Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..310s&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 Gullies are considered among the youngest geomorphic features on Mars based upon their stratigraphic relationships, superposition on steep slopes and distinctive morphology in unconsolidated sediment. Multiple formation hypotheses have been proposed, which can be divided into three broad classes: entirely dry mechanisms (e.g., [1,2]), wet mechanisms invoking groundwater or ground ice (e.g., [3,4]) and wet mechanisms invoking surficial meltwater (e.g., [5,6,7,8]). It has been difficult to differentiate between these hypotheses based upon past observations and it remains possible that gullies are polygenetic landforms. This study presents stratigraphic relationships in the depositional fan of a crater wall gully system that suggest: (1) multiple episodes of alluvial fan-style deposition, (2) very recent depositional activity that is younger than a newly recognized rayed crater, and (3) surficial snowmelt as the most likely source of these multiple episodes of recent gully activity. Gully-Fan Stratigraphy In Eastern Promethei Terra an ~5 km-diameter crater is observed with a well-developed gully system (Fig. 1) and several smaller gullies in its northnortheast wall. The large gully system (composed of a small western gully and larger eastern gully) shows evidence for incision into the crater wall country rock and has multiple contributory sub-alcoves and channels. The depositional fan associated with this gully system is bounded on its western side by a small arcuate ridge swell that is not observed on the eastern side of the fan. This ridge is interpreted as a moraine-like structure that may have bounded a glacially-formed depression into which the fan is deposited [8]. Similar depressions with bounding ridges are commonly observed in this latitude band (~30-50°S) in association with deeply incised gully alcoves [9,10,11]. This gully fan is composed of multiple lobes with distinct lobe contacts, incised channels, and cut-andfill deposits - all features similar to those seen in terrestrial alluvial fans [12,13]. The western portion of the fan is contained within the depression, while the younger eastern portion overlies and obscures any potential evidence of the ridge structure. A very striking and unusual feature of this gully fan is the large number of superposed impact craters; due to their density and similar diameter, we interpret these to be secondary craters from a large nearby primary impact crater. The depositional lobes of the fan can be divided into two groups: 1) those that predate the secondary crater population and 2) younger lobes that are superposed on the secondary craters. Numerous secondary craters (~1-25 m-diameter) superpose the lowermost stratigraphic lobe (Fig. 1, A), while at least three younger lobes (Fig. 1, C1, D1, and D2) directly superpose the cratered lobe. The emplacement date of these secondaries provides a robust maximum age for the youngest lobes of this fan, and therefore the most recent fluvial activity of the gully. Most gullies either have no superposed impact craters [3,7] or are too small to date with any certainty using crater counts [14]. Therefore, locating and dating the parent impact crater of these secondaries is critical to constrain the chronology and origin of gully systems. Rayed-Crater Source of the Secondary Craters Regional reconnaissance for the origin of the secondary craters led to the discovery of a previously unidentified rayed crater complex (consisting of an ~18 km-diameter outer crater and an ~7 km-diameter inner crater) approximately 175 km southwest of the gully system. Distinctive rays are observed in THEMIS nighttime thermal inertia data, but are not observable as albedo contrasts in THEMIS visible data, consistent with other identifications of young rayed craters on Mars [15,16]. The rims of both craters are distinct and consistent with the morphology of very young impact craters on Mars. The inner crater has a greater depth to diameter ratio than the outer crater (0.121 compared to 0.073), consistent with young Martian craters [17]. Both the outer and inner craters have classically-defined gullies, preferentially developed on their pole-facing walls. Polygons are observed in gully alcoves of the outer crater, but not in alcoves of the inner crater, implying a difference in substrate or thermal cycling time [18]. The outer crater is floored by ejecta from the inner crater and mantling deposits. There is no evidence of an underlying concentric crater fill deposit or other altered fill unit typical of older Amazonian altered craters [19]. The inner crater is floored by unconsolidated sediment and contains a small collection of dunes. No evidence of pits, hummocky texture or other sublimation features are observed indicating that the crater interior is not a periglacial terrain. We interpret the inner crater as younger than the most recent episode of mantling deposition (~0.4Ma) [20] due to the exposed spur and talus slope development on the equator-facing wall, a slope and orientation that preferentially preserves smooth mantle texture in this latitude regime [21]. One superposed crater (~45 mdiameter) is observed in HiRISE coverage. Using the technique of Hartmann and Quantin-Nataf [22], who dated Gratteri crater by counting small craters superposed on the floor, the inner crater is on the order of 100Ka. Based upon these observations and the relative proximity of secondary craters to the outer crater rim (making it unlikely they originated from the outer crater), the 7 km-diameter inner crater is the likely source of the rays and secondary craters of interest on the gully fan lobe. Acknowledgments: Special thanks to the Mars Recognisance Orbiter and HiRISE teams as well as the Odyssey and THEMIS teams. This research was funded by NASA. Conclusions This study has identified a gully system fan in Eastern Promethei Terra with morphology requiring multiple periods of activity for its construction. At least one lobe of the fan has retained a dense secondary crater population, while at least two episodes of activity post-date emplacement of the secondary craters. Approximately 175 km to the southwest, the likely parent rayed crater was discovered using THEMIS thermal inertia data. This 7 km-diameter crater is located within a morphologically older 18 km-diameter crater. We interpret the source crater as younger than the most recent obliquity-controlled glacial period (~0.4Ma), which is consistent with crater age dating of the floor as well. The multiple episodes of alluvial fan activity mapped in this study imply that gullies are not catastrophic landforms that formed in single events. Rather, multiple episodes of fluvial activity in the gully system are required to deposit and rework the alluvial fan that is observed. The alluvial fan morphology [10, 11] and sedimentary channel structures make dry mass-wasting processes implausible for the formation of this gully system. The multiple episodes of activity required by the fan stratigraphy documented here cast serious doubt on catastrophic groundwater discharge scenarios that are unlikely to generate episodic releases. Small amounts of surficial meltwater derived from snow and ice accumulation is suggested by the insolation geometries of gully systems and most plausibly can account for multiple periods of recent (<0.4Ma) activity required by these observations. This chronology is consistent with other evidence [11] that places gully formation in the waning stages of the ice ages that produced the latiduedependent mantles. References [1] Treiman, A. (2003) JGR 108, doi: 10.1029/2002JE001900. [2] Shinbrot, T. et al. (2004) PNAS 101, doi: 10.1073/mnas.03082511 01. [3] Malin, M. and Edgett, K. (2000) Science 288, doi: 10.1126/ science.288.5475.2330. [4] Heldmann, J. et al. (2007) Icarus 188, doi: 10.1016/j.icarus.2006.12.010. [5] Costard, F. et al. (2001) Science 295, doi: 10.1126/science.1066698. [6] Christensen, P. (2003) Nature 422, doi: 10.1038/nature01436. [7] Dickson, J. et al. (2007) Icarus 188, doi: 10/1016/j.icarus.2006.11.020. [8] Head, J. et al. (2008) Workshop on
Fassett Caleb
Head James
Schön Samuel
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