We now know that there is liquid water on the surface of Mars. Streaks of dark material stream down crater walls, appearing and disappearing with the seasons. Imaging from orbit has confirmed that these features contain hydrated salts, leading researchers to conclude that the water took the form of a salty brine, which would prevent it from evaporating instantly in Mars’ cold, thin atmosphere.
But a new paper released today suggests we may want to revisit the role of brine. The international team behind it tested what would happen if pure water flowed through sand under Martian conditions. Some of the water quickly boiled away, but it managed to spread a little further than expected and showed features similar to some captured from orbit.
There are some challenges to figuring out what’s happening on Mars. The first is that we don’t have any hardware near where the watery features form; all of our direct exploration must take place from orbit. Another challenge is that we do not know the nature of the water. At the pressure of Mars, pure water could boil at temperatures reached during the day and freeze at night, while salts could keep it liquid at prevailing temperatures.
The team behind the new work decided it could get a sense of what might be going on using a small-scale model. They took a small block of ice (70 g) and melted it into sand at 20 degrees Celsius; later they repeated the process with a salt-saturated ice cube. The melting happened under Mars-like atmospheric pressure and the sand was set up as a 30-degree downward slope to approach a crater wall.
With pure water, some of it eventually wetted the sand, darkening it slightly (similar to the appearance of the seasonal features on Mars). Any water that was not on the surface of grains of sand tended to evaporate, as expected. But the most vigorous boiling occurred just beyond the front of the moistened sand, where it met the atmosphere.
There, the authors calculated that the water vapor would reach speeds of 100 meters per second, which is capable of blowing grains of sand from the surface. This could move grains of sand up to 4mm in diameter and eject them at speeds of about a third of a meter per second. After enough sand was blown away, a trough formed; once that trough was deep enough, the walls of the trough would flow down to restore a gentle slope. This process would continue as long as there was fresh water, creating a repeating cycle of percolation and the dry, granular stream. Over time, this creates a series of ridges and dips along the stream’s path.
Salt brine, on the other hand, had much less impact. The water spread wider (unsurprisingly, since the water remained rather than boiling off), but did not flow further down. And while brine channels could form channels near where the ice melted, those were the only physical changes in the slope. Very little happened when water or brine flowed under an Earth-like atmosphere.
Overall, the authors conclude that we should not rule out the possibility that liquid water (as opposed to saline brine) plays a role in shaping Martian terrain. “A small amount of liquid water reaching the surface,” they write, “could have a disproportionate geomorphological impact.” That impact, they conclude, is greater than any brine can create.
This doesn’t mean we have the definitive explanation for the features we see on Mars. As mentioned above, those are tens of meters in size; those produced in this model are only tens of centimeters. Since we’re unlikely to be able to imagine that from orbit, we’ll have to wait until we can get hardware there to image them up close and see what the small-scale features look like.
Natural Geosciences2016. DOI: 10.1038/NGEO2706 (About DOIs).