Scientists think that the radar signals that suggest the presence of subsurface lakes located deep under the surface, could be emerging from clays, and not water.
The search for life on the Red Planet
The search for life beyond Earth’s orbit has become one of the biggest obsessions for astronomers and Mars is believed to be the most prime location for such a discovery. Life flourishes in the presence water and recent studies piqued global interest by suggesting the existence of subsurface lakes on the Red Planet.
Now, some scientists think that the radar signals that suggested the presence of water in these lakes located deep under the surface could be emerging from clays, and not water. Three papers published over the course of the last month have offered new insights into the mystery signals, drying up the lakes hypothesis.
In 2018, a team led by Roberto Orosei of Italy’s Istituto Nazionale di Astrofisica announced evidence suggesting the existence of subsurface lakes deep below the ice cap at Mars’ south pole. The team had studied data from a radar instrument aboard the European Space Agency (ESA) Mars Express orbiter that showed bright signals beneath the polar cap. These signals could be interpreted as liquid water, the scientists had argued.
The orbiter used radar signals to penetrate rock and ice, which changed as they’re reflected off different materials. However, researchers after conducting tests in a cold laboratory are now suggesting that signals were not from water.
Too cold for lakes
Researchers now say that many of these lakes may be in areas too cold for water to remain in a liquid state. Aditya R Khuller and Jeffrey J Plaut from Nasa’s Jet Propulsion Laboratory (JPL) analysed 44,000 radar echoes from the base of the polar cap across 15 years of observations. They found many of these signals in areas close to the surface, where it should be too cold for water to remain in liquid form.
Two separate teams further analysed the data to determine whether anything else could be producing those signals. While Carver Bierson of ASU completed a theoretical study suggesting several possible materials that could cause the signals, including clays, York University’s Isaac Smith measured the properties of smectites, a group of clays present all over Mars.
Clay, not water
Smith put several smectite samples, which look like ordinary rocks but were formed by liquid water long ago, into a cylinder designed to measure how radar signals would interact with them. He then doused them with liquid nitrogen, freezing them to minus 50 degrees Celsius, close to temperatures observed at the Martian south pole. Once frozen, the rock samples perfectly matched the radar observations made by the ESA’s Mars orbiter.
The team then looked for the presence of such clay on Mars using the MRO, which carries a mineral mapper called the Compact Reconnaissance Imaging Spectrometer. They found smectites scattered in the vicinity of the south pole’s ice cap. “Smith’s team demonstrated that frozen smectite can make the reflections no unusual amounts of salt or heat are required and that they’re present at the south pole,” JPL said.
Not the first such claim
The subsurface lake hypothesis is not the first to have garnered global eyeballs, in 2015 NASA’s Mars Reconnaissance Orbiter found what looked like streaks of damp sand running down slopes, a phenomenon called “recurring slope lineae.” Researchers had detected signatures of hydrated minerals on slopes where mysterious streaks were seen on the Red Planet. These darkish streaks appeared to ebb and flow over time.
However, repeated observations, using the spacecraft’s High-Resolution Imaging Science Experiment (HiRISE) camera, showed granular flows, where grains of sand and dust slip downhill to make dark streaks, rather than the ground being darkened by seeping water. The phenomenon existed only on slopes steep enough for dry grains to descend the way they do on faces of active dunes.
While it is impossible to confirm what the bright radar signals are without landing at Mars’ south pole, the latest studies have offered plausible explanations that are more logical than liquid water.