This 2016 image from NASA's Mars Reconnaissance Orbiter shows white material believed to be dusty water ice lining the edges of Martian gullies. Scientists believe dust particles within this ice act similarly to dust that falls onto glaciers on Earth, warming up in sunlight and causing subsurface pockets of meltwater to form.NASA/JPL-Caltech/University of Arizona
Scientists have yet to find evidence of life on Mars, but a new study from researchers at NASA's Jet Propulsion Laboratory, the University of Washington and other universities suggests microbes could find a potential home beneath layers of ice known to exist on Mars' surface.
In the study, published Oct. 17 in Communications Earth & Environment, authors showed that enough sunlight shines through surface ice for photosynthesis to occur in shallow subsurface pools of meltwater. Similar subsurface meltwater pools that form within ice on Earth have been found to teem with life, including algae, fungi, and microscopic cyanobacteria, all of which derive energy from the sun via photosynthesis.
"If we're trying to find life anywhere in the universe today, Martian ice exposures are probably one of the most accessible places we should be looking," said lead author Aditya Khuller at NASA's Jet Propulsion Laboratory, who will join the UW Applied Physics Laboratory as a senior research scientist in November.
Unlike Earth, Mars has two kinds of ice: frozen water and frozen carbon dioxide. The new study focused on water ice, largely formed from snow mixed with dust that fell during a series of Martian ice ages during the past million years. That ancient snow has since solidified into ice, still peppered with specks of dust.
Those dust particles are key to explaining how subsurface pools of water would form within ice when exposed to solar rays: dark dust absorbs more sunlight than the surrounding ice, causing the deeper ice to warm up and melt up to a few feet below the surface.
It's a matter of debate whether ice can actually melt and exist as a liquid on the surface of Mars due to the planet's thin, dry atmosphere, where water ice is believed to sublimate - turn directly into gas - the way dry ice does on Earth. But the atmospheric effects that make melting difficult on the surface wouldn't apply below the surface of a dusty snowpack or glacier.
This new paper uses computer modeling to suggest that dusty ice lets in enough light for photosynthesis to occur as deep as 10 feet (3 meters) below the surface. In this scenario, the upper layers of ice prevent the shallow subsurface pools of water from evaporating while also providing protection from harmful radiation. That's important given that, unlike Earth, Mars lacks a protective magnetic field to shield it from both the sun's ultraviolet rays and radioactive cosmic ray particles zipping around space.
The water ice that would be most likely to form these subsurface pools would exist in Mars' midlatitudes - between the latitudes of 30 degrees and 60 degrees - in both the northern and southern hemispheres.
"This latest paper examines the propagation of solar radiation into the ice, showing that just below the surface there is a zone that is safe from ultraviolet but still gets enough visible light to support photosynthesis," said co-author Steve Warren, professor emeritus of Earth and space sciences at the UW. "But of course photosynthetic organisms won't survive unless the ice in that zone can melt, at least occasionally."
At the UW, Khuller plans to continue working to determine where liquid water is likely to exist on Mars. The next step, Khuller said, will be to recreate some of Mars' dusty ice in a lab setting. Meanwhile, he and others are beginning to map out the most likely spots on Mars to look for shallow meltwater - scientific targets for possible human and robotic missions in the future.
Gary Clow at the University of Colorado Boulder is also a co-author on the new paper.
