An Eroded Martian Lacustrine Deposit(?)

"White Rock": An eroded Martian lacustrine deposit(?)

Steven H. Williams Department of Geology, Arizona State University, Tempe, Arizona 85287-1404 James R. Zimbelman Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, D.C. 20560

ABSTRACT The existence and location of ancient lake sites and sediments have important implications for Martian paleoclimate and exobiology. White Rock, an enigmatic crater interior deposit, may be the eroded remnant of such a lacustrine deposit. Stereogrammetric analysis of newly processed Viking images allows better determination of the dimensions of White Rock (12.5 x 15 km, thickiess 180 to 540 m, volume -40 kd), reveals differences in erosion patterns that may reflect differences in depositional environment, and allows the identification in or on the crater wall of a possible source region of the high-albedo White Rock material. If White Rock is the remnant of a once-larger deposit, then open-system circulation may have been required to deliver the required quantity of evaporites.

INTRODUCIlON other lacustrine features have been identi- An eroded crater deposit somewhat re- The existence and. persistence of lakes fied (Parker et al., 1989, 1993). sembling that at White Rock is found in the and oceans in the ~artianpast is one of the crater Becquerel; it may also have a lacus- most intriguing and controversial topics in BACKGROUND trine origin. Lee (1993) acknowledged the modem planetology (see Baker, 1991; Carr, Several possible small ocean basins and possible lacustrine origin of White Rock and 1991; Baker et al., 1991). The creation of other probable lacustrine sites have been identified other possible lacustrine deposits. large standing bodies of water reflects the identified on Mars (Parker et al., 1989; For- Additional evidence of evaporites in the prevailing nature and change of the paleo- sythe, 1990; Baker et al., 1991; De Hon and Martian surface environment comes from climate and may also have implications for Pani, 1992; Scott et al., 1992, Farmer et al., Viking lander X-ray fluorescence meas- exobiology (Clark, 1978; Clark and Van 1993; and others). We propose as a strong urements of considerable quantities of sul- Hart, 1981). candidate the unusual feature called "White fates and chlorides in the Martian soil (Banin Lacustrine deposits can be difficult to Rock," an eroded crater interior deposit et al., 1992; and others). Thermodynamic identify remotely. One approach is chemi- (Fig. 1). Its unusual shape and albedo con- considerations and other Viking lander ob- cal, based on evaporites having distinctive trast drew early attention (McCauley, 1974), servations suggest that magnesium sulfate reflection spectra. Previous Earth-based and although care has been urged in relylng on and sodium chloride would be the dominant spacecraft remote sensing (summarized in the albedo contrast to be important in inter- evaporites on Mars and that carbonates Soderblom, 1992) and Viking lander obser- preting White Rock (Evans, 1979). Regard- would be very rare, with the possible excep- vations (summarized by Banin et al., 1992) less of albedo contrast, the morphology of tion of siderite (Clark and Van Hart, 1981). have set limits on the globally averaged White Rock is that of a weakly indurated, Local conditions may let other chlorides and abundance of some sulfates, chlorides, and fine-grained deposit that has not undergone sulfates (including gypsum) precipitate in carbonates. Another approach uses the tra- any apparent postdepositional erosion by large quantities. ditional elements of photo interpretation: large-scale mass wasting, fluvial, and/or gia- location, shape, and association. Lakes cial action, but has been subjected to exten- ROCK formed in (what were then) topographic sive wind erosion (Ward, 1979). Ward (1979) A detailed examination of a newly proc- lows, and are commonly found associated also discussed the paleoclimatic rdca- essed stereograrn of the best Viking images with scour erosion or other evidence of wa- tions of White Rock being a lacustrine de- of White Rock (Fig. 2) and Viking images of ter impoundment and identifiable inlet posit and critiqued alternative formation and the surrounding terrain were the basis of this and/or outlet channels. Other morphologic evolution mechanisms for White Rock, sum- study. White Rock has a teardrop shape, in features diagnosticof standingwater include marized as follows. White Rock is not ice, general; its 15-km-long axis of symmetry is beach ridges, wavecut notches, tombolos because the surface temperatures do not al- oriented northwest-southeast. The width at and other sand spits and bars, and hanging low it and because no changes have been the broad, northwest end is 12.5 km, and it deltas. Lacustrine sediments are fine grained observed in White Rock over several years is characterized by several deep reentrants and typically are poorly indurated, and so of observation. There are no volcanic fea- in the tapered end that run parallel to the they erode into distinctive shapes and pro- tures in the vicinity of White Rock, so it is axis. A secondary set of grooves, perpen- duce distinctive deposits (Mabbutt, 1977). unlikely that it is an eroded remnant of an dicular to the deep reentrants, is best ex- The biggest obstacle to the use of morphol- ignirnbrite deposit that was once more exposed at the broad, thick end of the deposit. ogy to identify lacustrine deposits is that tensive. Nor is White Rock likely to be a The orientation of reentrants and grooves most features indicative of standing water wall landslide deposit, because that would suggests that some sort of rectangular joint have avery low preservation potential. Nev- fail to account for the albedo contrast be- system exists in the White Rock deposit ertheless, several possible shoreline and tween White Rock and its surroundings. along which preferential eolian erosion oc-

GEOLOGY, v. 22, p. 107-110, February 1994 ngun 1. Whb Rock (~ITUW) I8 located In unnvnrd cntrr 8ouUmst of Schbpurlli originaIlyafillingthe basin had the initial sa- Impact kdnIn Slnua Ikkrur mgkn of Mar8 (M -8". Ion# 99Sq. Othw MtM In vicinity y3pwr linity of terrestrial sea water (-3.5 %), then ~to~lntrrkrrdknd.pwlt.,knmorphologyotWhbRock~rlkdo~~2600km30fwaterwouldberequired to yield k of oc#1.r of it md Itr rurroundlngr .n uniqw. Shown morrlc Vlklng photograph8 northmd 40 km3 to cornor of MC-20 NE quadnngk. north I8 up of evaporites, equivalent filling the impact basin to a depth of -700 m. This depth should be considered a lower bound because the presentday White Rock ap curs. Several fragments of the White Rack try of the thickness of the White Rock de- pears to be the erosional remnant of a pre- deposit have fallen from its thick upwind posit. The stereogram was caliited as fol- viously more extensive deposit. The upper edge into locations where they have been lows. The 1.3-km-diameter crater located bound for evaporites required would have protected from subsequent destruction by due south of the tapered tip of White Rock filled the entire basin to a depth of -500 m, eolian erosion. No fragments are present was obsewed to have a parallax of -3 pixels comparableto the maximum height of White along the flanks of the tapered margin; pre- between the rim of the small crater and its Rock. The evaporite volume in that case is sumably, saltation enhanced by the deflee floor. If that crater is assumed to be typical 1900 km3, requiring 130000 km3 of terres- tion of local air flow results in relatively of Martian bawl-shaped craters, then it trial seawater equivalent, correspondingto a rapid destruction by eolian erosion. Trans- would have a depth of -270 m (Pike, 1980). depth in the basin of an impos.9i'ble 34 km. verse grooves found on the broad part of Therefore, each pixel of parallax represents Waters of sigdkantly higher salinity than White Rack occur only in its upper -360 m. -90 m in relative elevation. By that scale, terrestrial seawater would reduce the water The new high-resolution stereagram the thickest part of White Rock lies -540 m requhements somewhat, but not sdciently (Fig. 2) allows estimation by stereogmme- above the plains to the north and east, and to allow for an extensive White Rock de

GEOLOGY, February 1994 posit to have formed from the evaporation of a single filling of the basin with saline water. Terrestrial analogy raises the intriguing possibility that the large volume of water required for White Rock indicates that the impact basin was, at least temporarily, an open hydrologic system. Saline waters were added continually to the basin at a rate that matched approximately the evaporation loss. Dense brines that formed at the surface by evaporation would sink and precipitate salts and gypsum. When the in- put of new saline waters ceased, the body of water dried up, leaving behind one last deposit of evap&rites. The crude zonation thus produced may be responsible for the difference in the style of erosion observed on the transverse ridges on White Rock. Wind-eroded deposits in the interior of craters are common on Mars, yet the morphology and albedo contrast of White Rock are unique. It is therefore unlikely that any widespread geologic process was responsible for its formation, unless some sort of special local conditions were involved to make the White Rock environment different from that in other impact basins. Such a con- dition would be met if there were a local de- Fiaure 3. Thin. discontinuous line larrowsl of hlah-albedo material Is visible in wall of posit of older White Rock-type material that l&ct basin &ntalnlng White ROC^ whlchlles jtkl out of Image area, to nolth. Une may was available for incorporation into the represent at leasl part of origlnal source of White Rock material. Thls Viking orbiter frame, 826A38, has spatial resolution of 29 mlpixel; scale bar = 5 km. fonning White Rock deposit. Close examination of the wall of the crater containing White Rock reveals a thin, discontinuous layer of high-albedo material that may have high in the San Andres Mountains under- National Monument is famous. The signifi- been the source of, or at least contributed to, goes erosion by rainfall and running water cance of White Sands being an analog to the White Rock material (Fig. 3). If that is and is carried in solution to Lake Lucero, a White Rock arises from the potential of the the case, then White Rock may be grossly playa where it recrystallizes. The wind then original source of the White Rock material similar to White Sands, New Mexico, and remobilizes the gypsum, creating the bar- predating the crater that contains White the surrounding area. There, gypsum in beds chanoid ridges for which the White Sands Rock. If the analogy is apt, then conditions

GEOLOGY,February 1994 that allowed at least the local formation of Baker, V.R., Strom, R.G., Gulick, V.C., Kargel, how wet?: Lunar and Planetary Institute evaporite-type deposits prevailed in the J.S., Komatsu, G., and Kale, V.S., 1991, An- Technical Report 9343, part 1, p. 17. early part of Martian geologic history, a fact cient oceans, ice sheets and the hydrological Mabbutt, J.A., 1977, Desert landforms: Cam- cycle on Mars: Nature, v. 352, p. 589-594. bridge, MIT Press, 340 p. that can be used to constrain models of Mar- Banin, A., Clark, B.C., and WWe, H., 1992, McCauley, J.F., 1974, White Rock: A Martian tian geology and climate history. If the high- Surface chemistry and mineralogy, in Kief- enigma: National Aeronautics and Space albedo layer is part of a regional veneer, fer, H.H., et al., eds., Mars: Tucson, Uni- Administration Special Publication 320, postdating the basin, then it and White Rock versity of Arizona Press, p. 594-625. p. 170-171. Carr, M.H., 1991, The ancient ocean hypothesis: Parker, T.J., Saunders, R.S., and Schneeberger, can be much younger. White Rock is too A critique: Division of Planetary Sciences of D.M., 1989, Transitionalmorphology in west small for accurate dating by impact crater the American Astronomical Society, 23rd Deutemnilus Mensae, Mars: Implications population, but should have collected a few Annual Meeting, Palo Alto, California, Ab- for modification of the IowlandEupland craters if it had been exposed in its present stracts, p. 99. boundary: Icarus, v. 82, p. 111-145. form for an appreciable part of Martian Clark, B.C., 1978, Implications of abundant hy- Parker, T.J., Gorsline, D.S., Saunders, R.S., groscopic minerals in the Martian regolith: Pieri, D.C., and Schneebemr, D.M., 1993, history. I-, V. 34, p. 645-665. Coital g&morphologyof tie mart&north- Many lacustrine sediments have distinc- Clark, B.C., and Van Hart, D.C., 1981, The salts ern lai ins: Journal of Geo~h~Sid. - Research. tive infrared signatures and display numer- of Mars: Icarus, v. 45, p. 370-378. v. 98, p. 11,061-11,078. ous diagnostic small-scale features. The loss De Hon, R.A., and Pani, E.A., 1992, Flood surge Pike, R.J., 1980, Control of crater morphology by through the Lunae Planum outflow complei gravity and target type: Mars, Earth, Moon: of Mars Observer is sorely felt; the optical Mars: Lunar and Planetaw Science Proceed- Lunar and Planetary Science Pmcedngs, camera and the thermal emission spectrom- ings, v. 22, p. 63-71. V. 11, p. 2159-2189. eter would readily have been able to detect Edgett, K.S., and Christensen, P.R., 1991, The Scott, D.H., Chapman, M.G., Rice, J.W., and Martian lacustrine sites and materials, the particle size of martian aeolian dunes: Jour- Dohm, J.M., 1992, New evidence of lacus- gamma-ray spectrometer could have de- nal of Geophysical Research, v. %, trine basins on Mars: Amazonis and Utopia p. 22,765-~,n6. Planitia: Lunar and Planetary Science Pro- tected near-surface ice, and the laser altim- Evans, N., 1979, White Rock-An illusion?: Na- ceedings, v. 22, p. 53- 62. eter would have provided much better re- tional Aeronautics and Space Administration Soderblom, L.A., 1992, The composition and gional topographic control than is currently Technical Memorandum 80339, p. 28-30. mineralogy of the martian surface from spec- available. The need for improved Martian Fanner, J., Des Marais, D., Greeley, R., Land- troscopic observations: 0.3 pm to 50 pm, in heim, R., and Klein, H., 1994, Site selection Kieffer, H.H., et al., eds., Mars: Tucson, data remains unchanged, as does the ratio- for Mars exobiology: World Space Congress, University of Arizona Press, p. 557-593. nale for site targeting. We hope that future 29th COSPAR Meeting, Fkcmbgs, Ad- Ward, A.W., 1979, Yardangs on Mars: Evidence missions to Mars can soon help identify any vances in Space Research, v. 13/14(in press). of recent wind erosion: Journal of Geophys- Martian lacustrine materials and resolve the Forsythe, R.D., 1990, A case for martian salars ical Research, v. 84, p. 8147-8166. mystery that is White Rock. and saline lakes during the Noachian: Lunar and Planetary Science Conference, 21st, Manuscript received July 19,1993 Houston, Texas, Abstracts, p. 379-380. Revised manuscript received Nwember 4, 1993 ACKNO~MENTS Greeley, R., Skypeck, A, and Pollack, J.B., Manuscript accepted Nwernber 8,1993 This work was supported in part by the Plane- 1993, Martian aeolian features and deposits: tary Geology and Geophysics Program, National Comparisons with general circulation model Aeronautics and Space Administration. We thank results: Journal of Geophysical Research, the computer facility sm, in particular, Brian v. 98, p. 31S31%. Fessler, at the Lunar and Planetary Institute, led Lee, P., 1993, Briny lakes on Mars? Terrestrial by Kin Leung, for their assistance in processing intracrater playas and martian candidates the Viking mosaics. [abs.], in Squyres, S., and Kasting, J., eds., Workshop on early Mars: How warm and REFERENCES CmED Baker, V.R., 1991, Ancient oceans on Mars: Di- vision of Planetary Sciences of the American Astronomical Society, 23rd Annual Meeting, Palo Alto, California, Abstracts, p. 99.

Rinted in U.S.A. GEOLOGY, February 1994