Mass Extinctions and Supernova ExplosionsAuthor(s): Paul J. Crutzen and Christoph BrühlSource: Proceedings of the National Academy of Sciences of the United States of America,Vol. 93, No. 4 (Feb. 20, 1996), pp. 1582-1584Published by: National Academy of SciencesStable URL: http://www.jstor.org/stable/38626 .

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Proc. Natl. Acad. Sci. USA Vol. 93, pp. 1582-1584, February 1996 Astronomy

Mass extinctions and supernova explosions PAUL J. CRUTZEN AND CHRISTOPH BRUHL

Max Planck Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany

Contributed by Paul J. Crutzen, October 3, 1995

ABSTRACT In a recent contribution to this journal Ellis and Schramm [Ellis, J. & Schramm, D. N. (1995) Proc. Natl. Acad. Sci. USA 92, 235-2381 claim that supernova explosions can cause massive biological extinctions as a result of strongly enhanced stratospheric NO, (NO + NO2) production by accompanying galactic cosmic rays. They suggested that these NO, productions which would last over several centuries and occur once every few hundred million years would result in ozone depletions of about 95%, leading to vastly increased levels of biologically damaging solar ultraviolet radiation. Our detailed model calculations show, however, substantially smaller ozone depletions ranging from at most 60% at high latitudes to below 20% at the equator.

Ellis and Schramm (1) claim that nearby supernova explosions might have caused one or more of the mass extinctions identified by paleontologists by depleting the atmospheric ozone layer, thereby exposing organisms to potentially lethal solar UV irradiation. The discussion by Ellis and Schramm (1) is based on a number of simplifying assumptions and use of a formula going back to the first study on the topic by Ruderman (2), which in turn was based on studies about the effects of NO, (NO + NO2) additions to the stratosphere in the early 1970s. In this formula, independent of altitude, the ratio of the perturbed to unperturbed ozone concentrations, F, to the corresponding ratio X of NOx (NO + NO2) is given by

F = [(16 + 9X2)'/2 - 3X]/2. [11

Ruderman (2) claimed this expression to be an "idealization of the chemical kinetics which also gives a rough fit to numerical calculations."

Because the catalytic destruction of ozone by NO, via the pair of reactions (3)

NO + 03 -N02 + 02 [RI]

and

O + N2-NO+02 [R2]

is particularly dependent on the concentrations of oxygen atoms which are produced by photolysis of 03

O3+hv -> + 2 [R31

and removed via

O + 02 + M 03 + M, [R41

(M, arbitrary air molecule) the ozone destruction efficiency falls off rapidly toward lower altitudes, an effect which is not considered in Eq. 1. Also it appears that Eq. 1 was derived for relatively small deviations from the normal chemical state of the stratosphere, casting doubt on its applicability for situa- tions with much larger values of X. In addition, research has

The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. ?1734 solely to indicate this fact.

ppb N04-NO2 3 15 112

A) -70001.00

1 00 0 -

CA~~~~()

-g0 -6 1 50 0 50 sO 00 Lt todo [degresi

NO+N02 3 15 12 ratio

cn 2 (300

-0 0500 _

10005'

1 1000

90 -60 -30 0 30 60 90

Lotitude [deg rees]

FIG. 1. Calculated meridional cross section of N0r volume-mixing ratios in ppb (10 9) by volume for the present (Upper) and upper limits for the supernova-disturbed atmosphere (Lower) given as ratioes to the present (for mid-March).

shown that in the altitude range below about 25 km conversion. Of the NO O catalysts to HNO3 is substantially enhanced by the reaction of NON-derived N205 wi.th 1-20 on stratospheric sulfate aerosol:

N205 + H2? -3 2HN03 [R5]

Furthermore, it was shown in 1978 that the reaction

1102?+NO-OH + N02 [R6J

which leads to ozone production in the lower stratosphere, proceeds about 40 times faster than earlier measurements had indicated (4), thereby considerably reducing N0O-catalyzed ozone destruction. For a number of reasons Eq. 1 strongly overestimates ozone depletions resul.ting from N0r additionls to the stratosphere.

It is the aim of this study to present the results of a less parametric treatment of the issue, using an up-to-date, com-

Abbreviation: GCR, galactic cosmic r ays.

1582

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Astronomy: Crutzen and Briihl Proc. Natl. Acad. Sci. USA 93 (1996) 1583

ppb N0y 1 5 12

a-

k~~ 77iiQe'>

[L~~~17 - - 000

10 X O 15 000 15.0

U) 0,0 U) 71773000 --_

tL 100 .X ; 7 0

005o 010 090 1000 L -

90 60 30 0 30 60 90 Latitude [degrees]

NOY 3 15 12 ratio

.~~~~~~~ I

11

a- ? 0 5- 3 0D - 3 0

? 3 5.00 0 U),

7,00~~~~~~70 m- '1V-00 ' At.00O700

100 00

Latitude [degrees]

FIG. 2. Same as Fig. 1, but for all odd nitrogen compounds (NOr ? NO3 + 2N205 + 11N03).

prehensive, two-dimensional, height-, latitude-, and time- dependent model of stratospheric chemistry with a complete set of reactions representing currently known chemistry and reaction rates (5, 6). Otherwise, in this study we will hold ourselves as closely as possible to the numerical assumptions in the paper by Ellis and Schramm (1).

NO is formed by the action of galactic cosmic rays (GCR) on N2 and 02, leading in particular to the production of N atoms in their ground (4S) and electronically excited states (2D or 2p). Following rapid reaction with 02, the excited states always produce NO. However, as the reaction between ground state N and 02 iS rather slow, the "cannibalistic" reaction

N +NO-*N2?O [R7]

limits the amount of NO that can accumulate in the strato- sphere. The actual production of NO is thus significantly smaller than the production of: N atoms by the GCR. This fact was indeed considered by Ellis and Schramm (1), based on the formula derived earlier by Ruderman (2). In fact, as almost all NO production takes place at high geomagnetic latitudes (>60?) the formula of Ruderman (2) overestimates the loss of NOX by neglecting the absence of reaction R7 during polar- night conditions, when NO is converted to NO2 and higher oxides of nitrogen, in which case the N atoms mostly react with 02 to produce NO.

To obtain an upper limit estimate of the ozone loss by a supernova, we assume that actual NO deposition in the atmosphere equals total N production by GCR. Following Ellis and Schramm (1), the stratospheric NO production rate over a period of 300 years would then be almost 100 times larger than the current rate of NO production by GCR-i.e., globally averaged, i.. molecules cm-2.. 1. This NO is, however, largely

ppM 03 3 15 12

1 - Q4.000

6_ _4 000

U) I

Q ~ ~~~ 9 - 0 00

u) 44 ~~ ~~000 ---- ---------- -

4 000 ___

{.000 - ~ 2.ooo

- 00 8 - ( -*

L0,050 _ J 1000~~

-90 -60 30 0 30 60 90 Latitude [degrees]

03 3 15 12 ratio

_ -1 0 r 0.90

Q 0.70 .....-. 0 o o70

D ~ ~~~ , ~ ,o ,,1, XfD^6

-90 -60 -30 0 30 06 90 Latitude [degrees]

FIG. 3. CZalculated meridional cross sections of ozone volume- mixing ratios in ppm (10-6) by volume for mid-March. (Upper) Present atmosphere. (Lower) Minimum possible values for a supernova- disturbed atmosphere (presented as ratios to the present).

deposited at geomagnetic latitudes higher than about 60? (7), which, therefore, experience a production flux of 6 x 109 molecules cm-2 s- lasting for about 300 years (1). At lower latitudes, the areal deposition densities, although appreciably smaller, are still significant. The NO flux is introduced in our two-dimensional model as a function of latitude and altitude according to a figure given in Brasseur and Solomon (7) and using further data of Nicolet (8). Calculated meridional cross sections of NOX volume ratios for the present and the super- nova disturbed atmosphere for mid-March are presented in Fig. 1, showing up to a 20-fold increase in NOX mixing ratios near 100 hPa at high latitudes and less elsewhere. The corre- sponding enhancements for all odd nitrogen compounds [NOr - NOX + NO3 ? 2N2O5 ? HNO3] is shown in Fig. 2. Comparison between Figs. 1 and 2 shows that below 5 hPa (=35 kin) a large fraction of the odd nitrogen is present as compounds other than chemically active NON, despite their initial inputs as NO in the model. The globally averaged upper limit of NO production of 3.2 x i09 moleculescm-2 s1 by supernova GCR is about 100 times larger than the NO production by normal GCR and 20 times larger than the main source of stratospheric NOX provided by the oxidation of N2O (9). More than half of the NO production by GCR occurs below the 100 hPa level.

In Fig. 3 we present the meridional ozone distribution calculated for mid-March in the present atmosphere and the relative percent reductions in an atmosphere bombarded by the supernova GCR. We calculate depletions in ozone con- centrations up to 70% at high latitudes in the stratosphere. The corresponding total ozone changes, shown in Fig. 4, also maximize at high latitudes reaching 50% in the summer season.

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1584 Astronomy: Crutzen and Bruhl Proc. Natl. Acad. Sci. USA 93 (1996)

TOTAL OZONE difference (%) Supernova 9g o I . I I - I I-TT----

-40.0

60 40 0 -35.0 - -3010 -30.= 6 0 5 . - 2 5 0 . 2

_~~-20.0 2 -10 -20.0 -

(1) -~~~~~~~~~~~~~~~15.0 C7 -3 0 - 1 5 .0

-0 - 0=

-12.0 7.

-50 -15.0 -14.o

200 - -520.0 - \ /\

-30.0 ~~~~~~~~-3 . - 50 -0 300 3 5, 0 -40. -40.04

-50,0 ~ ~ 30 -90 1 I

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

FIG. 4. Seasonal and latitudinal distributions of the upper limits of relative total ozone changes (in %) between the present and a supernova-disturbed atmosphere.

In the tropics, total ozone is depleted by 10-20%. We repeat that the calculated ozone depletions are overexaggerated due to the neglect of NO destruction by reaction R7, which will be important during sunlit, summer-time conditions.

Our model results on ozone depletions are much smaller than the 95% reduction estimated by Ellis and Schramm (1) on the basis of Eq. 1. For the tropics, total ozone loss was calculated to be -15%. Although significant stress may well have been exerted on the high latitude biosphere, the effects on the tropical and subtropical biosphere may not have been strong enough to cause mass extinctions. Large ozone deple- tions at low latitudes would only be possible if the earth's magnetic field would be much disturbed, allowing much more GCR to reach lower latitudes, as is the case during magnetic reversals (10). The likelihood for a supernova event to coincide with a geomagnetic reversal is, of course, very small.

1. Ellis, J. & Schramm, D. N. (1995) Proc. Natl. Acad. Sci. USA 92, 235-238.

2. Ruderman, M. A. (1972) Science 184, 1079-1081. 3. Crutzen, P. J. (1970) Q. J. Roy. Meteorol. Soc. 96, 320-325. 4. Crutzen, P. J. & Howard, C. J. (1978) Pure Appl. Geophys. 116,

497-510. 5. Bruhl, C. & Crutzen, P. J. (1993) in The Atmospheric Effects of

Stratospheric Aircraft: Report of the 1992 Models and Measure- ments Workshop, eds. Prather, M. J. & Remsberg, E. E. (Natl. Aeronaut. Space Admin., Washington, DC), NASA Ref. Publ. No. 1292, Vol. 1, pp. 103-104.

6. DeMore, W. B., Sander, S. P., Golden, D. M. Hampson, R. F., Kurylo, M. J., Howard, C. J., Ravishankara, A. R., Kolb, C. E. & Molina, M. J. (1992) Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling (Jet Propulsion Lab., Pasadena, CA), JPL Publ. No. 92-20.

7. Brasseur, G. & Solomon, S. (1984) Aeronomy of the Middle Atmosphere (Reidel, Dordrecht, The Netherlands).

8. Nicolet, M. (1975) Planet. Sci. 23, 637-649. 9. Crutzen, P. J. & Schmailzl, U. (1983) Planet. Space Sci. 31,

1009-1032. 10. Reid, G. C., Isaksen, I. S. A., Holzer, T. E. & Crutzen, P. J.

(1976) Nature (London) 259, 177-179.

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