Liquid water on Mars is no surprise but here’s why it makes the planet tougher to explore

Water on Mars is exciting but laws of planetary protection state that we cannot send any craft from Earth near a potential site of life for fear of contamination.

Bengaluru: After years of speculation, researchers have finally discovered liquid water on Mars. But it is hardly a surprise.

Given how the Martian landforms have shaped, with mountains and valleys and riverbeds, researchers have known that the planet held historically large waterbodies. They have also detected water ice and seen drops of condensation on spacecraft. But a body of liquid water has remained elusive for decades.

Nearly every craft that has visited the planet has hunted for liquid water; the Mars Express craft that detected water itself had been looking for over 12 years. Liquid water is a prime factor that indicates habitability and potential microbial presence.

Scientists working on the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on the Mars Express spacecraft of the European Space Agency (ESA) used radar to scan the base of a polar icecap when they detected a body of water 2 km below the surface.

The waterbody, more than 1.5 km deep underground, managed to remain unfrozen because it’s briny — full of salt — and under immense pressure. The discovery, published in Science magazine, was made by a team of Italian astronomers from various institutions.

In the paper, the authors describe how between 2012 and 2015, they shot a series of radio pulses — 29 in total, one for each overhead pass the craft made. These waves dig into the icy shell and soil, and travel down until they encounter geological structures before bounding back to the craft. Using the patterns they form as they’re reflected, inferences can be made about what’s hiding underneath the surface. The intensity of brightness also reveals what the substance is that reflects the radar back; salt water is bright.

A screengrab from animation of the Martian poles with ice caps. | Credit: ESA
However, MARSIS had a bit of a problem: the dimensions of the objects that it measured were tough to calculate because the instrument was not calibrated. On Earth, it was simply too big to conduct tests which fired radar and revealed hidden shapes. So all measurements the team collected were relativistic and compared to other objects on Mars, which meant that arriving at the precise size of this subsurface water body took three years.

Coming to a conclusion
It’s been tricky arriving at this conclusion. Several craft have taken radar measurements and have arrived at no conclusive evidence of liquid water. But scientists always knew that the base of the polar caps were the best place to look for water as the pressures from the ice would raise the temperatures enough to keep the water liquid.

But considering Mars is so much colder than Earth, the only way for water to remain liquid is if it was filled with salt. Sodium, magnesium, and calcium have been found on the Martian surface and can reduce the melting point of water to −74°C, claim the authors.

But it is quite likely that this body of water isn’t a big lake. It could be a large deposit of slush mixed with soil and mud. Then again, not everyone even agrees that the body of water exists. Scientists working on NASA’s Mars Reconnaissance Orbiter (MRO), which is equipped with a similar radar instrument called SHARAD, say that they simply do not get any readings for subsurface liquid in the region at all. Further tests from several other instruments will tell us for sure in the near future.

What it means for us
Water on Mars is exciting. It is a prime candidate for habitability anywhere and the presence of water in general attracts a lot of attention for planetary missions. Mars might probably not be hospitable to life today, but when the planet was warm several millions of years ago, it might have hosted microbes — a theory scientists are struggling to confirm through various Martian missions that are currently looking for evidence of past life.

Discovery of water also means that the planet is less hostile for future life, if we colonise it. However, as of today, we’re limited by a very important law of planetary protection. The higher the chances of liquid water at a site, the more important it is to explore it.

But also, the more difficult it is to, because laws of planetary protection state that we cannot send any craft from Earth near a potential site of life for fear of contamination. The only way we can put life on the planet in the future is to ensure without a doubt that there had been no life on it in the past.

Not all’s impossible, though. Water could actually be more common on Mars than we thought. The ESA scientists also state that because of the size and location of the body, they are quite certain this isn’t the only lake. “There is no reason to conclude that the presence of subsurface water on Mars is limited to a single location,” they conclude.