EARLY HIRISE OBSERVATIONS OF ATHABASCA VALLES, MARS: A VOLCANIC LANDSCAPE

Martian ‘outflow' channel systems are broad, deep and commonly anastomotic valleys that emerge full-born from localized sources and decrease in depth distally [1]. They are morphologically similar to terrestrial catastrophic flood channels (e.g., the Channeled Scabland), and are widely believed to have formed by analogous processes. Athabasca Valles (~9°N, 156°E) is the youngest outflow channel system on Mars [2-3]. The paucity of impact craters on its floor indicates a surface age of 1.5-200 Ma [4], which suggests it was geologically active during the most recent epoch of Mars' history, the Late Amazonian. Fine-scale morphology is better preserved at Athabasca Valles than at other outflow channels. Previous studies have reported transverse dunes (purportedly of subaqueous origin) and longitudinal lineations (reputedly scoured by longitudinal vortices in turbulent floodwaters) within Athabasca Valles [e.g., 3].

The High Resolution Imaging Science Experiment (HiRISE) camera, onboard the Mars Reconnaissance Orbiter spacecraft, has acquired dozens of images of Athabasca Valles, many as stereo pairs. These images have excellent signal-to-noise ratios and pixel dimensions as small as 27 cm. The HiRISE data show that Athabasca Valles is a lava-draped channel system. Low-viscosity lava first flooded its channels, then drained downstream, deflating and draping the banks and floor of Athabasca Valles. The aforementioned transverse dunes exhibit lava surface textures rather than rounded boulders; the longitudinal lineations are terraces draped by lava, which grade into chains of hydrovolcanic (rootless) cones and craters. These hydrovolcanic constructs formed after deflation reduced the confining pressure enough for steam to vent through the lava. The available HiRISE data show no unambiguous channel erosion features that are unmodified by volcanism. Even the inferred high-stand of the floodwaters is demarcated by lobate lava flow margins. This suggests that either a bank-full lava flow overprinted the flood features or Athabasca Valles was carved by lava. We continue to investigate this conundrum.

[1] Sharp R. P. and Malin M. C. (1975) GSA Bull. 86, 593-609. [2] Tanaka K. L. and Scott D. H. (1986) LPS XVII, 865-866. [3] Burr D. M. et al. (2002) Icarus 159, 53-73. [4] McEwen A. S. et al. (2005) Icarus 176, 351-381.