Lunar and Planetary Science XXIX 1085.pdf PHOTOGEOLOGIC CONSTRAINTS ON THE EMPLACEMENT OF THE MEDUSAE FOSSAE FORMATION, MARS. J.R. Zimbelman1, A.K. Johnston1, and S.D. Patel2; 1CEPS/NASM MRC 315, Smithsonian Institution, Washington, D.C. 20560, [email protected]; 2Thomas Jefferson H.S. for Science and Tech., 6560 Braddock Rd., Alexandria, VA 22312. Geologic mapping of the Medusae Fossae Formation This work was supported by NASA grant NAGW- (MFF) has been underway over the past few years to 1390 to JRZ. SDP participated in this effort through the assess the numerous published hypotheses of origin for Mentorship Program of TJHSST. these enigmatic deposits [1]. The mapping effort has concentrated on three 1:500,000-scale Mars Transverse [1] Zimbelman, J.R., 1995, An examination of Mercator (MTM) quadrangles where there is good hypotheses of formation for the enigmatic massive contiguous coverage of Viking Survey mission images deposits in Amazonis Planitia, Mars, Lunar Planet. Sci. with 22 to 33 m/pixel resolution. Results from geologic XXVI , 1559-1560, Lunar and Planetary Institute, mapping of quadrangle MTM 05142 were presented at the Houston. GSA meeting in 1996 [2], and regional geologic mapping [2] Zimbelman, J.R., A.K. Johnston, and C.G. Lovett, of six 1:2,000,000-scale quadrangles covering much of the 1996, Geologic mapping of the Medusae Fossae MFF exposures was presented at the Fall AGU meeting in Formation within MTM quadrangle 05142 on Mars, Geol. 1997 [3]. Initial attempts at assessing the various Soc. Am. Abs. Prog. , 28(7) , A128. geologic hypotheses of origin were presented at the last [3] Zimbelman, J.R., A.K. Johnston, P.S. Russell, and two LPSCs [4,5]. Here we continue this on-going effort C.G. Lovett, 1997, Regional geologic setting of the to understand the MFF deposits by reporting results from Medusae Fossae Formation on Mars, Trans. Am. Geophys. our geologic mapping of MTM 00147, including some of Union , 78(46) , F411. our findings from the regional mapping mentioned earlier. All of these geologic observations will be used to assess [4] Zimbelman, J., D. Crown and D. Jenson, 1996, further the relative strengths and weaknesses of the Initial investigation of the enigmatic massive deposits in various hypotheses of origin [e.g., 5], to be reported at a Amazonis Planitia, Mars, Lunar Planet. Sci. XXVII , 1495- later date. 1496, Lunar and Planetary Institute, Houston. Our mapping to date has revealed that the global-scale [5] Zimbelman, J., D. Crown, J. Grant and D. Hooper, MFF units, labeled at the upper, middle, and lower 1997, The Medusae Fossae Formation, Amazonis Planitia, members of MFF [6], can be further subdivided when Mars: Evaluation of proposed hypotheses of origin, Lunar mapped at the 1:500,000 scale. In particular, both the Planet. Sci. XXVIII , 1623-1624, Lunar and Planetary upper and middle members can be divided into three units Institute, Houston. with clear stratigraphic superposition relations [3,4, this [6] Scott, D.H. and K.L. Tanaka, 1986, Geologic map of work]. The upper surface of most of these subunits appears the western equatorial region of Mars, U.S. Geol. Surv. to be relatively strongly indurated, forming a caprock to Misc. Invest. M ap I-1802-A , scale 1:15,000,000. the more friable underlying materials. Erosional patterns [7] Mars Orbital Laser Altimeter Press Release on the unit surfaces that comprise the upper member imply information from the first aerobraking pause. Groundtrack erosional winds from differing directions, suggestive of a locations available on the Web at: geologic record of changing climatic wind regimes [7], http://ltpwww.gsfc.nasa.gov/tharsis/mola.html and all of the subunit surfaces display distinct yardang [8] Mars Orbital Camera Press Release MOC2-19A trends that often can be correlated over distances of and -19B, showing a portion of the Medusae Fossae several hundred km. Regional mapping indicates that the Formation at very high resolution. Available on the Web subdivision of the global units also likely includes the at: http://www.msss.com/mars/global_surveyor/ lower member of MFF, which is concentrated well west of MTM 00147 [3]. The on-going Mars Global Surveyor camera/images/12_31_97_release/3104/index.html (MGS) mission is already returning important new data that will likely have an important impact on interpretations of MFF, such as the new topographic profiles from the Mars Orbital Laser Altimeter (MOLA) [7] and very high resolution images of MFF taken by the Mars Orbital Camera (MOC) [8]. Once MGS reaches its final mapping orbit, now scheduled for March 1999, these new data will play a crucial role in evaluating the likely origin of the MFF materials. We conclude that the distinct physical properties displayed by various portions of the subunits, in addition to the possibility that sufficient time may have elapsed between subunits for major shifts to take place in erosive wind regimes, represent important new data that must be accommodated by any successful hypothesis of origin for MFF. In collaboration with D. Crown, J. Grant, and D. Hooper, these observations are being applied to the various hypotheses of origin, and we anticipate soon being able to present ‘truth tables’ of observable characteristics for each hypothesis..