Å·ÃÀ¿Ú±¬ÊÓƵ

Hoke, Monica R. T. ¹ ; Hynek, Brian M. ²

¹ University of Å·ÃÀ¿Ú±¬ÊÓƵ, Dept. of Astrophysical and Planetary Sciences, LASP
² University of Å·ÃÀ¿Ú±¬ÊÓƵ, Dept. of Geological Sciences, LASP

Introduction: Valley networks (incised valleys thought to be akin to dried up river beds) provide a record for the location, quantity, source, and timing of flowing water on the surface of Mars. Analysis of valley network geomorphology, stream order, and drainage density helps to describe the form and extent of fluvial erosion during early martian history. Determining the ages of individual networks that appear to have formed primarily by precipitation and surface runoff through crater density analysis provides constraints on the timing and duration of warmer, thicker atmospheric conditions.

Method: Fourteen of the largest valley networks in the equatorial regions of Mars were mapped and analyzed using coregistered Thermal Emission Imaging System (THEMIS) daytime IR and Mars Orbiter Laser Altimeter (MOLA) data in ArcGIS. Drainage area was determined using a convex hull method that connects the outermost tributaries rather than drainage divides due to the ~3.5 Byr of post-formation impacts and geologic processes that may have erased small tributaries and altered the location of drainage divides. The density of impact craters on a planetary surface has been commonly used to estimate the ages of formation on the assumption that older surfaces have a greater number of craters. The network areas were individually crater-age dated following the method outlined by Tanaka [1982]. Craters 1 km in dia. and larger superposed on the valleys were counted and age estimations were calculated.

Results and Discussion:The results from this research place the end of precipitation-driven formation of these valley networks in the Late Noachian and earliest Hesperian epochs (~3.6 to 3.8 Bya). The difference in age between the oldest and youngest analyzed networks is 210±50 Myr. Within this range are networks that have distinctly separate ages and those that appear coeval. Both the morphologies and crater ages of these valley networks indicate some valleys experienced multiple periods of formation as regions of precipitation moved around, sometimes returning to previously rainy regions. The surprising lack of older networks could indicate valley formation in these regions did not occur earlier in time.

Regional variations in rainfall on Mars is be expected for periods of warmer, wetter climate [Colaprete, 2004], driven by changes in topography and relative locations of bodies of water, changes in atmospheric characteristics, and/or changes in orbit and obliquity. This variability in precipitation is especially important for a hypothetical early Mars without a large ocean to constantly supply water to the same region.

The timing of valley network formation on ancient Mars appears to coincide well with the accumulation of CO2 and H2O from Tharsis outgassing [e.g. Phillips et al., 2001], providing a possible explanation for the climate change necessary for stable liquid water on the surface of ancient Mars and the formation of dense valley networks.

Colaprete, A. et al., 2004, The effect of impacts on the early martian climate: Second Conference on Early Mars, abstract 8016.

Fassett, C. and J. Head, 2008, The timing of martian valley network activity: Constraints from buffered crater counting: Icarus, v. 195.

Phillips, R. et al., 2001, Ancient geodynamics and global-scale hydrology on Mars: Science, v. 291, p. 2587-2591.

Tanaka K., 1982, A new time-saving crater-counting technique, with application to narrow features: NASA Tech. Memo. 85127, p. 123-125.