We've known for a while that there are frozen water deposits just beneath the surface of the Martian poles, but now it seems that there are seasonal streams of liquid water that wind across the red planet's surface. Today, a group of scientists announced their discovery in Science, and told io9 what this liquid water might be like — and whether it changes the prospects for life on Mars.
The watery streams were spotted by NASA's HiRISE Martian orbiter in several different locations on the Martian surface. They look like thin, dark rivulets coming down from the tops of craters - much like mountain streams on Earth. The dark streaks appeared seasonally over a period of three years, so the researchers are relatively certain that they're seeing what might be equivalent to a spring thaw on Earth. But what kind of water could maintain liquid form on the surface of Mars, which is such an icy planet? The answer is that it's probably extremely brackish and packed with salts that lower the freezing temperature of water.
Researchers on the project dubbed these seasonal flows "recurring slope lineae" or RSLs for short.
University of Arizona astronomer Shane Byrne, who worked on the discovery, said:
What the specific salts are is an open question at the moment. NaCl [common table salt] only depresses the melting point by about 21 degrees Celsius, so it's likely that a different, more-effective, salt would be required to keep water liquid under all the circumstances where we've seen this activity. The Phoenix lander discovered perchlorate salts at a different location on Mars so they are a good candidate salt in this case too.
Added Byrne's Georgia Tech colleague James Wray, "We don't have a good way to estimate the salt content from orbit, but . . . some sites with these flows could have water no more salty than Earth's seawater (~3.5% salt content)." Though there are a lot of good possibilities for what kind of salts might be allowing this water to remain liquid, we won't know until we can actually get a sample or examine them using an instrument that can do a chemical analysis.
The scientists said that sites with RSLs show new streaks of water for a few months every Martian year (about 1.9 Earth years), but it's hard to say how long these seasonal trickles have been occurring since we've only had HiRISE observations for 3 Martian years.
Here, at left, you can see the features that the researchers have identified as RSLs in the Newton Crater. This image shows the crater during the off-season when there is no water.
And here at left you can see the same region during the wet season. Long, tan fingers of brackish water flow down from the crater top into the land below. This pattern has been documented over the past three Martian years in at least three other regions of the planet.
The next big question is where this water comes from. Could these flows be left over from an ancient Martian ocean that's disappeared or gone underground? Byrne said it's too early to say, but there are a few leading possibilities:
Where the water is coming from really is the core mystery here. It may be delivered from the sub-surface along cracks in the rocks (which would imply a larger reservoir of liquid nearby) or small amounts in the atmosphere might freeze onto the surface during winter. We're really right at the beginning of the investigation into these features so expect more findings in the years to come!
So it could be coming from underground, or it might be from the Martian equivalent of melting snow.
The point is that we now have compelling evidence that there is water on the surface of Mars, and possibly more flowing beneath the surface of the planet. This is an entirely new discovery, and could be a gamechanger when it comes to the planet supporting life. Could it mean that there's a better chance that we'll find some kind of extraterrestrial life on Mars? Byrne isn't ruling it out:
I think this is something the scientific community will have to chew over for years to come and perhaps one day we will send a landed mission to investigate these sites. Life on the Earth has evolved to fill every ecological niche possible so if life ever existed on Mars then these locations might be the best place to go looking for it despite the fact that it would still be a challenging environment by terrestrial standards.
Wray emphasized that liquid water isn't evidence for life, but adds that the water flows "could help shape the future search for microbial life on the planet, as subsurface pockets of brine are indeed one type of plausible habitat for microbes." We won't know more until we can land on the surface and investigate further.
Another big question is whether this water bodes well for an Earth colony on the planet. Have we found a good water source for our future colonists. Probably not. The amount of water we've seen so far is really small, and only in a few locations. Byrne pointed out that we've already found the best source of water for a colony, and that's from ice right beneath the Martian sands:
Much more useful to future astronauts are the vast deposits of water ice in the shallow subsurface that cover half of Mars. An astronaut who needed water could go collect it with a shovel. This ice is usually on the cold (pole facing) slopes in the mid-latitudes as opposed to these RSL features that HiRISE has been monitoring which are on the warmest slopes.
So, given the evidence gathered so far, we're not likely to see a Martian colony erected at the edges of these seasonal streams of water — unless they turn out to be trickles from a vast, underground sea. More likely our future colonists will get their water with picks and shovels. Ice mining may be the first big industry on Mars.
Additional reporting by Keith Veronese
Images courtesy of NASA/JPL/University of Arizona