Valley networks on Mars, created by flowing water over 3 billion years ago, have always been regarded as critical evidence of liquid water's existence on the Red Planet. Researcher Alexander Morgan of the Planetary Science Institute has used impact craters to determine the maximum timescales for their formation, offering new insights into Mars' climatic history.
Close-up of an unnamed valley network on Mars. Craters are marked with blue and red circles. Red craters are more recent than the valley network, while blue craters are older. Dotted circles indicate less certainty of overlapping with the valley network. The black dotted line represents the mapped valley network.
(a) overview of the valley system. The entire basin is outlined in white; high-altitude areas that have experienced less erosion are highlighted in black.
(b) detailed view of the area indicated in (a).
Credit: MOLA MEGDR, NASA/USGS; THEMIS mosaic, ASU/NASA/USGS; CTX, NASA/MSSS. Morgan points out that the conditions allowing for the existence of rivers on Mars were likely highly intermittent, switching between prolonged dry periods and brief episodes of fluvial activity. These conclusions are based on studies of erosion rates on ancient Mars and suggest that Martian rivers eroded very slowly, at a pace akin to certain regions of the Atacama Desert in Chile.
One hypothesis is that erosion could have been hindered by the accumulation of large pebbles in riverbeds, preventing further erosion. Another explanation is that the rivers flowed very rarely, possibly less than 0.001% of the time, primarily activating during warming episodes caused by volcanic activity or changes in the planet's axial tilt and orbit around the Sun.
Morgan's study provides a more nuanced perspective on the Martian climate, moving away from overly general descriptions like "warm and wet" or "cold and icy." Like Earth, Mars has experienced significant climatic variations throughout its history.
This research also highlights the similarities between Martian and Earth rivers in terms of short- and long-term variability. On Earth, rivers are affected by upstream rainfall or snowmelt in the short term, and by long-term climate changes. On Mars, rivers would have functioned similarly, with short-term variations due to storms or snowmelt, and long-term variations due to shifts in the planet's rotation and orbit around the Sun.