DMB
3/9/09
Summary of: Irwin, R.P. et al. (2008), Fluvial valley networks on Mars, in: River Confluences,
Tributaries, and the Fluvial Network, edited by S. Rice, A. Roy, and B. Rhoads, pp. 409–430,
John Wiley, West Sussex, U.K.
Martian valley classification (based on size morphology and network planform)
1. ‘Fretted’ valleys: gridded planform, little evidence of through-flowing water
2. Outflow channels
3. Valley networks; subdivisions are as follows (with decreasing valley spacing and length):
a. Large, widely spaced, sinuous stem valleys (~10 km wide, ~1 km deep 50%), and
increase in width downslope with tributaries much smaller than the stem
b. Small valley networks with closely spaced tributaries and similar width to higher order
segments downslope
c. Dense, sub-parallel slope valleys
**True dendritic patterns (reflects full network development under homogenous geological
conditions) have not been seen on Mars (what does it mean to have a “true dendritic”
pattern?)
 Most valleys have a flat-floored or V-shaped cross-sectional profile: V-shaped suggests late,
rapid downcutting by runoff along a steep gradient, with little subsequent modification
 Associated fans and deltas: implies standing bodies of water
Morphometry
**There is a significant difference between Martian drainage basins and the ideal quasiequilibrium condition.
**Martian valley networks appear underdeveloped relative to terrestrial counterparts
 Horton (1945) order 3-4: indicates short length with limited tributary development
 Bifurcation ratios of 2.9 to 7.6 (similar to terrestrial networks)
 Most martian valleys have low sinuosity
 Why few meandering valleys?
 Most follow steepest topographic gradient, regardless of local structure
 Drainage density (total length of valleys per unit area): 10-2 to 10-1 km/km2 (this is one to
three orders of magnitude less than typical terrestrial values)
 Possible reason for low drainage density: poor development of headwater tributaries
 Implications: High infiltrations due to cratering? Limited period when Earth-like climate
prevailed?
 Junction angles (angle at which channels merge) are small and are irregular, perhaps due to
immaturity
Geologic, topographic and regional distribution:
 Mostly in cratered southern highlands; evident but poorly developed in mid to high latitudes
 Sparse below -1500 to -1900 m elevation
 Any implications for mantling/resurfacing, climatic or base-level controls here?
 Some valleys originate at crater rims or sharp crests where no upslope aquifers were
available
Age
 Noachian to Early Hesperian
DMB
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3/9/09
Valley network activity cotemporaneous with heavy bombardment, and with a period of
more rapid erosional modification and infilling of Noachian craters (in contrast to the
relatively fresh morphologies of Hesperian and Amazonian aged craters)
Question: Were valley networks continuously active over a long period or did they
experience multiple regional to global scale reactivations?
 Irwin et al. (2005)
Water Sources
1. Ground-water sapping: spring discharge causes weathering of the aquifer material and
undermining the surface which extends the valley headward; could get volcanic intrusions
causing varying heat fluxes with permafrost interactions.
 Evidence for: theatre-headed tributaries/valleys
 Evidence against: 1) recharge amounts, 2) most valley networks formed in cratered,
rather than volcanic terrains, 3) water must have ubiquitous, temporally long-lived
and volumetrically abundant, 4) theatre headwalls are not unique to groundwater
sapping (get in waterfalls), 5) climatic models and morphometric analyses of yield
ambiguous results
2. Overland Flow (pluvial activity, snow-melt, etc.):
 Need thicker, warmer atmosphere to support more intense rainfall or snowmelt and longdistance flow without freezing
 Against: climatic models find that a thick CO2 greenhouse can not warm Mars above a
global average temp of 0°C
Scaling relationships
 Width, depth, and velocity on Mars would be 1.27, 1.18, and 0.67 times their value on Earth
and discharge would be multiplied by 0.62, based upon the following:
 Empirically, the mean flow depth (H) and the channel width (W) are related by H  W0.69
 Combined equation of the unit-balanced continuity and Darcy-Weisbach equations: Q =
HWV = H1.5W(8gS/f)0.5 (Q=discharge, g=grav. accel., S=slope, f=friction factor
 Alternatively, can use Q = 1.9W1.22
 Equations for critical shear stress and bed shear stress suggest that smaller particles would
remain suspended much longer in Martian rivers, increasing their transport rate by a factor of
~1.5
 **Lower bed shear stress and particle settling velocities on Mars would reduce the abrasion
rate of channel bedrock
 Implications for erosional estimates?
Summary points
 Events causing Martian networks may have been discontinuous and not intense enough to
fully regrade the landscape into a set of mature drainage basins
 Result: valley network length, order, network structure, junction angles, sinuosity,
drainage density, cross-sectional and longitudinal profiles, and watershed topography still
reflect crater topography
 Tributary development inhibited by short longevity of Earth-like conditions, inefficient
fluvial abrasion under lower gravity and possible high infiltration capacities
 Erosion rates declined suddenly