EPOXI at Comet Hartley 2 Michael F. A'Hearn, et al. Science 332, 1396 (2011); DOI: 10.1126/science.1204054

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Prior remote sensing showed that Hartley 2’s nucleus has an average radius 1/5 that of comet Tempel 1’s nucleus (5, 6), yet it releases more gas EPOXI at Comet Hartley 2 per unit time at perihelion, even when allowing for the smaller perihelion distance of Hartley 2 Michael F. A’Hearn,1* Michael J. S. Belton,2 W. Alan Delamere,3 Lori M. Feaga,1 (1.059 versus 1.506 AU). This puts Hartley 2 in a Donald Hampton,4 Jochen Kissel,5 Kenneth P. Klaasen,6 Lucy A. McFadden,1,7 different class of activity than that of Tempel 1 or Karen J. Meech,8 H. Jay Melosh,9,10 Peter H. Schultz,11 Jessica M. Sunshine,1 any of the other comets visited by spacecraft Peter C. Thomas,12 Joseph Veverka,12 Dennis D. Wellnitz,1 Donald K. Yeomans,6 (fig. S1). The two comets have very different Sebastien Besse,1 Dennis Bodewits,1 Timothy J. Bowling,10 Brian T. Carcich,12 surface topography (Fig. 1), but whether the dif- Steven M. Collins,6 Tony L. Farnham,1 Olivier Groussin,13 Brendan Hermalyn,11 ferent topography is related to the hyperactivity is Michael S. Kelley,1 Michael S. Kelley,14 Jian-Yang Li,1 Don J. Lindler,15 still being investigated. Carey M. Lisse,16 Stephanie A. McLaughlin,1 Frédéric Merlin,1,17 Silvia Protopapa,1 James E. Richardson,10 Jade L. Williams1 The Nucleus Spin state and variations. The rotation state of the Understanding how comets work—what drives their activity—is crucial to the use of comets in nucleus distinguishes the morning from the eve- studying the early solar system. EPOXI (Extrasolar Planet Observation and Deep Impact Extended ning terminator and, in comparison with longer- Investigation) flew past comet 103P/Hartley 2, one with an unusually small but very active nucleus, term coma observations, allows the number and taking both images and spectra. Unlike large, relatively inactive nuclei, this nucleus is outgassing relative strengths of active areas to be determined. primarily because of CO2, which drags chunks of ice out of the nucleus. It also shows substantial Knowledge of the nuclear spin can also put con- differences in the relative abundance of volatiles from various parts of the nucleus. straints on internal distribution of mass in the nucleus, internal energy dissipation, and the mag- omets are the fundamental building blocks The flyby spacecraft carries the High Resolu- nitude of the net torque. of the giant planets and may be an im- tion Instrument (HRI), which combines a visible- The large variations in brightness in Fig. 2, Cportant source of water and organics on wavelength camera with a pixel size of 2 mrad reduced to a measure of the amount of dust Earth. On 4 July 2005, the Deep Impact mis- and a set of filters with a near-infrared (near-IR) leaving the nucleus, show a period of roughly 18 sion carried out an impact experiment on comet (1.05to4.85mm) spectrometer with an entrance hours, but the spacing of peaks in the light curve on November 11, 2011 9P/Tempel 1 (1, 2) to study differences between slit of 10 mrad by 256 mrad, with 512 spatial shows a clear pattern that repeats every three cy- the comet’s surface and the interior. Although pixels along the slit. Spectral maps were created cles. We interpret this [supporting online mate- the impactor spacecraft was destroyed, the flyby by scanning the slit across the comet while tak- rial (SOM) text] as an excited state of rotation, spacecraft and its instruments remained healthy ing a sequence of spectra. The Medium Resolu- with each cycle corresponding to precession of in its 3-year, heliocentric orbit after completion tion Instrument (MRI) has a pixel size of 10 mrad the long axis of the nucleus around the angular of the mission. The Deep Impact flyby spacecraft and a different but overlapping set of visible- momentum vector, with a period of 18.34 hours was retargeted to comet 103P/Hartley 2 as part of wavelength filters (3, 4). at encounter. The pattern of three cycles is due an extended mission named EPOXI (Extrasolar to an approximate commensurability between Planet Observation and Deep Impact Extended Encounter with Hartley 2 this precession and the roll around the long axis www.sciencemag.org Investigation). The closest approach to Hartley 2 was 694 km at with a period of 27.79 hours (55.42 hours is 13:59:47.31 UTC on 4 November 2010, 1 week also possible; the ambiguity does not affect any after perihelion passage and at 1.064 astronom- conclusions in this paper). The orientation of 1Department of Astronomy, University of Maryland, College ical units (AU) from the Sun. Flyby speed was the angular momentum vector is not yet tightly –1 Park, MD 20742-2421 USA. 2Belton Space Exploration Ini- 12.3 km s , and the spacecraft flew under the constrained but is within 10° of being perpen- tiatives LLC, 430 South Randolph Way, Tucson, AZ 85716 USA. comet with a somewhat northward trajectory in a dicular to the long axis. This excited state also 3Delamere Support Services, 525 Mapleton Avenue, Boulder, 4 solar system reference frame. Because instru- implies a nodding motion of the long axis rel- CO 80304, USA. Geophysical Institute, University of Alaska– Downloaded from Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775–7320, USA. ments are body-mounted on the spacecraft, the ative to the angular momentum vector, but the 5Max-Planck-Institut für Sonnensystemforschung, Max-Planck- spacecraft rotated to keep the instruments pointed observed near-axial symmetry of the shape lim- Strasse 2, 37191 Katlenburg-Lindau, Germany. 6Jet Propulsion at the comet. Observations of the comet were its this to an amplitude of <1°. The precession Laboratory, 4800 Oak Grove Drive, Pasadena CA 91109, USA. carried out for 2 months on approach (5 Sep- period is increasing at 0.1% per period near 7Code 600, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA. 8Institute for Astronomy, University of Hawaii, tember to 4 November) and for 3 weeks on de- perihelion, which is an unusually high but not 2680 Woodlawn Drive, Honolulu, HI 96822, USA. 9Lunar and parture (4 to 26 November), during which more unprecedented rate of change for a comet. The Planetary Library, University of Arizona, 1629 East University than 105 images and spectra were obtained. roll period is decreasing. These changes are Boulevard, Tucson, AZ 85721–0092, USA. 10Department of Earth and Atmospheric Sciences, Purdue University, 550 Sta- 11 dium Mall Drive, West Lafayette, IN 47907, USA. Department Fig. 1. Comparison of a of Geological Sciences, Brown University, Providence, RI 02912, smallpartof(left)Tem- USA. 12Department of Astronomy, 312 Space Sciences Build- ing, Cornell University, Ithaca, NY 14853, USA. 13Laboratoire pel 1 with (right)Hartley d’Astrophysique de Marseille, Universitéde Provence and CNRS, 2 at approximately the 13013 Marseille, France. 14Planetary Science Division, NASA same image scale and Headquarters, Mail Suite 3V71, 300 E Street SW, Washington, with nearly identical in- 15 DC 20546, USA. Sigma Space Corporation, 4600 Forbes Bou- struments. (Left) Impac- levard, Lanham, MD 20706, USA. 16Johns Hopkins University– Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, tor Targeting Sensor (ITS) MD 20723, USA. 17LESIA, Observatoire de Paris, UniversitéParis image iv9000675, 9.1 m –1 7, Batiment 17, 5 place Jules Janssen, Meudon Principal Cedex pixel .(Right)MRIim- 92195, France. age mv5004032, 8.5 m –1 *To whom correspondence should be addressed. E-mail: pixel . Sun is to the right. [email protected]

1396 17 JUNE 2011 VOL 332 SCIENCE www.sciencemag.org RESEARCH ARTICLES presumed to be due to torques produced by the because the waist is no longer a gravitational low forms that make up the knobby terrain. There is outgassing. for lower densities. The upper limit is not well marginally resolved mottling of the smooth regions Nuclear shape. The nucleus is bi-lobed in shape determined, but a four-times increase above the on the larger lobe and better resolved mottling (Fig. 1), with a maximum length of 2.33 km. The best-fitdensityto880kgm−3 requires modest and local topography of 10- to 30-m scale in the shape is well constrained by stereo viewing of porosity for pure ice and substantial porosity for waist. The darkest regions of the larger lobe are nearly half the object and for much of the re- plausible rock and ice mixtures. slightly more sharply bounded than is the darker mainder sampled by silhouettes against the light Any interpretation rests on the character of band within the waist. The elevated forms that scattered from the coma (Table 1). This bi-lobed the surface at the waist, which is mottled on constitute the rough terrain are in some areas shape is crudely similar to that of comet Borrelly horizontal scales of 10 to 30 m and has some aligned along boundaries of albedo markings and (7) but is both relatively and absolutely smoother. isolated cases of local relief >10 m. The mot- follow much of the southern edge of the waist. The rotation is slow enough that gravity is suf- tling, local topography, and generally gradational Many of the elevated forms exhibit two to three ficient to hold the two lobes together for any bulk boundary distinguishes the waist from the best- times higher albedo than the average, which is a density of >100 kg m–3. observed flow on Tempel 1 (1) and the ponded much greater range than seen on Tempel 1. Ragged, Constraints on density. The fast (12.3 km s–1) materials on (433) Eros (10) and Itokawa (8), at somewhat sinuous, narrow depressions are visi- flyby did not permit determination of the nuclear least locally, but does not rule out the possibility ble at high-incidence angles near the southern end, mass from the spacecraft’s trajectory. The smooth that the overall shape of the waist is approxi- about 10 m deep, up to 90 m wide, and extend- shape of the “waist” region connecting the two mately an equipotential. If the surface in this re- ing for over 250 m. lobes might indicate material collecting in a grav- gion approaches an equipotential and was formed The average geometric albedo of the nucleus itational low, such as observed on asteroid (25143) by flows or by deposition similar to those on other is ~4%. Within the larger lobe area are several Itokawa (8). Collection could proceed by in-falling objects, it has subsequently evolved, suffering sev- roughly equidimensional spots <80 m across that material landing in the gravitational low and/or eral meters of erosional etching. These facts sug- appear even darker than the larger, more elon- by in situ fluidization of regolith induced by out- gest that the equipotential assumption may not be gated “dark” areas mentioned above (Fig. 3E). flowing gas (9). If some form of frictionless, reliable and that, unlike the case for Tempel 1 in The darkest unshadowed spots are less than half fluidized flow is responsible for the formation or which a different and more direct approach was the average brightness. Although we cannot rule modification of this region, it should represent a possible (11), the density might be considerably out steep-sided holes for the dark spots near the “flat” surface so that it lies along an equipotential higher than deduced under our assumptions. terminator, they generally occur in regions that with respect to the combined forces of both grav- Geology of surface. The nucleus has two pri- are smooth in stereo and show no shadow sig- ity and rotation. Under these assumptions, which mary terrain types (Fig. 3A): knobby terrain char- natures. Thus, local albedos span at least a factor on November 11, 2011 may not be valid, the density of the nucleus can acterized by rounded to angular elevated forms of 4, compared with <2 on Tempel 1. be estimated by fitting potential contours to the up to 50 m high and 80 m wide and relatively Jets occur in all terrains but are clustered in observed geometry of the waist (SOM text). smooth regions occupying both the waist (Fig. the rough topography of the smaller lobe and We assumed internal homogeneity, a preces- 3C) between the two lobes and parts of the larger mid- to northern part of the larger lobe (Fig. 3). sion period of 18.34 hours, and a wide range of lobe (Fig. 3B). The elevated forms appear to be Such clustering of jets has also been seen at densities. The variance is minimized for a bulk the larger members of a population, with most comet 1P/Halley (12). At least some jets appear density of 220 kg m−3 (fig. S4). Even this min- examples near or below our practical mapping to originate at or near large, bright, elevated forms. imum residual leaves large-scale slopes of up to a resolution (~12 m) (Fig. 3D). The smoother areas Jets also originate beyond the terminator in areas few degrees relative to the equipotential. A rea- have darker central regions, are elongate, and with no direct sunlight, such as along the lower www.sciencemag.org sonable lower limit for the density is 180 kg m−3 are partially bounded by strings of the elevated edge of the larger lobe in Fig. 4. Even our res- olutionof10to12misnotsufficienttoclearly resolve the morphology of the sources of the jets. This comet lacks a population of depressions, such as those that dominate 81P/Wild 2 (13)or those scattered across Tempel 1 (1, 14). The knobby terrain is similar to some of the rougher areas on Tempel 1. The smoother regions on Hartley Downloaded from 2 do not show the striations that are suggestive of flow markings on the best-observed such re- gion on Tempel 1, and they are more gradation- ally bounded. The combination of terrains is very different from Tempel 1 or Wild 2, and this comet lacks exposures of thick, internal layers that were prominent on Tempel 1.

Nuclear Activity CN anomaly. Gaseous CN abundances were mea- sured routinely from the start of observations on 5 September (SOM text). The long-term CN gas production gradually increased from 6 × 1024 s–1 on 5 September to a peak of 3 × 1025 s–1 at peri- helion (28 October), after which it decreased again to ~2.4 × 1025 s–1at closest approach and Fig. 2. Variation of visible flux with time in a 191-km square aperture at the comet. Flux is proportional – to the amount of dust leaving the nucleus if physical parameters of the dust do not change (no changes in 1.2 × 1025 s 1 on 25 November (fig. S5). During dustcolororotherpropertiesappearinourdatanor are any reported by Earth-based observers). The most of the encounter, the CN production varied dotted box is expanded later (Fig. 7). Near closest approach, the flux from the nucleus is too large to allow periodically with the precession of the nucleus, as accurate photometry of the coma. did the grains and other gases.

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Table 1. Properties of Hartley 2’s nucleus. Fig. 3. (A)Hartley2,im- age mv6000002, ~7 m –1 Volume 0.82 T 0.08 km3 pixel . Sun is to the right, Diameter 0.69 to 2.33 km and the positive rotation- Surface gravity 0.0013 to 0.0033 cm s−2 al pole is at the smaller Precession period 18.34 T 0.04 hours (right) end. The view is from latitude ~–33°. (B) (November 4) Relatively smooth region Geometric albedo 4% (average) T of larger lobe. (C)Thewaist Mean radius 0.58 0.02 km between the two lobes. Cross section 0.43 to 1.59 km2 −3 (D) Knobby terrain, char- (Assumed density 220 kg m ) acterized by rounded to T Roll period 27.79 or 55.42 0.1 hours angular elevated forms up to 50 m high and 80 m wide. (E)Anexampleofa roughly equidimensional The production of CN also exhibited an anom- spot <80 m across that alous increase, by a factor of ~7, between 9 and appears even darker than 17 September, after which it slowly decreased, the larger, more elongated returning to the long-term trend line by 24 Sep- “dark” areas. tember (fig. S5). There was a very weak increase in dust release [DA(q)fr < 10 cm] above its trend in that same period. There was no corresponding increase of water (15). Integrating over the period of 9 to 24 September and subtracting a baseline gas production rate of 7 × 1024 s−1, we found that ~2 × 1031 CN radicals (~800 tons) were released in the anomaly. The long-duration, gradual increase and de- Escape velocity is poorly defined near the surface as ones to which radar is sensitive, is very small on November 11, 2011 crease of gaseous emission without a correspond- because of the rotating, elongated shape as well as compared with that detected by radar (18, 19), ing increase in the dust production is atypical of as the uncertainty in the mass of the nucleus. We which presumably is detecting much darker cometary outbursts, which have sudden onsets estimate that 10 to 20% of the chunks are mov- chunks over a much larger field of view. and are usually accompanied by considerable ing at less than their local escape velocity. Heterogeneity of dominant volatiles. Spectral dust. The increase is unlike the activity observed Sizes were estimated from the brightness of scans of the comet were obtained from 1 October at 9P/Tempel 1 or the behavior of any other com- the chunks (SOM text). The apparent flux from to 26 November, including several in which the et. The spatial profile of CN during the anomaly each of >104 individual chunks ranges from nucleus was spatially resolved. Figure 5 is from a was very different from that during the rest of ~10−13 to ≲10−11 Wm−2 mm−1 in the HRI image scan taken 7 min after closest approach, with an the observations and suggests formation of CN in Fig. 4, which is similar to that measured in the MRI image taken at nearly the same time. Red www.sciencemag.org from some extended source other than photo- MRI image taken close in time (Fig. 4). Sam- boxes show regions where we have extracted the dissociation of HCN. This could be grains too dark pling below 10−12 Wm−2 mm−1 is incomplete. We two spectra shown in Fig. 6, both of which have to scatter much sunlight, such as HCN polymers considered two extreme cases for the scattering had the continuum manually removed. The ratio (16) or the CHON grains found at 1P/Halley (17). properties: icy chunks scattering with the albedo of the H2ObandtotheCO2 band varies spatially They would need to be lifted by something abun- and phase function of Europa (24, 25) and dirty by 2.9 times. In these maps (Fig. 5), the emission dant and volatile. chunks scattering with the albedo and phase bands of H2OandCO2 are both somewhat op- Large chunks. Radar observations in October function of the nucleus of comet Tempel 1 (26). tically thick, implying that variations in column showed an ensemble of particles greater than a The range of measured fluxes corresponds to density could be larger than in brightness. Downloaded from few centimeters (18, 19). Although clouds of radii of 10 to 150 cm if they are dirty chunks and The maps show a water vapor–rich region large particles had been reported previously from 1to15cmiftheyareice(nearlypure).Belowthe extending roughly perpendicular to the waist of radar measurements of other comets (20), the minimum size, the chunks blend into the back- the nucleus and presumably arising from the location relative to the nucleus and the com- ground of unresolved, smaller chunks or grains. waist. This region has relatively little CO2,rel- position are not known. Previous searches with Because meter-sized objects are at the extreme of atively little gaseous organics, and thus far, no remote sensing for an icy grain halo in comets what can be lifted by gas drag and because the detectable water ice. The region of the jets off the have rarely been successful, with the only detec- smaller, unresolved chunks are demonstrably icy end of the smaller lobe of the nucleus is rich in tions being at large heliocentric distances (21–23). (see below), we argue that the largest chunks are CO2, organics, and water ice but has a lower col- At EPOXI’s close approach, individual icyandroughly10to20cminradius. umn density of water vapor than above the waist. chunks were seen near the nucleus in many The size distribution implied by the fluxes of There is substantial ice in jets emanating from images from both MRI and HRI (Fig. 4). The the discrete chunks is unusually steep. Most of beyond the terminator along the lower edge of motion of the spacecraft allowed determination the mass and most of the cross section are in the the larger lobe of the nucleus. The boundaries of the positions of individual chunks and their smallest grains. The discrete chunks contribute and the direct association with the major units of motions. A sample of 50 chunks has been followed roughly 4% of the total surface brightness in the the nucleus seen here are dramatic and suggest in many different MRI images and, other than innermost coma (<5 km), and those ≳5cm(com- very different histories for the waist and the re- one at 28 km, all were found to be within 15 km pleteness limit) are widely spaced at 4 × 10−8 m−3. mainder of the nucleus. The coincidence of strong of the large end of the nucleus. The motions are If we extrapolate the size distribution (SOM text), absorption bands of ice with jets that are bright in all very slow, with 80% moving at <0.5 m s−1 and >100% of the surface brightness is accounted for the continuum suggests that jets are bright when the fastest moving at <2 m s−1.Thisimpliesmin- just with chunks >0.5 mm. The total cross section highly reflective ice is present in the jets and con- imum life times of 104 s for many of the chunks. of discrete chunks, roughly the same size chunks versely, that jets are usually fainter than the nucleus

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Fig. 4. (Left)OriginalHRIimage (left, hv5004024, E-66s, range 915 km). (Middle) Deconvolved image. (Right)MRIcontextim- age (mv5004029) showing the location of the HRI field above the large lobe of the nucleus. Ar- rows indicate projected direc- tions to the Sun and Earth.

because they are of optically thin, relatively dark material. Theoretical calculations of scattering by icy grains (SOM text) show that the predominant scattering grains must be smaller than 10 mm. However, >100% of the surface brightness can be accounted for by extrapolating the chunks to a size of 0.4 mm, implying that the chunks are fluffy aggregates or clusters of ~1-mm solid grains. Either most of the aggregates of order 1 mm have broken up, or they mimic the scattering of the small grains. This result is very similar to the result obtained at Tempel 1 after the impact (no ice was on November 11, 2011 observed before the impact). Those grains were predominantly micrometer-sized (27). The sim- ilarity between excavated material from Tempel 1 and ambient outgassing from Hartley 2 suggests that the constituent grains of solid ice are on order of a micrometer in most comets. On the basis of calculations of life times (28–30) for the <10-mm solid components, the ice must be nearly pure for the grains to persist. www.sciencemag.org The detection of strong absorption by ice, the detection of very large chunks in the coma, the concentration of all species other than H2O vapor away from the waist of the nucleus, and the relatively smooth surface of the waist lead Fig. 5. Relative spatial distribution in the coma of Hartley 2. The red boxes (5 by 5 pixels; 52 m pixel–1) us to suggest that the material at the waist has indicate regions sampled to produce the spectra in Fig. 6. Panels labeled CO2, Organics, and H2OVapor been redeposited as a mixture of dirty grains and are maps of the total flux in the relevant emission bands. The panel labeled H O Ice is a map of the depth 2 Downloaded from fluffy, icy aggregates that have not yet sublimed. of the ice absorption feature at 3 mm. Each panel has been individually linearly stretched. All spectral The warmth of the dirty grains then leads to sub- images are from a scan at E+7 min, hi5006000. Sun is to the right. limation of the icy grains just below the surface. We conclude that this aspect of the chemical het- erogeneity of the nucleus of Hartley 2 is probably In Fig. 7, we compare a portion of the vi- most certainly primordial, unlike the ambig- evolutionary. sual light curve with the variation of CO2 and uous interpretation for the heterogeneity of To determine the absolute abundance ratio, H2O from the spectral scans. The scale is arbi- Tempel 1 (34). we considered a spectral map made three ro- trary, so only relative variations are meaningful. Summary and Conclusions tations (55 hours) earlier, when both the preces- The CO2/H2O ratio varies by a factor of 2 be- sion and roll orientations were the same. Spectra tween maxima and minima. The lower portion Comet 103P/Hartley 2 differs in many ways from were extracted from 120- and 600-km boxes, of Fig. 7 shows images of the CO2 and the H2O 9P/Tempel 1 and is an ideal example of hyper- both centered on the brightest pixel of thermal from the spectral maps. The red line indicates active comets, ones that produce more H2Oper emission (a better proxy for the nucleus than a the position of the nucleus as defined by the unit time than should be possible by sublimation reflected light center). In an aperture of 600 by peak thermal pixel. Close inspection shows that from the small surface area of their nuclei. Super- 600 km centered on the nucleus (fig. S8), and CO2 is more sunward (up in the figure) than volatiles, specifically CO2 in the case of Hartley –1 assuming an outflow speed of 0.5 km s ,we H2O near the maxima, reflecting the different 2, are the primary drivers of activity. The super- found average production rates Q(H2O) = 1.0 × spatial distributions. This suggests that the volatiles drag out chunks of nearly pure water-ice, 28 −1 27 −1 10 s and Q(CO2) = 2.0 × 10 s for ~20% CO2/H2O ratio is less in the large lobe of the which then sublime to provide a large fraction of fraction of CO2. This is higher than the fraction nucleus than in the small lobe, but this is a very the total H2O gaseous output of the comet. Other obtained in previous measurements of the global tentative conclusion until the rotational state is hyperactive comets include 46P/Wirtanen and production of CO2 in this comet (31–33). fully understood. If true, this heterogeneity is al- 21P/Giacobini-Zinner.

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sistent with any model of the outer protoplan- etary disk. This is also different from Tempel 1(~1)(36) and Halley (<1) (37). A large anom- aly in CN, too slow to be called an outburst, is unexplained.

References and Notes 1. M. F. A’Hearn et al., Science 310, 258 (2005). 2. M. F. A’Hearn, M. R. Combi, Eds., Icarus 191, 1 (2007). 3. D. L. Hampton et al., Space Sci. Rev. 117,43 (2005). 4. K. P. Klaasen et al., Rev. Sci. Instrum. 79, 091301 (2008). 5. O. Groussin, P. Lamy, L. Jorda, I. Toth, Astron. Astrophys. 419, 375 (2004). 6. C. M. Lisse et al., Publ. Astron. Soc. Pac. 121,968 (2009). 7. L. A. Soderblom et al., Science 296, 1087 (2002). 8. A. Fujiwara et al., Science 312, 1330 (2006). 9. M. J. S. Belton, J. Melosh, Icarus 200, 280 (2009). 10. M. S. Robinson, P. C. Thomas, J. Veverka, S. Murchie, B. Carcich, Nature 413, 396 (2001). 11. J. E. Richardson, H. Melosh, C. Lisse, B. Carcich, Icarus 190, 357 (2007). Fig. 6. Absolute spectra in the coma of Hartley 2 at CA+7 min. The black curve is the water-rich but ice- 12. H. U. Keller et al., Astron. Astrophys. 187, 107 (1987). 13. D. E. Brownlee et al., Science 304, 1764 (2004). free spectrum from the box in the H2O vapor panel in Fig. 5, above the waist of the nucleus. The red curve is the ice-rich spectrum from the box in the H O ice panel in Fig. 5, the jet region near the small end of the 14. P. C. Thomas et al., Icarus 187, 4 (2007). 2 15. M. R. Combi et al., Astrophys. J. 734, L6 (2011). nucleus. 16. C. N. Matthews, R. Ludicky, Adv. Space Res. 12,21 (1992). 17. J. Kissel et al., Nature 321, 336 (1986). 18. J. K. Harmon et al., IAU Circ. 9179, 1 (2010). 19. J. K. Harmon et al., Astrophys. J. 734, L2 (2011). on November 11, 2011 20. J. K. Harmon et al.,inComets II, M. C. Festou et al., Eds. (Univ. of Arizona Press, Tucson, AZ, 2005), p. 274. 21. M. F. A’Hearn, E. Dwek, A. T. Tokunaga, Astrophys. J. 248, L147 (1981). 22. H. Campins, G. H. Rieke, M. J. Lebofsky, Nature 301,405 (1983). 23. M. S. Hanner, Astrophys. J. 277, L78 (1984). 24. W. M. Grundy et al., New Horizons Team, Science 318, 234 (2007). 25. B. Buratti, J. Veverka, Icarus 55, 93 (1984).

26. J.-Y. Li et al., Icarus 187, 141 (2007). www.sciencemag.org 27. J. M. Sunshine et al., Icarus 190, 284 (2007). 28. H. Patashnick, G. Rupprecht, Icarus 30, 402 (1977). 29. M. S. Hanner, Icarus 47, 342 (1981). 30. H. I. M. Lichtenegger, N. I. Kömle, Icarus 90,319 (1991). 31. H. A. Weaver et al., Astrophys. J. 422, 374 (1994). 32. L. Colangeli et al., Astron. Astrophys. 343, L87 (1999). 33. J. Crovisier et al., ESA SP 427, 161 (1999). 34. L. M. Feaga, M. Ahearn, J. Sunshine, O. Groussin, Downloaded from T. Farnham, Icarus 190, 345 (2007). 35. H. A. Weaver et al., Astrophys. J. 734, L5 (2011). 36. L. M. Feaga et al., Bull. Am. Astron. Soc. 38,450 (2007). 37. D. Bockelée-Morvan et al.,inComets II,M.C.Festouet al., Eds. (Univ. of Arizona Press, Tucson, AZ, 2005), p. 391. Acknowledgments: Data from EPOXI will be released through NASA’s Planetary Data System in 2011. Image IDs in this paper are part of the final ID in the archive. This work was supported by NASA’s Discovery Program contract NNM07AA99C to the University of Maryland and Fig. 7. Light curve in visible light compared with the light curve in H2OandCO2. The curve for grains is task order NMO711002 to the Jet Propulsion Laboratory. an expanded version of the box in Fig. 2. The data points for gas are derived from spectral maps. The lower The work was supported by the home institutions of portion shows an expanded view, with images of the CO2 and H2O for each point in the dotted box. The several of the scientists, particularly by the University centroid of reflected continuum in each scan has been aligned with the red, horizontal line. Sun is up in of Maryland. The contributions of O. Groussin and the small images of gas. F. Merlin to this project were funded in part by the Centre National d’Etudes Spatiales.

Supporting Online Material In Hartley 2, H2O sublimes from the waist both refractories and icy chunks from the periph- www.sciencemag.org/cgi/content/full/332/6036/1396/DC1 with a much lower content of CO2 and barely any ery of the active regions. CO2/H2Ovariesbya SOM Text trace of icy grains. We tentatively interpret the factor 2, probably between one end and the other. Figs. S1 to S8 waist as a secondary deposit of material, although From HST measurements (35), CO is <0.3%, Table S1 the mechanism of redeposition remains unclear. implying a ratio of CO2/CO that is >60, which 9 February 2011; accepted 5 May 2011 The most likely mechanism involves fallback of is a far more oxidized environment than is con- 10.1126/science.1204054

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