COMETARY SCIENCE

Subsurface properties andearly activity of comet67P/Churyumov-GerasimenkoSamuel Gulkis,1‡ Mark Allen,1 Paul von Allmen,1 Gerard Beaudin,2 Nicolas Biver,3

Dominique Bockelée-Morvan,3 Mathieu Choukroun,1 Jacques Crovisier,3

Björn J. R. Davidsson,4 Pierre Encrenaz,2* Therese Encrenaz,3* Margaret Frerking,1

Paul Hartogh,5 Mark Hofstadter,1 Wing-Huen Ip,6 Michael Janssen,1

Christopher Jarchow,5 Stephen Keihm,1 Seungwon Lee,1 Emmanuel Lellouch,3

Cedric Leyrat,3 Ladislav Rezac,5 F. Peter Schloerb,7,1 Thomas Spilker1†

Heat transport and ice sublimation in comets are interrelated processes reflectingproperties acquired at the time of formation and during subsequent evolution. TheMicrowave Instrument on the Rosetta Orbiter (MIRO) acquired maps of the subsurfacetemperature of comet 67P/Churyumov-Gerasimenko, at 1.6 mm and 0.5 mm wavelengths,and spectra of water vapor. The total H2O production rate varied from 0.3 kg s–1 in earlyJune 2014 to 1.2 kg s–1 in late August and showed periodic variations related to nucleusrotation and shape. Water outgassing was localized to the “neck” region of the comet.Subsurface temperatures showed seasonal and diurnal variations, which indicated that thesubmillimeter radiation originated at depths comparable to the diurnal thermal skin depth.A low thermal inertia (~10 to 50 J K–1 m–2 s–0.5), consistent with a thermally insulatingpowdered surface, is inferred.

The Rosetta Orbiter spacecraft carries 12science instruments to study comet 67P/Churyumov-Gerasimenkowith a long-rangegoal of better understanding the origin ofour solar system. Comet 67P/Churyumov-

Gerasimenko is a typical Jupiter family comet(JFC). JFCs are believed to have been injectedfrom theKuiper belt outside the orbit of Neptunebecause of gravitational interactions with thegiant planets or collisions. Currently, comet 67Pis in a low-inclination orbit with a semimajor axisof 3.46 AU and an orbital period of 6.45 years.Its present orbital configuration was achievedin February 1959 during a close encounter withJupiter that converted its perihelion distancefrom 2.7 AU to 1.28 AU (1). Its prior history isuncertain. According to some statistical studies,comet 67P might have been subject to mass ero-sion driven by thermal sublimation with deple-tion of the surface materials up to a depth of afew kilometers (2). As a consequence, a dust man-tle would gradually build up and cover the nu-cleus surface, where ice mixtures of H2O, CO2,and CO would be found (3).

The instrument suite onRosetta was chosen tostudy physical and chemical properties of thenucleus, the evolution of the outgassing from thenucleus, and development of the coma as a func-tion of the heliocentric distance, as well as theinteraction of the solar wind with the targetcomet. The development of the gas and dust comasalong the cometary orbit depends critically onthe energy transport through the dust mantleand the differential outgassing of different spe-cies according to their volatility. The MicrowaveInstrument on the Rosetta Orbiter (MIRO) (4)has been designed to study heat transport in thenucleus, outgassing from the nucleus, and devel-opment of the coma as interrelated processes.The measurements and monitoring of the inter-play between gas emissions and surface temper-ature evolution by MIRO therefore bring keyinformation to the temporal evolution of the sub-surface ice-dust complex. This work reports onearlymeasurements of comet 67Pwith theMIROinstrument from June to September 2014.The MIRO instrument consists of a 30-cm di-

ameter, offset parabolic reflector telescope fol-lowed by two heterodyne receivers. Center-bandoperating frequencies of the receivers are near190 GHz (1.6 mm) and 562 GHz (0.5 mm).Throughout this Report, we refer to the twobandsas the mm (millimeter) and smm (submillimeter)bands, respectively. Broadband continuum chan-nels are implemented in both the mm and smmbands for measurements of near-surface temper-atures and temperature gradients in the nucleusof comet 67P. The smm band also includes a veryhigh resolution spectrometer (44 kHz channelwidth) for measuring spectral lines of H2

16O,H2

18O, and other molecules in the coma. Here,

we report the first measurements of water H216O

and H218O in the coma and subsurface thermal

measurements of the nucleus obtained duringthe time interval from June to September 2014.We refer to the June-July observations as pre-rendezvous observations, characterized here asthe timewhen the nucleuswas unresolved by theMIRO smm beam and the entire coma could beobserved at once. The August-September periodis identified as the postrendezvous time interval,during which both the nucleus and coma wereobserved with spatial resolutions that variedfrom 500 to 40 m.Observations of comet 67P with MIRO began

on 24May 2014. The instrument mode was dual-continuumand spectroscopic. Initial observations

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1Jet Propulsion Laboratory/California Institute of Technology,4800 Oak Grove Drive, Pasadena, CA 91109, USA. 2LERMA,Observatoire de Paris, PSL Research University, UPMCUniversité Paris 06, CNRS, UMR8112, F-75014 Paris, France.3LESIA-Observatoire de Paris, CNRS, UPMC, UniversitéParis–Diderot, 5 place Jules Janssen, 92195 Meudon, France.4Department of Physics and Astronomy, Uppsala University,Box 516, SE-75120 Uppsala, Sweden. 5Max-Planck-Institut fürSonnensystemforschung, Justus-von-Liebig-Weg 3, 37077Göttingen, Germany. 6National Central University, Jhongli,Taoyuan City 32001, Taiwan. 7University of Massachusetts,619 Lederle Graduate Research Tower, Amherst,MA 01003, USA.*Distinguished Visiting Scientist at the Jet Propulsion Laboratory†Retired. ‡Corresponding author. E-mail: samuel.gulkis@jpl.nasa.gov

A

B

Fig. 1. Sample spectra of the 1(1,0) – 1(0,1) tran-sitions of H2

16O and H218O lines at 556.939 GHz

and 547.676 GHz, respectively, obtained withMIRO in nadir-viewing geometry.All spectra seento date are biased toward negative gas velocities(toward the spacecraft), which is interpreted aspreferential outgassing from the day side of thenucleus. (A) Line observed in emission on 23 June2014 UTwhen the Rosetta distance to the cometis D = 128,000 km, the heliocentric distance is rH =3.84 AU, the spatial resolution isW = 260 km, andthe phase (Sun-comet observer) angle is F = 32°.(B) (Inset plot) Lines observed in absorptionagainst the nucleus subsurface thermal emissionon 19 August 2014 UT (D = 81 km, rH = 3.5 AU,F =37°, W = 160 m). The line intensity is expressed inthe main-beam TB scale. The velocity scale isgiven relative to the nucleus velocity.The spectralresolution is 24ms–1.TheH2

16Ospectrumof23 Juneis consistent with an expansion velocity of 0.68 T

0.02 km s–1. The optical thickness of the H216O

line observed on 23 June is 0.3.On 19 August, theH2

16O line is optically thick near the line center(opacity > 10), whereas the H2

18O line is opticallythin (opacity < 0.1).

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were carried out with the instrument pointed inthe nadir direction of the comet and off of thecomet to provide background reference.In the June-July 2014 time interval, the Rosetta

spacecraft approached the comet from 550,000 kmto 973 km. The phase angle of the observationsduring this period ranged from 35° to 1°, and theheliocentric distance changed from3.95 to 3.62AU.TheMIRO smm single-pixel, one-dimensional (1D)field of view ranged from 2400 to 4.2 km, largerthan the nucleus radius of 3.8 km. It was possibletomeasure andmonitor the totalwater outgassingrate of comet 67P without spatial mapping dur-ing this time interval.Water vapor was first detected with MIRO on

6 June 2014, when the comet was at 3.92 AUfrom the Sun. A weak emission line from the1(1,0) – 1(0,1) rotational transition of H2

16O at thefrequency of 556.936 GHz was detected, whichthen rapidly increased as the distance of Rosettato the comet decreased (Fig. 1A). A large blueshift (frequency increase) of the spectral line inthe comet rest velocity frame (about –0.4 km s–1 inthe June period) was observed, indicative of wateroutgassing, essentially toward the hemispherefacing Rosetta and the Sun. This suggests largetemperature contrasts between daytime andnight-time, with temperatures on the night side toolow for rapid sublimation of water ice. The waterproduction rate displayed periodic variations onshort time scales, typically by 50% in magnitudewith two maxima andminima per period (Fig. 2),which are correlated with the 12.4-hour rotationof the nucleus (5). This variation is related to thecomplex shape of the nucleus, for which the totalarea exposed to the Sun varies during the rotation(5). Both the lineDoppler shift and time variationssuggest that the nucleus regions exposed to solarillumination were the main source of water va-por in the June-July period. However, surfaceinhomogeneity in ice content andmaterial proper-ties may also play a significant role. Indeed, localjet activity was observed by the OSIRIS camera(5), specifically in the so-called “neck” region of67P, where the spectral and reflectance proper-ties indicate the presence of water ice (5, 6). Sim-ple models considering either a homogeneousdistribution of water ice on the surface or oneconcentrated in the neck region (Fig. 2) canexplain the observed time variation in a first ap-proximation (7).As mentioned earlier, in the June-July time

period, the nucleus was unresolved and it waspossible to sample the inner coma in a single-pixel field of view. The spectra observed duringthis time period show blue-shifted water lines(Fig. 1) in emission, which are indicative of wateroutgassing mainly into the hemisphere facingthe sun. Therefore, for purposes of calculatingthe mean total water production rate over onenucleus rotation using this data set, we used theapproximation of hemispheric outgassing.We in-ferred from radiative transfer calculations (8, 9)that the mean total water production rate overone nucleus rotation was 1 × 1025 molecules/s(0.3 kg s–1) in early June and reached 2 × 1025

molecules/s (0.6 kg s–1) at the end of July 2014.

For more localized production patterns, highervalues are derived as a result of higher opacities.We estimate that the above values might under-estimate the actual production rate by a factor oftwo, at most. Based on the rotational variationsin Fig. 2, we estimate the outgassing variabilityto be on the order of T20%.We expect to be ableto refine this estimate, as we are able to betterdefine the parameters that enter in the radiativetransfer calculations, i.e., the temperature andvelocity profiles in the coma and the global ex-tent of the outgassing.Rosetta was at a distance of 100 km from the

comet on August 6 and continued to approachrapidly. The nucleus was well resolved at thattime (and throughout August and September),with a spatial resolution of <200 m in the smmchannel. When viewing the resolved nucleusthrough the coma, the continuum radiation (fromthe nucleus) was observed to be in absorption atthe H2

16O and H218O spectral line frequencies

present in the coma. Absorption of the nucleusthermal emission in the coma provides clear evi-dence that the excitation temperatures of the ab-sorbingwatermolecules are lower than the nucleusbrightness temperature (TB) (9). Starting in earlyAugust, the spectral line due to H2

18O provided amore sensitive measure of the water productionrate than the H2

16O line, which became opticallythick (Fig. 1B). Therefore, we focus here on theH2

18O observations to derive quantitative infor-mation on water production for this time period.The frequency of the peak intensity of the waterlines is diagnostic of the gas expansion velocity,

whereas the width contains information on theradial profiles of the gas velocity and temperature.The water column density along the MIRO

line of sight exhibited considerable variability,as it varied by a factor of 5 or more over a timescale of 10 hours during the August 7.4 to 9.5UTC period (Fig. 3). Considering both nadir andlimb observations acquired from August 6 toSeptember 1, we estimate the mean water pro-duction for this period (heliocentric distanceranged from 3.6 AU to 3.46 AU) to be about 4 ×1025 molecules/s. In this time frame, the MIROfootprint unevenly covered nucleus latitudesfrom 0° to 80° N and longitudes from 0° to 360°(Fig. 3 and figs. S1 and S2). However, maximumcolumn densities are generally observed aboveregions of cometocentric coordinates at latitudesof 65° to 70° and longitudes of 0° to 100°, i.e.,close to the North pole and to the neck, specif-ically at the place where the nucleus presents itsdeepest concavity (the neck is the region connect-ing the small head and large body of the nucleusin reference to the ducklike shape of 67P). This isin line with water outgassing mostly from theneck region and responsible for the prominentdust jet seen in OSIRIS images. Water produc-tion from icy grains has been observed in a num-ber of comets (10), and therefore, sublimating icygrains from this dust jet could also be significantcontributors to the observed water production.To our surprise, the highest water column den-sities are often observed above shadowed neckregions where the nucleus thermal continuum—and, consequently, the surface and subsurface

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Fig. 2. Variations of the total water production rate as the nucleus rotates.The rates shown aboveare based on MIRO H2

16O observations between 13 and 15 July 2014. Based on coarse maps, the pointingdirection of the MIRO field of view was 300 arc sec southwest from the nucleus center at the time of theobservations. In one nucleus rotation period (12.4 hours) (5), the water production rate exhibits twominima and two maxima.The rotation period of 67P (12.4 hours) is shown with a red bar. Early models toexplain the variations are found in (7).

CATCHING A COMET

temperatures—is low (Fig. 3 and fig. S2). It ispossible that observed gases are originating fromnearby insolated regions. Inversely, low column

densities are measured above some illuminatedregions (on the body and on the head), againsupporting an activity driven by more than solar

insolation and confirming the inhomogeneity inice distribution observed by other Rosetta in-struments (5, 6).

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Fig. 3. Water column density and expansion ve-locity measured by MIRO from 7.4 to 9.5 August2014 UTC by using nadir H2

18O spectra. Signifi-cant spatial variability is seen, with maxima cor-relatedwith lines of sight near the neck region of thenucleus. (A) Column density (blue dots with errorbars), nucleus thermal continuum temperatures(red curve), and cosine of the incidence angle in thecenter of MIRO beam (dotted black curve). Largedrops in the smmcontinuum (red) temperature areobserved when the MIRO line of sight crosses thelimb. (B) Mean water expansion velocity given bythe velocity at maximum absorption. (C) (Bottom)Illumination maps of the nucleus at the specifiedtimes The SHAP2 digital shape model provided bytheOSIRIS teamwas used.The submillimeterMIRObeam position is shown by the red dot on the illu-mination maps.The x,y plane is perpendicular tothe MIRO line of sight and contains the comet-Sun line.

Fig. 4. Illumination map of the digital shapemodel SHAP2 with contour plot of the MIROsubmillimeter antenna temperature. MIRO seessubsurface brightness features correlated with sur-face slopes, with regions facing the Sun generallybrighter (warmer) than those tilted away from theSun. This is an indication of rapid response to solarillumination and, hence, a low thermal inertia. Ex-cellent overlap of the MIRO and shape model limbsis obtained on the Sun-lit (right) side of the nu-cleus.The left side of the nucleus is in polar night atthis time, and the shape model—based on visibleimaging—cannot determine its true shape here.Theextension of the MIRO data beyond the limb on thenight side indicates the true size of the nucleus inthis region. The shape model is presented as anillumination map with the intensity given by thecosine of the solar illumination angle projected ontoa plane perpendicular to the line joining the space-craft to the barycenter of the comet.

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Line profiles provide information on the kin-ematics of the gas flow. Note that the line-peakvelocity (–0.6 to –0.75 km s–1) is observed to behigher along lines of sight where higher columndensities (Fig. 3) are recorded, whereas the linewidth decreases. Therefore, projection effects ofthe gas flow are minimized for these lines ofsight, and the line-peak velocity should be closeto the expansion velocity. These findings alsodemonstrate that the observed column-densityenhancements are related to larger sublimationfluxes from the surface and not to local densityenhancements caused by shocks between flowsthat would decelerate the gas.Continuum observations from the MIRO 0.5-

and 1.6-mm bands were geo-referenced to theSHAP2 digital shape model provided by theOSIRIS team. The map of the antenna temper-ature (Fig. 4) was derived by scanning the MIRObeam across the nucleus and then combining thescans into amap using the “on-the-fly” techniquedeveloped for mapping with single-dish radiotelescopes (11). The antenna temperature con-tours correlate well with illumination patternson the nucleus surface, which are, in turn, de-termined by topography and orientation withrespect to the Sun.Subsurface temperatures, which determine the

MIRO smm and mm thermal fluxes, may havethree components. (i) The diurnal component—variation with local solar time—is confined to ashallow layer defined by the thermal 1/e pene-tration depth—on the order of 1 to 2 cm for 67P’s12.4-hour rotation period and plausible thermalproperties of a particulate regolith. (ii) An annualsignal, dependent on obliquity, orbital period,and perihelion location, with latitude-dependent

amplitude and a 1/e attenuation depth of theorder 1 m. The seasonal variations in tempera-tures determine the MIRO emissions from pene-tration depths below the diurnal layer. (iii) The“static” or “subseasonal” component is the ap-proximately isothermal layer beneath the sea-sonally varying layer, and its temperature dependson the latitude-dependent orbit variation ofthe solar flux.Plotting the MIRO mm and smm brightness

temperatures as a function of the local solar timeand latitude (Fig. 5) reveals three key features ofthe nucleus thermal environment. (i) Both the smmand mm channels reveal sensitivity to the diur-nal heating cycle, peaking in early to midafter-noon, although with amplitudes diminished by afactor of approximately 2 to 4 from the smm tomm channel. (ii) The observed 40- to 50K smmday-night peak-to-peak variations represent 35%or more of the predicted surface temperaturevariations for a completely insulating surface(zero thermal inertia) at the September Sun dis-tance. For realistic nonzero thermal inertia values,the ratio of the smm TB/surface temperaturepeak-to-peak values will be higher. This resultimplies that the smm 1/e penetration depth lies ator above the 1/e thermal attenuation depth fordiurnal variations. (iii) Seasonal effects are appar-ent in the reduction of the diurnally averaged TBvalues from the 20°N to 30°N latitude bin to the20°S to 30°S latitude bin. We conclude that, on aglobal scale, theMIRO smm emissions originateprimarily at depths comparable to the diurnallayer thickness, whereas the mm emissions orig-inate at depths roughly three times the thermalattenuation depth, consistent with electrical ab-sorption properties, which scale inversely with

wavelength. The latitude dependence of themea-sured brightness temperatures is consistent withexpectations from seasonal variations of the solarflux given 67P’s orbital parameters.Temperature depth profiles were computed by

solving the 1D heat equation for a range of valuesof the thermal inertia, and the resulting antennatemperatures in the mm and smm bands werecalculated from evaluating the transfer of the ther-mal radiation emitted in the subsurface region.Comparing these theoretical antenna tempera-tures with MIRO observations leads to an esti-mated thermal inertia in the range of ~10 to50 J K–1 m–2 s–0.5 (12).With this range of values, large (up to ~50 K)

differences between theMIROmeasured daysidesubsurface temperatures and the temperature atthe surface can be inferred, highlighting the keyinsulating role played by the dusty surface: Itlargely restricts the amount of heat transferredto the interior, including ices within. This certain-ly contributes greatly to the longevity of 67P and,probably, of comets in general. The importanceofmeasuring the temperatures below the surfaceof a comet—and particularly below its diurnallayer—is illustrated by these data.MIRO’s spectroscopic and continuumobserva-

tions provide interrelated data on heat transport,sublimation of ices, and outgassing by providingdata on the nucleus subsurface temperature, lo-cation, time, and fluxes of water being emittedfrom the nucleus surface. Less than 1% of the 67Pnucleus surface is needed to explain the watergas production rates discussed earlier if water icewere located on the surface (13). A substantialportion of the measured outgassing originatesfrom the neck of the nucleus.

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Fig. 5. Brightness temperaturesas a function of local solartime, from MIRO continuummeasurements of September2014, are shown for effectivelatitude bins of 20° to 30° N(black data points) and 20° to30° S (red data points). Thesolid curves are diurnal sinusoidalfits to the data. Both effectivelatitudes and the local solar timeare computed from the shapemodel–derived surface orienta-tion at the MIRO beam center.Only points for which the MIRObeams lie entirely within thenucleus are included. The data arerestricted to the 100° to 200°longitude band in order to elimi-nate data in the neck region,where extreme shadowingconditions obscure the interpre-tation of the diurnal heatingcurve. (A) Submillimeter data.(B) Millimeter data.

A B

CATCHING A COMET

REFERENCES AND NOTES

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2. O. Groussin, G. Hahn, P. L. Lamy, R. Gonczi, G. B. Valsecchi,The long-term evolution and initial size of comets 46P/Wirtanen and 67P/Churyumov–Gerasimenko. Mon. Not. R.Astron. Soc. 376, 1399–1406 (2007). doi: 10.1111/j.1365-2966.2007.11553.x

3. H. L. F. Houpis, D. A. Mendis, W.-H. Ip, The chemicaldifferentiation of the cometary nucleus: The processand its consequences. Astrophys. J. 295, 654–667 (1985).doi: 10.1086/163409

4. S. Gulkis et al., MIRO: Microwave instrument for the Rosettaorbiter. Space Sci. Rev. 128, 561–597 (2007). doi: 10.1007/s11214-006-9032-y

5. H. Sierks et al., On the nucleus structure and activity ofcomet 67P/Churyumov-Gerasimenko. Science 347,aaa1044 (2015).

6. F. Capaccioni et al., The organic-rich surface of comet67P/ Churyumov-Gerasimenko as seen by VIRTIS/Rosetta.Science 347, aaa0628 (2015).

7. See supplementary materials on Science Online, Models usedfor the water production observed on 13 to 15 July 2014.

8. V. Zakharov, D. Bockelée-Morvan, N. Biver, J. Crovisier,A. Lecacheux, Radiative transfer simulation of water rotationalexcitation in comets: Comparison of the Monte Carlo andescape probability methods. Astron. Astrophys. 473, 303–310(2007). doi: 10.1051/0004-6361:20066715

9. S. Lee, P. von Allmen, L. Kamp, S. Gulkis, B. Davidsson,Non-LTE radiative transfer for submillimeter water lines inComet 67P/Churyumov-Gerasimenko. Icarus 215, 721–731(2011). doi: 10.1016/j.icarus.2011.07.007

10. K. J. Meech et al., EPOXI: Comet 103P/Hartley 2 observationsfrom a worldwide campaign. Astrophys. J. 734, L1–L9 (2011).doi: 10.1088/2041-8205/734/1/L1

11. J. G. Mangum, D. T. Emerson, E. W. Greisen, The On The Flyimaging technique. Astron. Astrophys. 474, 679–687 (2007).doi: 10.1051/0004-6361:20077811

12. See supplementary materials on Science Online, Method fordetermining the thermal inertia.

13. See supplementary materials on Science Online, Ice coverageand content on 67P.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the assistance and supportof the entire MIRO project teams at the Jet PropulsionLaboratory, the Max-Planck-Institut für Sonnensystemforschung,LESIA-Observatoire de Paris, and LERMA-Observatoire deParis–Meudon. We thank Y. Anderson, T. Koch, R. Nowicki, andL. Pan for their efforts in scheduling, operations, and supportof the MIRO instrument. The authors recognize here the enormouscontributions that our friend and colleague, Lucas Kamp, madeto the MIRO science and engineering efforts before his recentdeath. The authors acknowledge support from their institutionsand funding sources. A part of the research was carried out at theJet Propulsion Laboratory, California Institute of Technology,under a contract with the National Aeronautics and SpaceAdministration. A part of the research was carried out at theMax-Planck-Institut für Sonnensystemforschung with financialsupport from Deutsches Zentrum für Luft- und Raumfahrt andMax-Planck-Gesellschaft. Parts of the research were carried out byLESIA and LERMA, Observatoire de Paris, with financial supportfrom Centre National d'Études Spatiales (CNES) and CNRS/InstitutNational des Sciences de l'Univers (INSU); at the National CentralUniversity with funding from the Taiwanese National Science

Counsel grant 101-2111-M-008-016; at the University ofMassachusetts, Amherst, USA; and at Uppsala University withfinancial support from the Swedish National Space Board.We acknowledge personnel at ESA's European Space OperationsCenter (ESOC) in Darmstadt, Germany, and at ESA andNASA/JPL tracking stations for their professional work incommunication with and directing the Rosetta spacecraft, therebymaking this mission possible. We acknowledge the excellentsupport provided by the Rosetta teams at the ESOC in Germanyand the European Space Astronomy Center (ESAC) in Spain.Special mention is made of C. Vallat, whose superb efforts havemade many difficult MIRO observing sequences possible;B. Grieger, whose calculations and visualizations have enabled andoptimized our science return; and N. Altobelli and M. Küppers,whose early liaison support and later oversight responsibilities havegreatly benefited the mission as a whole. The authors thankHolger Sierks and the OSIRIS team for permission to use the SHAP2shape model for analysis purposes and for permission to publishseveral images here. All MIRO data are released through the PSAarchive of ESA and the PDS archive of NASA. The data reported hereare scheduled for delivery to the archive on 19 May 2015 and areavailable on request until then.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/347/6220/aaa0709/suppl/DC1Supplementary TextFigs. S1 to S4References (14–17)

12 October 2014; accepted 27 December 201410.1126/science.aaa0709

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DOI: 10.1126/science.aaa0709, (2015);347 Science

et al.Samuel Gulkis67P/Churyumov-GerasimenkoSubsurface properties and early activity of comet

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