Mars could have been as cold in its past as it is now
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S ome of the clearest signs that ancient Mars once sported huge amounts of flowing water on its surface are provided by networks of rivers. These valley networks have long-since dried up, but their distinctive forms carved into the landscape are both abundant and widespread across the southern uplands of Mars. They tell us that during the period in the planet’s history when they formed – the late Noachian, which was around 4.1 billion to 3.7 billion years ago – the Martian environment was very different from what we find today.
But what exactly was the nature of the early Martian climate? Was it once very Earth-like, with conditions warm enough for a genuine water cycle of rainfall and evaporation, resulting in long-term lakes and rivers? Or was ancient Mars the same very cold, icy, planet we know today, with average temperatures well below the freezing point, the valley networks having been formed by short-lived melting episodes triggered by a volcanic eruption or asteroid impact?
Has Mars ever been much warmer and wetter, or has it always been inhospitably cold, bleak and dry? The answer to this question is not only of interest to planetary climatologists, but is also important for determining the chances of Mars ever having developed life of its own.
The preserved remains of the valley networks might also be able to provide insights into the ancient Martian climate. In recent years planetary scientists have used orbiter imagery and surface models to estimate the volume of liquid water that would have been necessary to carve out the valley networks, and over what timescales. The problem is, these calculations vary quite widely.
One research group estimated that eroding all of the valley networks would have required a volume of water equivalent to submerging the entire Martian globe to an average depth of almost 5km. If true, this would indicate that the flowing water must have been recycled many times down the valleys by evaporation and rainfall, and thus that ancient Mars was a great deal warmer. On the other hand, different studies calculated a required volume of water equivalent to only 3-100m globally, which could easily be explained by transient river activity in an otherwise very cold and icy climate.
These earlier estimates were based on certain assumptions and limited data sets and vary greatly from each other. So, to try to bring some clarity to the issue, Eliott Rosenberg and his colleagues at the Department of Earth, Environmental and Planetary Sciences, Brown University, have used more complete data for the volumes of the valley networks and an improved method for estimating the liquid flow that would have been required. They find that the minimum amount of liquid water needed to carve out the valley networks is around 640m globally.
This latest calculation sits in the middle of previous estimates. And it also seems to be consistent with either a warm, rainy ancient Mars, or a cold and icy climate. Intriguingly, the locations of many of the valley networks and lakes are where climate models predict ice layers would form in a cold scenario, and so they could be explained by meltwater running off surface ice. Perhaps ancient Mars was not as habitable as we might have thought.
LEWIS DARTNELL was reading… The volume of water required to carve the Martian valley networks: Improved constraints using updated methods by Eliott N Rosenberg, et al. Read it online at: www.sciencedirect. com/science/article/pii/S0019103517305900
A NEW STUDY FINDS THAT THE MINIMUM AMOUNT OF LIUID NEEDED TO A CARVE OUT MARS'S VALLEY NETWORK IS AROUND 640M GLOBALLY
An aerial image – taken by the Mars Reconnaissance Orbiter – of Mars’s Uzboi Vallis, believed to have been formed by running water
LEWIS DARTNELL is an astrobiology researcher at the University of Westminster and the author of The Knowledge: How to Rebuild our World from Scratch (www.the-knowledge.org)