NASA / JPL-Caltech / ESA / DLR / FU Berlin / MSSS
Mars clearly once had a lot of water – there are just far too many features that clearly formed in a watery environment for that to be a matter of debate. What was less clear was how much of that water was liquid and for how long. While some features clearly indicate that liquid water was present for a long time, others probably formed under glacier ice.
It is not clear whether the differences are a matter of timing – a wet period followed by an icy, for example – or because of regional differences in Mars’ climate. It’s hard to tell in part because we can not get Mars’ climate models to produce a climate that is wet long enough to form many water features.
To try to place some restrictions on what the ancient Martian climate might look like, a team of planetary scientists decided to take a cautious look at some of the once watery features identified on the surface of the red planet. Timothy Goudge, Caleb Fassett, and Gaia Stucky de Quay (yes, that’s a planetary scientist named Gaia) identified a series of lakes, and used the features of the lakes to put some restrictions on the precipitation they fiede.
From top to bottom
How does a lake tell us about rainfall? By elevating the land to study the basin where it is located, it is possible to determine how large an area it once ran. Given that and the volume of the lake, it is possible to determine how much precipitation was needed to fill it. But how do you determine the volume of a lake that has not existed for billions of years?
While the researchers can not determine the exact volume, they can set limits. A number of the lakes were drained by water flowing, indicating that their volume should have reached at least that level, providing a lower limit on the volume of the lake. Others have never drained, and so deliver a maximum volume for this lake – higher than the surrounding terrain and it would be flooded. Overall, they analyzed 54 of the former type of lake and another 42 of the latter.
The researchers then made a simple balance between the precipitation and the evaporation rate, with the mathematics agnostic on the question of whether the precipitation formed as rain or snow. The evaporation rate would also depend on how dry or humid the climate was, with dry climates setting an upper limit for how much water should be introduced and waterlogged species, allowing less precipitation to achieve the same thing.
The researchers then knew how large an excessive precipitation event would have to be to overcome the evaporation and complete the wash. This is not the kind of solid supply of water needed to balance evaporation and possibly feed a stream of waste; rather, it is the amount of water that needs to flow more than that to fill the lakes somewhere between these upper and lower boundaries.
The researchers call these “runoff episodes” and refer to their Duration as “unconstrained.” All they can say is that “this episode must have been continuous and provided enough water to fill and break lakes with open basins. [those with runoff streams] but not lakes with closed basins. “
Within borders
Calculating these upper and lower limits for different lakes produces a frequency distribution for both the upper and lower limits of the precipitation required to produce appropriately large runoff events. And that tells us a few important things. The first is that there was a lot of downfall involved in these events. The lower limit is a little over four meters, and the upper limit is 159 meters. Again, this was over a completely undefined time period, but it is still a significant amount of precipitation.
The other thing that is very clear is that the rainfall was not evenly distributed – Mars did not have a single climate. The researchers were able to identify areas that were likely to have received larger or smaller amounts of precipitation.
But a number of important things are not yet clear. Another is how long these events lasted – the authors call them a “quasi-continuous runoff episode.” Another is how many of them were there. While we have some data for Gale Crater, where we accidentally dropped a rover, it is generally not clear how long most of these basins remained filled with water. Finally, we do not know if the precipitation fell as rain or if there was some or heavy snow that then washed in periods of temporary heat in the basins.
However, by providing some limits on what may have been early in the history of Mars, the study provides some limitations that climate models must meet when attempting to understand the past of the red planet. With enough of these boundaries, it will be easier to get a clear picture of the distant past.
Geology, 2020. DOI: 10.1130 / G47886.1 (About DOIs).
