Proceedings of the National Acacdemy of Sciences: PNAS 2002 99: 1780-1783.

Oded Aharonson, Maria T. Zuber, Daniel H. Rothman, Norbert Schorghofer, and Kelin X. Whipple

Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139
Communicated by Sean C. Solomon, Carnegie Institution of Washington, Washington, DC, December 27, 2001 (received for review November 15, 2001)

Measurements acquired by the Mars Orbiter Laser Altimeter on
board the Mars Global Surveyor indicate that large drainage
systems on Mars have geomorphic characteristics inconsistent with
prolonged erosion by surface runoff. We find the topography has
not evolved to an expected equilibrium terrain form, even in areas
where runoff incision has been previously interpreted. By analogy
with terrestrial examples, groundwater sapping may have played
an important role in the incision. Longitudinally flat floor segments
may provide a direct indication of lithologic layers in the bedrock,
altering subsurface hydrology. However, it is unlikely that floor
levels are entirely due to inherited structures due to their planar
cross-cutting relations. These conclusions are based on previously
unavailable observations, including extensive piece-wise linear
longitudinal profiles, frequent knickpoints, hanging valleys, and
small basin concavity exponents.

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