Seth Kadish
Geo 292
Long-term precipitation and late-stage valley network formation: Landform simulations of
Parana Basin, Mars
C. J. Barnhart et al.
Focus of the paper: How did late-stage valley networks form? What climatic/environmental
conditions were necessary for fluvial activity to incise these widespread Noachian features?
The basic outline of their approach:
 Digital mapping (MOLA): All valley networks in the Parana Valles drainage catchment
(PDC). Digitized in 3D space.
 Landform evolution simulations (MSLM): Simulates long-term changes via weathering,
mass-wasting, fluvial, eolian, and lacustrine processes. “MSLM simulates channel
incision by calculating terrestrially equivalent mean annual flood discharges as a function
of contributing area, and subsequently by calculating incision rate as a function of
discharge.” NB: “The actual erosional time scale on Mars might be different if the
frequency of flood discharges is not the same as on Earth.” What evidence do we have
that it would or would not be?
 Qualitative and quantitative comparison of model surfaces to the actual surface. Surfaces
used include actual surface (AS), initial conditions surface (ICS), which is impossible to
reconstruct but theirs looks pretty good (Figure 7), and the model output surface (MOS).
 Three parameters explored: Discharge scaling (α values – tested 0.3, 0.5, and 0.7),
downstream loss via evaporation and infiltration, and surface crust induration (ISC
formation balances lack of vegetation/bioturbation with impact gardening).
Parana Valles/Basin:
Watershed-defined drainage network with high drainage density and deeply incised channels.
Strong regional slope to the NW. Impact interruptions (some large crater rims breached, others
not) suggest the network formation period was extensive.
Simulations and Analyses:
 72 simulations covering end-member scenarios. Run past the point of current erosion.
 Use chi-square distribution for statistical analyses (Equations 5 and 6).
 Sample of results:
Quantitative Results:
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Severe overerosion or undererosion leads to large chi-square values.
The discharge exponent and the ISC have a much greater effect on the chi-square values
than the evaporation ratio.
Discharge estimate: 125 to 4682 m3/s.
Formation time scale: 500,000 to 700,000 mean annual flood size flows. For arid or
semi-arid terrestrial weather, this would take 105 – 106 years. On Earth, flood conditions
sustained for 2% of the year, so continuous flow at mean annual flood stage for 103 – 104
years could form Parana Valles. Episodic discharge would increase this to 107 years.
Qualitative Results:
 Compared simulated DEMs to actual PDC shaded relief, and evaluated on a variety of
characteristics (pg. 13).
 Do not consider crater interiors, alluvial and fluvial deposits, sedimentation, and
deposition patterns.
 Best qualitative fits had an α = 0.5 (semi-arid), whereas the best quantitative fit (lowest
chi-square) was 0.3. Non-zero, modest ISC provides best fit. Higher evaporation ratios
were marginally better than low or zero ratios, but the variable had minimal effect on the
resulting DEMs.
Implications:
 At least 104 years of sustained liquid on the surface in the form of episodic precipitation
with long repose times.
 Geomorphology implies that seasonal episodic floods controlled valley formation and
that it occurred sequentially with evaporation/infiltration.
 Paucity of crater rim exit breaches implies that flood stage discharge did not lead to
generalized filling and breaching of craters.
 Contrary to the hypotheses of previous researchers, valley networks in PDC were not
formed in a few several-year-long massive deluges. Not due to brief impact-induced
climate excursions.
Questions:
 Does this study appropriately use terrestrial values/conditions? Is this a weakness in the
results of their model, especially regarding discharge estimates and formation timescales?
 Do we expect these results to be consistent with all valley networks?
 How do we contrast their finding of well-preserved crater rims (non-breached) with other
studies that show numerous breaches by channels? Is Parana Valles a special case, or is
it the norm?
 Why does evaporation have such minimal effect on the results? Is this at all relevant to
Heldmann’s finding that a minimal amount of water in Martian gullies is lost to
evaporation? Is evaporation on Mars less important than we think?
 What conditions are necessary to form the ICS and how long does it take to form? Is it
similar to terrestrial desert pavement? Would we expect a Martian ICS to form under
semi-arid conditions?