Going with the Flow
Using terrestrial tools for modeling a Martian ocean
The hydrologic network referenced in the accompanying article was developed in a standard way using the geoprocessing tools in the Hydrology toolbox that are part of the ArcGIS Spatial Analyst extension. The basic hydrologic modeling and workflow applied to the Martian DEM were identical to those used on DEMs describing locations on Earth.
After visually examining the data, Kasei Valles was chosen as the most promising area for hydrologic analysis. The DEM for that area was clipped to the combined spatial extent of the local polygons of the geomorphic units representing older channel material, older floodplain material, teardrop shaped bars or islands, and ridged plain material.
The resulting subset of the DEM was then filled (i.e., a routine process that enables a DEM to be used for further hydrologic functions; it facilitates continuous flow by removing local imperfections in the original DEM) with the Z-limit parameter of 150 meters. Subsequently, the initial hydrologic analysis functions, such as Flow Direction and Flow Accumulation, were run. The density of the stream network was established by setting the threshold value of 10,000 cells to the output raster derived using the Flow Accumulation tool. All cells that accumulated flow from more than 10,000 cells were saved as a raster representing a river network. The cells that had value on the Flow Accumulation output raster less than 10,000 cells were assigned a value of NoData and were not part of the network. The raster network was transformed into a vector format, a linear network using the Stream to Feature function.
The outline of the hypothetical Martian ocean was generated using the CON tool, which performs a conditional if/else evaluation on each input cell of an input raster. The threshold representing "sea level" was set to -3,900 meters. All cells of the Martian DEM within the expanded given extent of the northern lowlands (Vastitas Borealis) with an elevation lower than -3,900 meters were saved to a raster representing this conjectured ocean.