Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p22a..02d&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P22A-02
Other
[5415] Planetary Sciences: Solid Surface Planets / Erosion And Weathering, [5419] Planetary Sciences: Solid Surface Planets / Hydrology And Fluvial Processes
Scientific paper
New bright gully deposits were observed on Mars in 2006 [1], and several similar deposits were reported in 2007 [2]. As gullies may have formed due to the action of liquid water, these new deposits are of considerable interest. Images from HiRISE have shown many more fresh-appearing deposits associated with gullies. In order to better understand recent gully activity, we have searched >1400 HiRISE images for fresh deposits, focusing on the southern mid-latitudes; we exclude dune gullies, which are discussed elsewhere [3]. Very fresh deposits (distinct from other gully deposits in tone and without obvious modification such as development of eolian ripples) have so far been found at almost 50 sites between 29 and 55 S. Those poleward of 40 S latitude show no obvious orientation preference. Equatorward of 40 S, recent deposits show a strong tendency to face the pole, consistent with typical gully orientations at this latitude. Mass wasting should not show orientation preferences unless slopes are systematically steeper in one direction. In fact, steep slopes in the southern mid-latitudes are more likely to face the equator [4], but gullies might locally provide very steep slopes prone to failure. We have noted ten sites where the formation time can be constrained by a preexisting image without the deposit, including the two reported by [1]. Seven of these are on pole-facing slopes. Formation times for eight of these deposits are known to better than one Mars year. These formation intervals tend to exclude late spring and summer; this is consistent with the season reported for dune gully activity [3], but may be affected by non-random image times. Many of the observed new deposits could be consistent with thin coatings, but two deposits in Gasa Crater (35.7 S, 129.4 E) have before-and-after HiRISE coverage showing changes on fans and the distal part of the channels. These include movement of boulders and visible topographic changes, indicating that those deposits are more than thin veneers. This may indicate that gullies are forming today rather than being relict features from a different climate with only superficial recent modifications. These observations are consistent with a model in which current gully activity is driven by CO2 frost causing mobilization of surface material in winter [e.g. 5]. Seasonal CO2 frost is found on pole-facing slopes at relatively low latitudes, and a broader range of orientations closer to the pole. As some of the fresh deposits are found on equator-facing slopes where frost is unlikely, dry granular flow is also possible [6]. These features are not uncommon, and so indicate a significant component of recent gully evolution, in an epoch in which surface liquid water is very unlikely. While these observations do not exclude a role for liquid water in the initiation and erosion of gullies, we suggest further investigation of the role of CO2-driven or other current seasonal processes in gully formation and evolution. [1] Malin et al. (2006) Science 314, 1573-1577. [2] McEwen et al. (2007) Science 317, 1706-1709. [3] Diniega et al. (2009, this conference). [4] Kreslavsky and Head (2003) GRL 30, doi: 10.1029/2003GL017795. [5] Ishii and Sasaki (2004) LPSC XXXV, abstract 1556. [6] Pelletier et al. (2008) Geology 36, 211-214.
Byrne Shane
Diniega Serina
Dundas Colin Morrisey
McEwen Alfred S.
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