Enlarge /. A simulated view of Gale Crater Lake about 150 km in diameter on Mars about 3 billion years ago.
NASA / JPL-Caltech / ESA / DLR / FU Berlin / MSSS
Clearly Mars once had a lot of water – there are simply far too many features clearly formed in an aqueous environment for this to be an issue. What is less clear is how much of this water was liquid and how long. While some features clearly indicate that liquid water was present for a long time, others likely formed under glacial ice.
It is not clear whether the differences are a question of timing – for example a rainy season followed by an icy one – or due to regional differences in the Martian climate. In part, it's hard to tell because we can't get climate models of Mars to produce a climate that's humid long enough to form many watery features.
In an attempt to narrow down the appearance of the ancient Martian climate, a team of planetary scientists decided to carefully study 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 number of lakes and used the lakes' features to limit the rainfall that fed them.
How does a lake tell us about rainfall? By examining the elevation of the land around the basin the lake is in, you can find out how large is the area that it once drained. Given this and the volume of the lake, it is possible to determine how much rainfall it took to fill it. But how do you determine the volume of a lake that has not existed for billions of years?
While researchers cannot determine the exact volume, they can set limits. Some of the lakes have been drained by outflowing water, which indicates that their volume must have reached at least this level, which gives a lower limit to the lake volume. Others have never been drained and thus offer the maximum possible volume for this lake – higher than the surrounding area and it would have overflowed. In total, they analyzed 54 of the former lake type and another 42 of the latter.
The researchers then performed a simple trade-off between precipitation and evaporation rate, the math being independent of whether the precipitation formed as rain or snow. The rate of evaporation would also depend on how dry or humid the climate was, with dry climates setting an upper limit on water input and wetter climates allowing less rainfall to achieve the same.
The researchers then figured out how large an excess rainfall event would have to be to overcome evaporation and fill the basin. This is not the type of stationary water supply that is required to make up for evaporation and possibly feed an outlet stream. Rather, it is the amount of water that would have to flow beyond that to fill the lakes somewhere between those upper and lower limits.
The researchers call these "drainage episodes" and describe their duration as "casual". All they can say is that "this episode must have been sufficiently continuous and provided enough water to fill and breach open-basin lakes (those with drainage streams), but not closed-basin lakes."
Calculating these upper and lower limits for different lakes gives a frequency distribution for both the upper and lower limits of the precipitation required to generate runoff events of suitable magnitude. And these tell us a couple of important things. The first is that there was a lot of rainfall in these events. The lower limit is just over four meters and the upper limit is 159 meters. Again, this was over a completely undefined period of time, but there is still a significant amount of rainfall.
The other thing that is very clear is that precipitation wasn't evenly distributed – Mars didn't have a single climate. The researchers were able to identify areas where more or less precipitation has likely fallen.
But some important things are still not clear. Again, one is how long these events lasted – the authors call them a "quasi-continuous runoff episode". Another is how many of them there were. Although we have some data for the Gale Crater where we accidentally dropped a rover, it is generally not clear how long most of these pools were filled with water. After all, we do not know whether the precipitation fell as rain or whether some or all of the snow was snow that was then washed into the pools during periods of temporary warmth.
By placing some boundaries on what must have happened at the beginning of Martian history, it offers some limitations that climate modellers must face when trying to understand the red planet's past. With enough of these limits, it becomes easier to get a clear picture of the distant past.
Geology, 2020. DOI: 10.1130 / G47886.1 (About DOIs).