Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..402k&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
Introduction The Medusae Fossae Formation (MFF) is a complicated and discontinuous formation located in the southern parts of Elysium Planitia and Amazonis Planitia (130°-230°E and 12°S-12°N), covering an area of approximately 2.1 x 106 km2 and having an estimated volume of 1.4 x 106 km3 [1]. It is thought to have been deposited during the Amazonian period [2,3]. However, much of the cratering record may have been erased as friable units were eroded and long-buried terrains exhumed [4-6]. The formation is characterized by large accumulations of fine-grained, friable deposits and evidence of large amounts of erosion. There are many theories regarding the emplacement of this formation; recently the literature has focused on three possibilities: ignimbrites, ash fall, and aeolian dust. Some modified and inverted fluvial channels have been found within the deposit [7,8], (Fig. 1), indicating that there was some fluvial activity during or after the emplacement of the MFF. If the MFF is among the youngest surficial deposits on Mars [9], it is implied that meandering, channelized flow must have extended into the Amazonian, a significant constraint when considering the atmospheric evolution of the planet through time. Because of the wide implications that these findings have for the evolution of Mars and the Martian atmosphere, it is instructive to re-examine the evidence for the Amazonian age of the MFF. The initial conclusion comes from two main arguments: the relatively few superposed craters on the unit, and the superposition of the MFF on young lowland lava deposits [1, 9]. Using new high resolution data, we reexamine the relationships both within the MFF and with respect to adjacent units. Cratering Record The cratering record of the MFF and other easily eroded units has often been deemed unreliable [4, 10, 12], but it continues to be cited as evidence for the formation's young age. Throughout the MFF, pedestal craters, inverted craters, and remnant knobs can be found which trace the progression of crater erosion and suggest that a large number of craters originally superposed on the unit have been erased or degraded beyond recognition (Fig. 2). Stratigraphy Because lava units are emplaced in a geologically short period of time, cease to evolve after emplacement, and are relatively resistant to erosion, they are useful age markers for adjacent units such as the MFF which are more difficult to date. Early descriptions noted that lowland lava flows embay the formation in places [12], but later works have placed the MFF at the top of the stratigraphic column because it overlies lowland lavas [9]. We have observed that Cerberus lavas embay the formation in Northern Aeolis Planum and Zephyria Planum, though eroded Medusae Fossae Formation has been blown out onto the lava where it collects as sand dunes. In other places direct contacts are lacking, but ancient contacts may be recognized where lava units embayed the MFF after which the MFF was eroded from these areas, leaving irregular depressions and negative yardang patterns in the embaying lava units (Fig. 3). These boundaries are found in most places where there is a contact with a lava unit, including northwestern Aeolis Planum, north and east of Eumenides Dorsum, and in north-eastern Gordii Dorsum. In between Eumenides Dorsum and Amazonis Planum, several lobes of lava from Arsia Mons, mapped as Late Hesperian/Early Amazonian (AHt3) by [2], end abruptly in a negative yardang pattern, in close proximity to an eroding patch of MFF. Additionally, the large fan on the southeastern flank of Apollinaris Patera appears to embay the MFF yardangs at its base (Fig. 4). Discussion The Medusae Fossae Formation is a mobile unit which both erases craters on its own surface as well as sheltering nearby units from incoming projectiles, resulting in a misleadingly young age. The MFF appears to continuously erode into yardangs and reaccumulate as dunes and massive deposits. If it comes to a rest, the unconsolidated material gradually becomes indurated (perhaps through compression or interaction with the atmosphere) until it reaches a point where it can begin eroding into yardangs again. Lava flows, which remain relatively stable through time, make useful chronological markers for where the unit is and where it used to be. Recognition of remnant contacts between MFF and lava units is helpful in unravelling relationships between the MFF and lava units where direct contacts are not available. Lava unit contacts suggest that parts of the MFF may be older than previously hypothesized, perhaps Hesperian. This conclusion is consistent with the presence of fluvial channels within the deposit and relaxes the time constraint on its emplacement. References [1] Bradley, B.A. and Sakimoto, S.E.H. (2002) JGR, 107, E8. [2] Scott, D.H. and Tanaka, K.L. (1986) USGS Misc. Invest. Ser. Map I-1802-A. [3] Greeley, R. and Guest, J. (1987) USGS Misc. Inv. Series Map I-1802-B. [4] Schultz, P.H. and Lutz, A.B. (1988) Icarus 73, 91-141 [5] Schultz, P.H. (2006) Plan. Chron. Workshop, Abs. 6024. [6] Schultz, P.H. (2007) Science 318, 1080-1081. [7] Zimbelman, J.R. (2000) GSA Abs. Prog., 32(7), A303. [8] Edgett K.S. and Williams R.M.E. (2006) LPSC XXXVI, Abs. 1099. [9] Bradley, B.A. and Sakimoto, S.E.H. (2001) LPSC XXXII, Abs. 1335. [10] Tanaka, K.L. (2000) Icarus 144(2), 254-266. [11] Greeley et al. (2001) Space Sci. Rev. 96(1-4), 393-404. [12] Scott, D.H. and Tanaka, K.L. (1982) JGR 87(B2), 1179-1190.
Head James W.
Kerber Laura
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