Two research teams, using data from the ecu Space Agency’s Mars Express orbiter, have recently published results suggesting that what were thought to be subsurface lakes on Mars might not really be lakes in the least .
In 2018, scientists working with data from the Mars Express orbiter announced a surprising discovery: Signals from a radar instrument reflected off the red planet’s South Pole seemed to reveal a liquid subsurface lake. Several more such reflections are announced since then.
In a new paper published within the American Geophysical Union’s Geophysical Research Letters, lead author and grad student Aditya Khuller of Arizona State University’s School of Earth and Space Exploration with Jeffrey Plaut of NASA’s reaction propulsion Laboratory (JPL), describe finding dozens of comparable radar reflections round the South Pole after analyzing a broader set of Mars Express data. But many are in areas that ought to be too cold for water to stay liquid.
The question of whether the signals are liquid water or not is additionally being considered by a team of scientists led by ASU School of Earth and Space Exploration postdoctoral scholar Carver Bierson. Their research was also recently published in AGU’s Geophysical Research Letters and determined that these bright reflections could be caused by subsurface clays, metal-bearing minerals or saline ice.
Mars Express is that the second-longest-surviving continually active spacecraft in orbit around a planet aside from Earth, behind only NASA’s still-active 2001 Mars Odyssey. As Mars Express orbits Mars, it continues to supply important data on the red planet’s subsurface, surface and atmosphere.
Onboard this spacecraft is an instrument called the Mars Advanced Radar for Subsurface and Ionosphere Sounding, or MARSIS for brief . This instrument uses a radar sounder to assess the composition of the subsurface of Mars.
MARSIS has been collecting data around Mars since 2004, including the South Pole , allowing scientists to create a three-dimensional view of the south polar region. “We wanted to seem beneath the south polar ice and characterize the old terrain lying underneath using MARSIS data,” said Khuller.
In other recent studies using MARSIS data, researchers have found areas where the reflections below the surface are brighter than that of the surface, which isn’t what scientists would expect.
“Usually, radar waves lose energy once they travel through a cloth , so reflections from deeper down should be less bright than those from the surface,” said Khuller, who is concurrently on an internship at JPL under Plaut’s direction. “Although there are a couple of possible reasons for unusually bright subsurface reflections, these two studies concluded that a liquid water component was the explanation for these bright reflections, because liquid water appears bright to radar.”
The radar signals originally interpreted as liquid water were found during a region of Mars referred to as the South Polar Layered Deposits, named for the alternating layers of water ice, solid (frozen carbon dioxide) and mud that have settled there over many years. These layers are believed to carry a record of how the lean in Mars’ axis has shifted over time, even as changes in Earth’s tilt have created ice ages and warmer periods throughout our planet’s history. When Mars had a lower axial tilt, snowfall and layers of dust accumulated within the region and eventually formed the thick layered ice sheet found there today.
The areas originally hypothesized to contain liquid water span about 6 to 12 miles (10 to twenty kilometers) during a relatively small region of the Martian South Polar Layered Deposits. Khuller and Plaut expanded the look for similar strong radio signals to 44,000 measurements spread across 15 years of MARSIS data over everything of the Martian south polar region.
Unexpected ‘lakes’: A muddy picture?
The new, expanded study from Khuller and Plaut revealed dozens of additional bright radar reflections over a far greater range of area and depth than ever before. In some places, they were but a mile from the surface, where temperatures are estimated to be minus 81 degrees Fahrenheit (minus 63 degrees Celsius)—so cold that water would be frozen of the mars lakes, albeit it contained salty minerals referred to as perchlorates, which may lower the melting point of water.
“We’re not certain whether these signals are liquid mars lakes water or not, but they seem to be far more widespread than what the first paper found,” said co-author Plaut, who is additionally the co-principal investigator of the orbiter’s MARSIS instrument. “Either liquid water is common beneath Mars’ South Pole , or these signals are indicative of something else.”
Additionally Khuller noted a 2019 paper during which researchers calculated the warmth needed to melt subsurface ice during this region, finding that only recent volcanism under the surface could explain the potential presence of liquid water under the South Pole .
“They found that it might take double the estimated Martian geothermal heat flow to stay this water liquid,” Khuller said. “One possible thanks to get this amount of warmth is thru volcanism. However, we’ve not really seen any strong evidence for recent volcanism at the South Pole , so it seems unlikely that volcanic activity would allow subsurface liquid water to be present throughout this region.”
Khuller and Plaut’s next steps during this line of research are to research their discovery of a second, deeper layer under parts of the South Pole of Mars, which scientists think represents an older buried terrain called the Dorsa Argentea Formation. it’s thought to possess been modified by ancient glaciers once present across the region, and that they intend on trying to more accurately determine its composition and age.