Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..643d&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 While Amazonian fluvial landforms are not abundant on Mars, remote sensing data have revealed details regarding the role of ice in non-polar regions in the Amazonian. Evidence includes 1) deposits interpreted to be remnants of cold-based glaciers at low- and mid-latitudes [1-6]; 2) mantling deposits interpreted to be a desiccating layer of ground ice [7- 8]; 3) detection of hydrogen (inferred to be bound as water ice) in soil in the mid- and high-latitudes in each hemisphere [9-10]; and 4) viscous flow features interpreted to be the product of glacial-like flow along steep valley/crater walls [11-12]. The climate of Mars straddles the triple point, which motivated us to investigate the most-likely locations/microclimates for melting of these surface/near-surface ice features [13-14]: large-scale impact craters at low elevations and mid-latitudes, which provide 1) relatively high surface pressure; 2) increased solor insolation; and 3) potential residual thermal anomalies from the impact event. Lyot Crater, a ~215 km peak-ring impact basin in the northern lowlands of Mars (50°N, 30°E), provides an environment that meets these constraints. We analyzed recently obtained CTX data to document evidence of remnant glacial deposits and surface features that appear indicative of melting and drainage. Description The floor of Lyot exhibits several networks of sinuous valleys that have been incised exclusively into a pervasive stippled mantling unit (Fig. 1). Twenty separate networks are observed in CTX and THEMIS data, 15 of which occur in the eastern half of Lyot. The valleys range in length from short, 2 km long isolated valleys to 50 km long networks of multiple valleys that have widths that average ~250 m. Valley floors are smooth at CTX resolution, in contrast to the adjacent stippled mantling unit (Fig. 1). Profiles extracted from the Mars Orbiter Laser Altimeter (MOLA) data set show that, without exception, the valleys follow the local topographic gradient (Fig. 3). Regional slopes in the down-valley direction range from 0.36° to 6.12°, but most networks trend around the median for all valleys of 1.93°. Valleys start at a wide range of elevations, from ~-2883 m to ~-5684 m (mean = -3803.4 m). Valley walls appear uniformly fresh and no impact craters or ejecta blankets are observed on any of the valley floors (Fig. 1). Valleys emanate from the upslope margins of the stippled mantling unit along the crater rim and central peak ring and several terminate with depositional fans (Fig. 2). The valleys are superposed by the smoother mantling deposits observed on the flanks of isolated mesas, implying that valley formation occurred after the emplacement of the stippled mantling unit but before the deposition of the more-localized smooth mantling unit. Chronology Since we interpret the valleys as incising the stippled mantling unit, an accurate age for the stippled mantling unit provides a maximum age for valley formation. CTX imagery is the only data set that adequately resolves the stippled mantling unit in sufficient detail and spatial extent to perform accurate crater counts. Therefore we constrained our mapping of the unit to the three overlapping CTX frames in the eastern half of Lyot. We calculated the age for the stippled mantling unit using both the Neukum [15] and Hartmann [16] systems. In each system our counts yield a Middle Amazonian age, with a best-fit for our crater curve of ~1.5 Gyr in the Neukum [15] system and 0.78 Gyr in the Hartmann [16] system. This crater size-frequency determination is well-matched by production model isochrons and this young age is consistent with other stratigraphic constraints. Thus, we are confident that the valleys found in Lyot are Mid-Amazonian or younger. In either absolute age system, there appears to be a geologically significant (0.8 - 1.9 Gy) period of time between the formation of Lyot and the emplacement of the stippled mantling unit. Formation Numerical modelling has shown that Mars has undergone significant orbital excursions within the Amazonian, resulting in periods of high-obliquity [17]. At high obliquity, models predict an increase in peak surface temperature at the latitude of Lyot crater (50°N) [18-19]. For example, at 60° obliquity, Mischna et al. [18] found a maximum diurnallyaveraged temperature at 50°N at Ls=90 would be ~260°K, compared to ~220°K under current orbital conditions (obliquity = 25°), meaning that peak surface temperatures would be above 273°K for significant periods of time. Given these results and the high surface pressure at Lyot crater, surface conditions above the triple point of water are likely to have been achieved in the Middle- to Late-Amazonian at this location. Thus, insolation changes resulting from orbital variations is likely to have been the primary energy source for the melting of surface/nearsurface ice at Lyot. References [1] Squyres, S. (1978) Icarus, 34, 600-613. [2] Lucchitta, B. (1981) Icarus, 45, 264-303. [3] Head, J. and Marchant, D. (2003) Geology, 31, 641-644. [4] Pierce, T. and Crown, D. (2003) Icarus, 163, 46-65. [5] Head, J. et al. (2005) Nature, 434, 346-351. [6] Head, J. et al. (2006) EPSL, 241, 663-671. [7] Mustard, J. et al. (2001) Nature, 412, 411-414. [8] Head, J. et al. (2003) Nature, 426, 797-802. [9] Boynton, W. et al. (2002) Science, 297, 81-85. [10] Feldman, W. et al. (2002) Science, 297, 75-78. [11] Hartmann, W. et al. (2003) Icarus, 162, 259-277. [12] Milliken, R. et al. (2003) JGR, 108, 11-1. [13] Lobitz, B. et al. (2001) PNAS, 98, 2132-2137. [14] Haberle, R. et al. (2001) JGR, 106, 23317-23326. [15] Ivanov, B. (2001) Space Sci. Rev., 96, 87-104. [16] Hartmann, W. (2005) Icarus, 174, 294-320. [17] Laskar, J. (2004) Icarus, 170, 343-364. [18] Mischna, M. et al. (2003) JGR, 108, E6-5062. [19] Haberle, R. et al. (2003) Icarus, 161, 66-89.
Dickson James
Fassett Caleb
Head James
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