Understanding the past water budget of Mars has important implications for our understanding of geomorphologic features (and the potential development of life) elsewhere in our solar system. The discovery of valley networks by the Mariner 9 Mission to Mars sparked a long-lived debate over the amount and importance of water at or near the Martian surface. They are the most common drainage feature on Mars and are often cited as evidence for a warm wet early climate on Mars. However early interpretations of surface features suggested surface runoff from precipitation was an unlikely cause for the formation of the Martian valley networks. The relatively low number of valley networks, and their poorly developed drainage systems, suggested that groundwater was a more appropriate explanation than surface runoff. New evidence presented in a 2003 paper by Hynek and Phillips indicated that Martian valley systems are far more complex than first assumed – and that precipitation and surface runoff may have substantially contributed to the sculpting of the Martian landscape.
By utilising data from the Mars Orbiter Laser Altimeter (MOLA) and the Mars Orbiter Camera (MOC), Hynek and Phillips constructed a shaded relief map containing multiple layers of data using Geographical Information Systems (GIS) software. High-resolution topography allowed for accurate measurements of drainage basins, and by varying the image parameters in each GIS layer, far more valley systems on Mars could be identified than had previously been mapped. Figure 1 below compares the original mapping of valley networks on Mars with the maps created by applying this new technique.
Nearly all the regions of cratered highlands were re-examined and led to the increase in total mapped valley length and drainage density of all Martian drainage basins. As seen by the above figure, many previously identified individual valley segments can now be seen as part of larger, integrated, dendritic basins with lengths of up to thousands of kilometres long. This new evidence exposed much higher drainage densities over much of Mars than previously thought.
Many of the mapped lower-order systems are part of larger integrated basins that have features consistent with basins on Earth, and therefore possibly have been influenced by surface runoff. This new data then supports the theory that there was some degree of surface runoff, and by extension precipitation, on Mars in the past. Surface runoff is the simplest explanation for the integrated drainage basins, v-shaped valleys and drainage densities similar to terrestrial values. Early precipitation and surface runoff are probably necessary to explain the observed features of Mars and thus the climate must have been significantly different in the past.
What does that mean? Well perhaps there may have been rain on Mars in the past, which created these valleys, channels and basins. But if this was the case, it happened a very long time ago – between 3.8 to 3.6 billion years ago.
Authored by Oisin Byrne, Undergraduate, Department of Geography, Trinity College, Dublin.
Hynek, B.M. and Phillips, R.J. (2003) New data reveal mature, integrated drainage systems on Mars indicative of past precipitation. Geology 31 (9): 757-760.