Buried liquid water on Mars?

The question of whether there is water on Mars has been hotly debated for decades. It has generally been proven that water exists on Mars, though it’s mostly trapped in its ice form. For life to exist as we know it, liquid water is a vital component. Because Mars has such a thin atmosphere, liquid water on the surface isn’t stable and will immediately boil off. This makes it very tricky to even begin to try to observe liquid water. Recently however, using radar instruments, Italian researchers have discovered evidence of a large, liquid water lake beneath the glaciers at the southern pole of Mars.

Aboard the EU’s Mars Express spacecraft is a special instrument called a radar sounder. Radar sounders work by shooting specific frequencies of radar pulses at the surface of a planet and then observing the reflected signal. This can be thought of like clapping in a cave and listening to the echo that comes back to you. Depending on what the echo “sounds” like, scientists can gain a wealth of information about whatever surface they aimed the instrument at. Differing frequencies used by the instrument contain information about the surface rock, ice, or even what lies beneath the surface.

Figure 1: Radargram of the subglacial lake. In this figure, the x-axis is horizontal distance on the planet, and the y-axis is the time is takes the radar signal to bounce back to the spacecraft, which can be interpreted to understand subsurface topography. This figure can be thought of as a cross section. The subglacial lake can be seen at the basal reflection label. The SPLD label shows the South Polar Layered Deposit, which is an area of the south pole where pure water ice is formed in layers. Figure 2A from Orosei et al. (2018).

Using a radar sounder named MARSIS, Italian scientists made several flybys of the south pole region, which is mostly covered in water-ice glacier year round. What they found were bright subsurface radar reflections where, ordinarily, there should be nothing (fig.1). When you shoot a radar wave at a material, the target’s atoms will react by becoming electrically polarized to varying degrees (the atoms will have one side become more positive than the other, or vice versa). This degree of polarization can be unique for different materials, so it’s a great way to identify materials hidden in the subsurface. This quality of materials is known as dielectric permittivity. By analyzing the MARSIS radar reflections, the scientists discovered that the dielectric permittivity of the anomalous reflection matched that of water-bearing materials. By making several flybys of the area, the scientists were able to map out the subglacial lake and found it to be about 20 km wide and approximately 1.5 km beneath the surface (fig.2) 

Figure 2: This image shows the estimated size of the lake. Radar tracks are colored depending on subsurface elevations with warmer colors corresponding to higher elevations and cooler corresponding to lower elevations. The perceived lake is marked by the bold, black line. Figure 3A from Orosei et al. (2018).

Does this confirm that there’s a subglacial lake at Mars south pole? It’s very likely. (SHARAD, another radar sounder orbiting Mars, didn’t see the lake, though this may be to due to the different set of frequencies SHARAD uses). Does this confirm that there’s life on Mars? Not yet. For all we know, the lake could be completely inhospitable, or might not even be there in the first place! What this study does provide is compelling evidence that the secrets to extraterrestrial life in our solar system might be hiding under some ice, just one planet away!

Orosei, R., Lauro, S. E., Pettinelli, E., Cicchetti, A., Coradini, M., Cosciotti, B., … Seu, R. (2018). Radar evidence of subglacial liquid water on Mars. Science, 361(6401), 490 LP-493. Retrieved from http://science.sciencemag.org/content/361/6401/490.abstract