A novel, automated technique for delineating Martian
valley networks from digital terrain data is applied to the
Mare Tyrrhenum quadrangle on Mars, yielding a detailed
map for the entire quadrangle. The resultant average value
of drainage density for the Noachian part of the quadrangle
is D 0.05 km 1, an order of magnitude higher than the
value inferred from a global map based on Viking images,
and comparable to the values inferred from the precision
mapping of selected focus sites. Valleys are omnipresent in
Noachian terrain even outside the ‘‘highly dissected’’ Npld
unit. This suggests fluvial erosion throughout the Noachian,
implying widespread precipitation. The map of continuous
drainage density is constructed to study spatial variations of
D. This map reveals significant variations in degree of
dissection in Noachian on scale of >100 km. These
variations do not correlate with any terrain parameter and
their origin requires further study
