P UBLIC ATIO N S

Geo physical Research Letters

RESEARC H LETTER Pitte d terrai ns o n (1) Ceres a n d i m plicati o ns f or s hall o w 10.1002/2017 GL073970 s u bs urface v olatile distri b uti o n

Key P oi nts: H. G. Size m ore 1 , T. Pl atz 2 , N. Schorghofer 1 , T. H. Pretty ma n 1 , M. C. De Sa nctis 3 , • Fresh co mplex craters on Ceres host 1 4 4 4 5 6 disti nctive pitte d terrai ns t hat are D. A. Cr o w n , N. Sch mede mann , A. Neese mann , T. K n eissl , S. Marc hi , P. M. Sc he nk , m or p h ol o gi c all y si mil ar t o pitt e d M. T. Bl a n d 7 , B. E. Sc h mi dt 8 , K. H. G. Hughson 9 , F. T osi 3 , F. Za mbon 3 , S. C. Mest 1 , materials on Mars an d Vesta R. A. Yi n gst 1 , D. A. Willi a ms 1 0 , C. T. R uss ell 9 , and C. A. Ray mond 1 1 • Pitte d terrai ns o n Ceres likely f or m via t he ra pi d v olatilizati o n of m olec ular 1 Pla netary Scie nce I nstitute, Tucso n, Arizo na, US A, 2 Max Planck Institute for Solar Syste m Research, Göttingen, Ger many, H 2 O e ntrai ne d i n i m pact materials 3 4 • Pitted terrains may be co m mon Istit ut o di Astr o fi si c a e Pl a n et ol o gi a S p azi ali, I N A F, R o m e, It al y, De part ment of Eart h Scie nces, Freie U niversität Berli n, 5 6 m or p h ol o gical markers of v olatile-ric h Berli n, Ger ma ny, South west Research Institute, Boul der, Colora do, US A, Lu nar a n d Pla netary I nstitute, Housto n, Texas, near-s urface material i n t he aster oi d U S A, 7 US GS Astrogeology Science Center, Flagstaff, Arizona, US A, 8 Depart ment of Planetary and Space Physics, Georgia b elt I nstitute of Tec h nolo gy, Atla nta, Geor gia, US A, 9 De part ment of Eart h, Pla netary, a n d S pace Scie nces, U niversity of Califor nia Los A n geles, Los A n geles, Califor nia, US A, 1 0 School of Earth an d S pace Sciences, Arizona State University, Te m pe, Arizona, 1 1 Supporting Infor mation: U S A, Jet Pro pulsio n La boratory, Califor nia I nstitute of Tec h nolo gy, Pasa de na, Califor nia, US A • Supporting Infor mation S1 • Supporting Infor mation S2 A bstract Prior to the arrival of the Da wn spacecraft at Ceres, the d warf planet was anticipated to be Correspondence to: ice-rich. Searches for morphological features related to ice have been ongoing during Da wn ’s missi o n at H. G. Siz e m or e, Ceres. Here we re port t he i de nti fi catio n of pitte d terrai ns associate d wit h fres h Cerea n i m pact craters. T he size more @psi.edu Cerea n pitte d terrai ns ex hi bit stro n g mor p holo gical si milarities to pitte d materials previously i de nti fi e d o n Mars ( where ice is i mplicated in pit develop ment) and Vesta ( where the presence of ice is debated). We Cit ati o n: e mploy nu merical models to investigate the for mation of pitted materials on Ceres and discuss the relative Size m ore, H. G., et al. ( 2 0 1 7), Pitte d ter- rai ns o n (1) Ceres a n d i m plicati o ns for i m portance of water ice an d other volatiles in pit develo p ment there. We conclu de that water ice likely plays s hall o w s u bs urface v olatile distri b uti o n, an i mportant role in pit develop ment on Ceres. Si milar pitted terrains may be co m mon in the asteroid belt Ge o p hys. Res. Lett. , 4 4 , 6570 – 6 5 7 8, an d may be of interest to future missions motivate d by both astro biology an d in situ resource utilization. doi:10.1002/2017 GL073970.

Received 26 APR 2017 1. I ntr o d ucti o n Accepted 22 J U N 2017 Acce pte d article online 27 J U N 2017, The Da wn spacecraft entered orbit around d warf planet Ceres on 5 March 2015. Over the fi rst 16 months at Published online 15 J UL 2017 Ceres, the s pacecraft o btaine d mor phological, to pogra phical, mineralogical, ele mental, an d gravity data fro m a series of successively lo wer orbits. Fra ming Ca mera (FC) i mages fro m high altitude mapping orbit ( H A M O) and lo w altitude mapping orbit (L A M O) have provided global coverage of Ceres ’ s urface at pixel scales of 140 m/pixel and 35 m/pixel, respectively. In the FC data set we have identi fi e d disti nct pitte d terrai ns, characterized by clusters of round-to-irregular, apparently ri mless pits in s mooth i mpact materials. We descri be t he mor p holo gy, setti n g, a n d geo gra p hic distri butio n of t hese terrai ns, w hic h ex hi bit striki n g si mi- larities to pitte d materials previously discovere d by Da w n at Vesta [ De nevi et al ., 2 0 1 2; Re d dy et al. , 2012] a n d pitte d materials i de nti fi e d in lo w- an d mi d dle-latitu de Martian craters [ Tornabene et al ., 2 0 1 2; B oyce et al ., 2012]. We discuss likely for mation mechanis ms for the Cerean pitted materials, as well as i mplications for the geogra phic distri bution an d concentration of shallo w volatiles on Ceres.

2. Methods Pri or t o Da w n ’s arrival at Ceres it was a ntici pate d t hat t he d warf pla net ’s o ut er s h ell mi g ht b e w at er ic e-ric h [ e. g., McCord and Sotin , 2005, and references therein]. Further, ther mal si mulations predicted that ice in the shallo w subsurface could be protected against subli mation on billion year ti mescales by a dry silicate lag, varying in depth fro m tens or hundreds of meters near the equator to centi meters near the pole [e.g., ©2017. The Authors. Fanale and Salvail , 1989; Schorghofer , 2008, 2016]. Using FC i mages, we have carried out global searches T his is a n o pe n access article u n der t he for mor phological features that are potentially diagnostic of the presence of su bsurface ice base d on analo- ter ms of the Creative Co m mons Attribution- NonCo m mercial- No Derivs gies wit h a variety of s olar syste m o bjects [e. g., Sc h midt et al ., 2017]. Feat ures of partic ular i nterest i n t his Lice nse, w hic h per mits use a n d distri- search were pits and depressions, which have been linked to gas-phase volatile loss on Mars, Callisto, bution in any mediu m, provided the Euro pa, Mercury, Vesta, Trito n, a n d Pluto. We carrie d out a glo bal searc h for pits a n d de pressio ns duri n g eac h ori gi nal w ork is pr o perly cite d, t he use is non-co m mercial and no modi fi cati o ns mapping orbit (Survey, HA M O, and LA M O). Because Ceres ’ surface morphology is do minated by i mpact or a da ptations are ma de. structures, distinguishing bet ween features produced by volatile loss and features produced by pri mary

SIZE M ORE ET AL. PITTE D TERR AI NS O N (1) CERES 6570 Geo physical Research Letters 10.1002/2017 GL073970

a n d seco n dary i m pacts was a n o n goi n g co nsi deratio n i n our a nalysis of F C data. Our criteria for i de nti fi c ati o n of pits, de pressi o ns, a n d pit cl usters releva nt t o v olatiles were t he prese nce of (1) ri mless de pressi o ns, wit h (2) round-to-irregular, elongate, branching, or polygonal margins, and exhibiting (3) overlapping or repeating patter ns. T hese criteria were base d pri marily o n c haracteristics of t he Martia n “S wiss Cheese ” an d dissecte d ma ntl e t errai ns [ M ali n et al ., 2 0 0 1; Millike n a nd Mustard , 2 0 0 3; Byr ne a nd I ngersoll , 2003], the morphology of pitted materials inferred to be volatile-rich in craters on Vesta, Mars, and Mercury [ De nevi et al ., 2 0 1 2; Tornabene et al ., 2 0 1 2; Ble wett et al ., 2011], and revie ws of ice subli mation as a geo morphic process [e.g., Mangold , 2 0 1 1; M o ore et al ., 1996]. Follo wing co mpletion of our global searches, we interpreted the nature of i d e nti fi ed pits and depressions based on morphological co mparisons to other solar syste m bodies and esta blishe d nu merical mo dels of near-surface volatile trans port [ B oyce et al ., 2 0 1 2; Schorghofer , 2 0 1 6].

3. R es ults I n t he c o urse of o ur gl o bal searc hes, we i de nti fi e d four re gio ns wit h pits t hat met our criteria i n L A M O i ma ges a n d t hree a d ditio nal areas wit h a m bi guous de pressio ns pote ntially co nsiste nt wit h our criteria. All pitte d ter- r ai ns i d e nti fi e d in L A M O data were associate d with relatively fresh craters, pro m pting us to carry out focuse d re peat searc hes for pitti n g i n all t he most pristi ne Cerea n craters. T he results of t hese searc hes are su m mar- ize d in Ta ble 1 an d Figure 1 an d descri be d in detail belo w. Ika pati, Da ntu, Haula ni, a n d Ku palo craters ex hi bit clusters of pits t hat are disti n guis ha ble fro m near by s mall i mpact craters, based on their lack of high standing ri ms, their rounded but irregular margins, and their tendency to occur in overlapping clusters on other wise s mooth, lightly cratered terrain. Ikapati and Dantu craters host t he most exte nsive pit clusters o n Ceres (Fi gure 2). Pits occur o n fl oor materials and in s mooth ejecta exterior to t he crater ri m at bot h locatio ns. Pits ex hi bit a ra n ge of de pt hs wit hi n i n divi dual clusters (ty pically tens of meters but exten ding to de pths > 150 m i n t he lar gest exa m ples), a n d are ge nerally co nical or fl ute d i n pr o fi l e, r at h er t h a n fl at- fl o ore d. I n t he i nteri or of Ika pati crater ( de pt h 4 k m, dia meter 50 k m) m ulti- ple cl usters are distri b ute d acr oss s m o ot h fl at-lyi n g material i n t he s o ut heaster n half of t he crater (Fi g ure S1 i n the su p porting infor mation) [ Pre usker et al. , 2016]. Exteri or t o Ika pati, pit cl usters are prese nt i n t he s m o ot h ejecta po n de d i n to po gra p hic lo ws associate d wit h preexisti n g lar ge craters. Pit clusters o n t he Ika pati ejecta consist of s maller in divi dual de pressions than those in the crater interior. In divi dually, these s maller pits ( dia meters < 0.5 k m) are less easily distinguished fro m co mparably sized i mpact craters because their margins/ri ms are not resolved; ho wever, the pits appear incised in surrounding s mooth i mpact material a n d t heir clusteri n g o n po n de d material is texturally disti nctive fro m i m pact craters. T he distri b uti o n a n d m or p h ol o gy of pits at Da nt u ( de pt h 4 k m, dia meter 126 k m: Fi g ure S3) is si milar t o Ika pati. S mooth deposits cover most of the Dantu fl oor, a n d s moot h ejecta is po n de d in to po gra p hic lo ws to t he sout h- w est of t h e D a nt u ri m. W e i d e nti fi e d pit cl usters i n b ot h i nteri or a n d exteri or s m o ot h material. As i n Ika pati, t he lar gest pits ass ociate d wit h Da nt u (0.4 t o 0.9 k m) occ ur in t he crater interi or o n s m o ot h fi ll mat erial; pits o n ej ecta m at eri als ar e s m all er (t y pi c all y < 0.3 k m). Pits i n t he Da nt u i nteri or are cl ustere d ar o u n d t he ce ntral peak/ri n g co m plex an d s mooth materials on the crater fl o or. S mall i m pact craters, de fi n e d b y t h eir r o u n d s h a p e a n d r ais e d ri ms, are more a bu n da nt in t he Da ntu re gio n t ha n at Ika pati. T he hi g her- back grou n d crater de nsity a n d t he pre- valence of in divi dual pits with dia meters much s maller than 1 k m makes distinction bet ween “pits ” and i mpact craters more a m bi guous at Da ntu t ha n at Ika pati, particularly exter nal to t he Da ntu ri m. The s mooth fl o or material of Ha ula ni crater ( de pt h 2.5 – 3 k m, dia meter 34 k m; Fi g ure S2) is exte nsively pitte d. I n divi d ual pits are < 70 to ~400 m in dia meter, tens of meters deep and exhibit the sa me morphological characteristics as those in Dantu an d Ika pati. In Haulani, pit clusters are a djacent to an d so meti mes su per- posed by lobate and sinuous mass- wasting features sourced fro m the crater walls. Other pits are incised in materials t hat have fl o wed out ward fro m the central peak co mplex, superposing other pitted materials on t he sout her n crater fl o or. I n t he n ort h west q ua dra nt of t he crater, pits a n d fl o or fract ures occ ur i n cl ose pr oxi- mit y [ Buczko wski et al ., 2016a]. Exteri or t o Ha ula ni, m uc h of t he ejecta is exte nsively fl ui dize d, wit h material that is s mooth, sinuous, and exhibits localized lobate margins and ponding (Figure S2); ho wever, we did n ot i de ntify a ny pits or pit cl usters o n t he Ha ula ni ejecta. Kupalo crater (depth 2.5 k m, dia meter 26 k m; Figure S4) provides the fi nal exa mple of una mbiguous pitting o n Ceres. K u pal o is fl at fl oore d a n d hosts exte n de d pit clusters, wit h a distri butio n si milar to t hat see n o n t he H a ul a ni fl oor. In the crater interior, pit clusters and/or pitted texture in other wise s mooth material are

SIZE M ORE ET AL. PITTE D TERR AI NS O N (1) CERES 6571 Geo physical Research Letters 10.1002/2017 GL073970

T a bl e 1. Su m mary of Pit Occurrence in Well-Preserved I mpact Craters a De pt h Dia meter M ax Pit A p pr ox. Crater Parent Crater Latitude Longitude ( k m) ( k m) Pits? Di a m et er ( m) A g e ( m a) Pit S etti n g

Urvara 45.66 249.24 5 to 6 170 degraded 2054.8 130 – 2 4 0 ( 8/+20) Ma s mooth fl o or Da nt u 24.3 138.23 ~4 126 abundant 1102.8 72 – 1 5 0 ( 4/+9) Ma smooth fl o or a n d ejecta Occator 19.82 239.33 ~4 92 few, ambiguous 2 1 ( ± 0. 4) M a wall terrace a n d ejecta Ika pati 33.8 45.61 ~4 50 abundant 866.5 19 – 4 3 ( 2/+10) Ma smooth fl o or a n d ejecta Azacca 6.66 218.4 ~2.5 49 few, degraded 742.1 76 (±10) Ma s mooth fl o or H a ul a ni 5.8 10.77 2.5 to 3 34 abundant 3 7 2. 9 1. 7 – 2. 7 ( 0.2/+0.7) smooth fl o or K u p al o 39.44 173.2 ~2.5 26 abundant 3 2 6. 2 < 4. 5 M a ( dif fi cult to date) s mooth fl o or J uli n g 35.9 168.48 ~1.75 20 none < 2. 5 M a ( dif fi c ult t o dat e) O x o 42.21 359.6 < 1 10 none 0. 5 1 ( ± 0. 2) M a a Crater age esti mates are based on the Lunar Derived Syste m (L D M) and are discussed in detail by Sch mede mann et al. [2017] and Willi a ms et al. [ 2 0 1 7]. Pit dia meters are deter mined via the technique described by K n eissl et al. [ 2 0 1 1].

ass ociate d wit h si n u o us, l o bate, a n d strea mli ne d patter ns. We di d n ot i de ntify pit cl usters or pitte d text ures i n the Ku palo ejecta, des pite its s mooth texture, sinuosity, pon ding, an d lo bate margins. O ur glo bal searc h also reveale d t hree less pristi ne craters, Urvara, Azacca, a n d Occat or i n w hic h t here is p ossi ble evide nce of de graded pits. In the Urvara ( depth 5 – 6 k m, dia meter 170 k m) interior, t wo groups of hu m mocky de pressions dissect the s mooth fl oor material where the s mooth material abuts the rough, high-standing cen- tral peak material. Scr uti ny of t he Urvara fl oor i n L A M O i mages reveale d t hat there are fe w distinct, steep- walle d pits in either region. Instead, these areas consist of muted overlapping depressions with sloped walls and variable de pths; t he bou ndaries bet ween in divi dual depressions do not rise to the elevatio n of t he surroun di ng terrai n. T he setti n g of t his terrai n is si milar t o pitte d areas i n t he Da nt u, Ha ula ni, a n d K u pal o i nteri ors. Exteri or t o Urvara, ejecta materials are too de graded to allo w for a meanin gful assess me nt of w hether pits are present. Azacca crater (depth 2.5 k m, dia meter 49.9 k m) may also sho w evidence of degraded pits, although features at this location are much more a m biguous. The large-scale mor phology of the crater is broa dly si milar to both Ikapati and Dantu; the density of superposed craters is much higher. Azacca is a fl at- fl o ore d crater, wit h a central peak and pro minent fl o or fract ures. We i de nti fi e d t wo clusters of irregular, interconnecte d de pres- sions and a single large isolated depression with irregular margins ( ~1 k m by 400 m) on the Azacca fl o or (Fi gures S7c – S7f). T heir setti n g a n d sizes are si milar t o pits i de nti fi ed else where. We also observed a s mall nu mber of a mbiguous depressions on terrace material in Occator Crater (depth 4 k m; dia meter 92 k m; Figure S6), but not on the extensively modi fi e d crater fl oor. The possible presence of degraded pits in less-pristine craters suggests that pits may be a co m mon feature of fresh Cerean i mpact materials but that subsequent s mall i mpacts and mass wasting rapidly obscure the m.

Fi g ur e 1. Gl o bal distri b uti o n of pitte d crater materials o n Ceres. Pitte d materials si milar t o t h ose i de nti fi e d in Vestan an d Martia n i m pact craters [ D e n e vi et al., 2 0 1 2; Tornabene et al ., 2012] have bee n i de nti fi e d i n Ika pati, Ha ula ni, Da nt u, a n d Ku palo craters (re d circles) on Ceres. De pressions in Urvara, Occator, an d Azacca may be degra de d pits (orange triangles).

SIZE M ORE ET AL. PITTE D TERR AI NS O N (1) CERES 6572 Geo physical Research Letters 10.1002/2017 GL073970

Fi g ur e 2. Pit i mages and topographic pro fi les in (a, c, and e) Ikapati and (b, d, and f) Dantu. I mages are excerpted fro m a global L A M O mosaic provided by DLR. Topographic pro fi les are derived fro m stereo pairs of L A M O i mages using ISIS i mage processing soft ware [ Becker et al., 2 0 1 5].

T wo pro minent fresh craters, Juling and Oxo, sho w no evidence of pits. Juling (depth 1.75 k m, dia meter 20 k m) is nota ble for its proxi mity to Ku palo, its near-co m plete lack of su per pose d craters, an d its lo bate ejecta a n d i nteri or mass- wasti n g feat ures. J uli n g is i n fi lled by hu m mocky-to-lobate material, and lacks both

SIZE M ORE ET AL. PITTE D TERR AI NS O N (1) CERES 6573 Geo physical Research Letters 10.1002/2017 GL073970

a fl at fl o or a n d i de nti fi e d pits. Ox o ( de pt h < 1 k m, di a m et er 1 0 k m) is si mil ar t o J uli n g i n t er ms of siz e, l a c k of su per pose d i m pacts, a n d t he prese nce of exte nsive lo bate features i nterior a n d exterior to t he crater ri m. T he Oxo materials inclu de extensive bright de posits, an d Oxo is one of t wo locations on Ceres for where there is s pectral evi de nce of s mall qua ntities of water ice ex pose d at t he surface [ Co mbe et al ., 2016]. T he a bse nce of superposed i mpact craters makes the nondetections at both Juling and Oxo una mbiguous within the li mits of L A M O i mage resolution, but does not rule out the presence of pits < 60 m i n dia meter. The mor phology an d setting of Cerean pitte d terrains are re marka bly si milar to pitte d terrains i denti fi e d o n Vesta and Mars and described by Tor nabe ne et al. [2012] and De nevi et al. [2012] (Fi gures 3a – 3c). At C er es, Vesta, and Mars, individual pits have conical or fl uted depth pro fi les, appear ri mless (see discussion by T or n a be ne et al. [2012]) an d/or overla p with a djacent pits for ming share d boun daries. On all three planetary b o dies, pits are ass ociate d wit h pristi ne craters, a n d de gra de d exa m ples are see n i n less pristi ne craters, s u g- gesting that they are ra pi dly degra de d. In divi dual pits an d pit clusters are incise d in s mooth crater fi ll a n d ejecta materials. Co m monly, this s mooth material sho ws localized or pervasive evidence of fl o w — e. g., e mbay ment relationships with high-standing terrain, lobate margins, strea mlining, and/or ponding in topo- gra p hic lo ws. At Haula ni a n d Ku palo, as i n Tooti n g Crater o n Mars, pits occur o n a n d are over pri nte d by lo bate fl o ws. On Ceres, as on Mars and Vesta, the largest pits and the densest concentrations of pits occur in the ce ntral portio ns of t he s moot h crater fi ll de posit, an d/or along the contact bet ween pitte d materials an d the be d rock of t he ce ntral u plift (see Cerea n exa m ples i n Fi gures S2 – S5). I n c o m plex Martia n craters, t he de nsest clusters of pits occur o n t he lo west elevatio n surfaces i n t he crater i nterior a n d at locatio ns w here t here are mini mal effects of slu mping and terracing; on Ceres Ikapati and Urvara exhibit si milar trends. On Mars, Vesta, a n d Ceres, pit cl usters are s o meti mes ass ociate d wit h or cr ossc ut by fract ures si milar t o fract ures o bserve i n lar ge l u nar craters [ Heather and Dunkin , 2 0 0 3; Tornabene et al ., 2 0 1 2; De nevi et al ., 2 0 1 2; Buczko wski et al ., 2016b]. On Mars and Vesta, s maller pits occur where ejecta is ponded on terraces or in topographic lo ws exterior to t he source crater. O n Ceres, we see a clear exa m ple of t he latter at Ika pati a n d a possi ble exa m ple of t he f or mer at Occat or. Fi nally, t he Cerea n pits — like their Martian an d Vestan counter parts — obey a po wer la w-relati n g pit dia meter to host crater dia meter [ Tor nabe ne et al ., 2 0 1 2; De nevi et al ., 2 0 1 2] ( Fi g ur e S 8). There are so me li mitations to the parallels that can be dra wn bet ween the pitted materials on the three b o di es. De nevi et al. [2012] re porte d a disti nctive t her mal si g nature associate d wit h t he Vesta n pitte d terrai ns. So me Martian craters with pitte d materials have characteristic ther mal signatures, others do not [ M ali n a n d E d gett , 2 0 0 1; Tor nabe ne et al. , 2 0 0 6; Pre blic h et al ., 2 0 0 7; Tornabene et al ., 2012]. We di d not fi n d a ther mal signature associate d with the Cerean pits (Figure S12) [ C a pri a et al. , 2 0 1 4; T osi et al. , 2014]. T he relatively lo w resolution of F C L A M O i mages at Ceres (35 m/ pixel versus 17 m/ pixel at Vesta) prevents the detaile d ana- lysis of in divi dual pit mor phology carrie d out by Tor nabe ne et al. [2012] at Mars ( HiRISE i mages have pixel scales of ~28 c m/pixel). I mage resolution also introduces a mbiguity in the identi fi c ati o n of i n di vi d u al Cerea n pits as disti nct fro m s mall craters at locatio ns w here pits are s mall a n d/or back grou n d crater de nsities are high. Ho wever, the nu merous parallels in morphology and setting bet ween Cerean, Vestan, and Martian pitted materials suggest that the features on all three bodies are analogous and likely share a co m mon for mation process.

4. Disc ussi o n O n Mars, T or n a be ne et al. [2012] an d B oyce et al. [2012] attri bute d t he disti nct mor p holo gy a n d distri butio n of pitte d materials to the ra pi d devolatilization of i m pact materials follo wing crater for mation. De nevi et al. [2012] reached si milar conclusions for Vesta. We also prefer a rapid posti mpact for mation model at Ceres, on physical and morphological grounds. Long-ter m subli mation of ground ice — a physical process co m- monly linke d to the develo p ment of ri mless pits an d de pressions — is u nlik ely t o be res p o nsi ble f or pit pr o d uc-

tion on Ceres for t wo reasons. First, modeled H 2 O subli mation rates for Ceres are too slo w to produce the o bserve d relief during the short ti me since the for mation of the host craters. We o bserve pits > 100 m deep in craters that for med in the last 100 Myr (Table 1). We carried out si mulations to deter mine the maxi mu m relief t hat coul d be pro duce d by su bli matio n of grou n d ice over t he past Gyr o n Ceres, a n d fou n d it to be less than 4 m (Figure S9) [ Schorghofer , 2016; Riv ki n et al. , 2010]. Second, no identi fi e d class of solar syste m morphological features produced by long-ter m subli mation is analogous to the Cerean pits. “H oll o ws ” o n Mercury have li mited si milarities (length scale and crater fl oor setting), but hollo ws are elongate,

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Fi g ur e 3. C o m paris o n of pitte d terrai ns o n Mars, Vesta, a n d Ceres. (a) Pitte d terrai n wit hi n t he 10.3 k m dia meter Z u nil Crater (7.70° N/166.19°E) on Mars as seen in HiRISE i mage ESP_038250_1880 (0.25 m/pixel). (b) Pitted terrain within the 67.6 k m dia meter Marcia Crater (8.98° N/339.55°E) on Vesta as seen in Fra ming Ca mera i mages 14,662 (19.7 m/ pixel). Pitte d terrains o bserve d on Ceres are highlighte d in (c) Haulani Crater, ( d) Ika pati Crater, an d (e) Dantu Crater; Fra ming Ca mera na dir mosaic (30 m/ pixel). (e) Li nes i n dicati n g t he s hare d bor ders of t he Da ntu pits; see Fi gure 3 of B oyce et al. [2012] for co m- paris o n. Scale bar i n dicates 1 k m i n all i ma ges. N ote t hat n ort h is n ot u p.

bra nc hi n g, fl at- fl oored, associated with bright halos, and occur in degraded craters [ Ble wett et al ., 2 0 1 1] (Fi gure S10). T he Cerea n pit clusters, like t he Martia n pitte d materials descri be d by Tor nabe ne et al. [ 2 0 1 2], als o e x hi bit li mit e d si mil ariti es t o a bl ati o n h oll o ws or s u n c u ps [ cf., Mangold , 2011], but t here is no evi de nce for the extensive, nearly pure surface ice deposits required for suncup develop ment on Ceres. B oyce et al. [2012] developed conceptual and physical models for the develop ment of Martian pitted materi- als, base d o n o bservati o ns at Ries crater, Ger ma ny [ McE we n et al ., 2 0 0 7; Tor nabe ne et al ., 2 0 0 7, 2 0 1 2; M o u gi nis- Mark a nd Garbeil , 2 0 0 7; M orris et al ., 2010]. In the Boyce conce ptual mo del, melt- bearing i m pact breccia with

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e ntrai ne d v olatiles is de p osite d i n a crater fl oor and on the surrounding terrain soon after an i mpact. Water vapor begins to escape fro m the deposit and establishes a series of preferential path ways that transport vapor and fi ne- grai ne d material to t he surface. I ncrease d fl o w velocities for m larger vent pi pes an d dra w in additional gas via the Venturi effect. Flo wing gas and entrained clasts erode the vent walls. Patterns of regularly s pace d pits with su btle share d ri ms are left when the gas su p ply is de plete d. This conce ptual mo del explains the shape of individual pits, their occurrence in clusters, and the relationships bet ween crater dia meter, pit locatio n, a n d pit size o bserve d o n Mars, Vesta, a n d Ceres (Fi gure 3). The ulti mate source of the pit-for ming gas on Ceres is a matter of so me interest for evaluating pre- Da wn pre dictio ns of a bu n da nt s hallo w ice. B oyce et al. [2012] esti mate d that lo w-latitu de Martian target materials likely c o ntai n 4 – 10 wt % water in the for m of pore space ice, and an additional ~4 – 7 wt % water bound in minerals. Based on the assu mption that gas pressures in the melt-bearing layer should equal or exceed the overburden pressure to drive fl o w, B oyce et al. [2012] calculate d out fl o w velocities that would exhaust the

H 2 O supply in the days i m mediately follo wing the i mpact. They e mphasized that the volatile supply might be exhausted on shorter ti mescales due to gas fl o w exploiting preferred path ways once Venturi pipes were esta blis he d. A p plyi n g t he B oyce et al. [2012] nu merical model of pit-for ming gas fl o w to Ceres (Fi gure S10) suggests that fl o w velocities and the ti me fra me for pit for mation are broadly co mparable on Ceres and

Mars; ho wever, the total mass of H 2 O required for pit develop ment is s maller on Ceres than on Mars, due t o Ceres ’ lo wer gravitatio nal acceleratio n. Is the gas driving Cerean pit develop ment sourced fro m ground ice? De nevi et al. [2012] esti mate d that water bound in exogenic chondritic material was suf fi cient to produce pitted materials on Vesta, which is not thought to have endogenic sources of water. Sc ully et al. [2015] su bse quently suggeste d a role for s mall quan- tities of ice. I n co ntrast to Vesta, Ceres likely hosts su bsta ntial e n do ge nic volatiles. Data fro m t he Ga m ma Ray and Neutron Detector ( GRa N D) are broadly consistent with predictions of ther mal stability models, which indicate that ice should be present in regolith pore spaces at depths of centi meters near the poles (inside the depth range of GRa N D sensitivity) and depths of a fe w meters near the equator (not detectable by G R a N D) [ Schorghofer , 2 0 1 6; Pretty ma n et al ., 2017]. GRa N D data also suggest that bound “w at er ” ( molecular

H 2 O, hy droxi des, an d hy droxyl grou ps) is present in the “dr y ” e quatorial regolith at deci meter de pth scales [Pretty m a n et al ., 2017]. Data fro m the Visi ble an d InfraRe d Ma p ping S pectro meter (VIR- MS) in dicate localize d mi dlatit u de ex p os ures of water ice [ Co mbe et al ., 2016], as well as a m monia- and O H-bearing mineral phases

distri b ute d gl o bally [ DeS a nctis et al ., 2015]. VIR- MS data do not provide evidence for H 2 O- bearing mineral p hases at t he surface, w here p hylosilicates, car bo nates, a n d salts are likely desiccate d by ex posure to vacuu m a n d s u nli g ht. H o w e v er, t h er m al e v ol uti o n m o d els [ e. g., C astill o- R o gez et al ., 2016], GRa N D, a n d VIR- MS data col- lectively suggest that pit-for ming volatiles can be source d fro m both minerals an d ice on Ceres. Which reser- voir is do minant de pen ds critically on the peak te m perature of the brecciate d layer in which pits develo p. Hydrodyna mic i mpact models indicate that at the average i mpact speed a mong main belt asteroids (5 k m/s) [M arc hi et al ., 2016], the material at the botto m of craters is heate d to less than ~200 – 3 0 0 K [ M arc hi et al ., 2013]. A s mall fracti o n of tar get material — na mely, ejecta — is heated bet ween ~300 K and ~800 K [ M arc hi et al ., 2013]. Based on ther mogravi metric analysis of meteorites [e.g., Kolod ny et al ., 1 9 8 0; Gare n ne et al .,

2 0 1 4], C O 2 fro m carbonates is unlikely to be liberated in Cerean i mpact-heated material, due to its high te m perature threshol d for release ( ~1035 K). Hy droxi des an d hy droxyl grou ps in phyllosilicates can be li ber- ated above ~473 K and ~673, respectively, and may make minor contributions to pit production. Molecular

H 2 O i n t he re g olit h (i.e., a ds or be d water, water of hy drati o n i n salts, a n d crystalli ne ice) ca n rea dily va p orize at 300 K an d a bove an d likely do minates pit-for ming volatile fl o ws o n Ceres. Ice may be t he key player, give n

t hat crystalli ne ice is t he o nly f or m of m olec ular H 2 O that has been detecte d on Ceres ’s urface, a n d give n t hat t he re g olit h has a greater ca pacity t o st ore H 2 O as pore-s pace ice than as a dsor bate an d water of hy dration. O n Mars, pitte d crater materials are c o n fi ne d to mi d dle a n d lo w latitu des; pit for matio n is t hou g ht to cease at hi g h latit u des ( > 5 0 o ) due to hi g h ice co nce ntratio ns [ Tornabene et al ., 2 0 1 2; B oyce et al ., 2012], w hic h vary fro m ~50 to 100 vol. % [e.g., M ell o n et al ., 2 0 0 9; Feld man et al ., 2008]. Base d on esti mates of the total mass

of H 2 O that could be released fro m the pore space of a ~100 m brecciated layer, and on analogies with Mars, we conclude that the presence of pitted crater materials on Ceres places a rough upper li mit on ice concentration in the target material of ~40 – 50 vol %. This upper li mit is consistent with other lines of morphological evidence constraining ice content. The large-scale morphology of Cerean craters is

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analogous to i m pact structures on icy satellites, in dicating weak, ice-rich target materials [ Schenk , 2 0 1 5]; ho wever, crater topography on Ceres persists over billion year ti mescales, indicating a signi fi c a nt sili c at e co mponent to the upper crust and ice volu me fractions < 4 0 % [ Bla nd et al ., 2016]. The association of Cerean pits with lobate ejecta and lobate mass wasting features much more extensive than those observed o n Vesta also su g gests a role for mo dest qua ntities of ice [ Sc ully et al ., 2 0 1 5; Hughson et al ., 2 0 1 6; Sch midt et al ., 2017]. Thus, pitte d crater materials are strongly suggestive of ice in the Cerean regolith, an d their prese nce at lo w a n d mi d dle latitu des su p ports a t heoretical para di g m i n w hic h s hallo w ice is prese nt glo bally [F a n ale a n d S alv ail , 1 9 8 9; Schorghofer , 2016], but ice is not a volu metrically do minant co m ponent of the u p per meters t o kil o meters of t he re g olit h [ Bl a n d et al ., 2 0 1 6; Pretty ma n et al ., 2 0 1 7]. The discovery of pitted i mpact materials on Mars, Vesta, and no w Ceres suggests surprising si milarities in posti mpact surface modi fi cation on the three bo dies. Si milar pitte d materials may be co m mon in the asteroi d belt. U n dersta n di n g t he distri b uti o n a n d a b u n da nce of v olatiles, partic ularly water ice, i n t he re g olit h of r ocky bodies will be of on-going interest for both astrobiology and in situ resource utilization. More detailed modeling of pit develop ment processes is needed to elucidate the roles of i mpact te mperature, volatile source, an d volatile a bun dance in the for mation of pitte d i m pact materials in the inner solar syste m.

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