth unique, rare or common?
Abiogenesis: the Miller-Urey Experiment• 1952: Stanley Miller and Howard Urey.• Water (H2O), methane (CH4),
ammonia (NH3), and hydrogen (H2).• Heat, sparks.• Over 20 amino acids synthesized!
Note: Racemic mixture (equal parts L- and R-isomers).
Biology is homochiral.
Terrestrial Life• Organic compounds (carbon based).• Proteins based on 20 amino acids.• Enzymes = catalysts.• Common genetic code based on nucleotides:
RNA and DNA.• Chirality: biological amino acids are left-handed.
• Symmetry breaking: why L, not R?
Related question: Why does the universe favor matter over anti-matter? Another case of symmetry breaking.
