On Earth, solar radiation can transmit down to multiple meters within ice, depending on its optical properties. Organisms within ice can harness energy from photosynthetically active radiation while being protected from damaging ultraviolet radiation. On Mars, the lack of an effective ozone shield allows approximately 30% more damaging ultraviolet radiation to reach the surface in comparison with Earth. However, new research shows that despite the intense surface ultraviolet radiation, there are radiatively habitable zones within ice in the Martian mid-latitudes, at depths ranging from a few centimeters for ice with 0.01-0.1% dust, and up to a few meters within cleaner ice.
The white edges along these gullies in Mars’ Terra Sirenum are believed to be dusty water ice. Khuller et al. think meltwater could form beneath the surface of this kind of ice, providing a place for possible photosynthesis. Image credit: NASA / JPL-Caltech / University of Arizona.
“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 Dr. Aditya Khuller, a researcher at NASA’s Jet Propulsion Laboratory.
Mars has two kinds of ice: frozen water and frozen carbon dioxide.
Dr. Khuller and colleagues looked at water ice, large amounts of which formed from snow mixed with dust that fell on the surface during a series of Martian ice ages in the past million years.
That ancient snow has since solidified into ice, still peppered with specks of dust.
Although dust particles may obscure light in deeper layers of the ice, they are key to explaining how subsurface pools of water could form within ice when exposed to the Sun.
Dark dust absorbs more sunlight than the surrounding ice, potentially causing the ice to warm up and melt up to a few feet below the surface.
Mars scientists are divided about whether ice can actually melt when exposed to the Martian surface.
That’s 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 Martian surface wouldn’t apply below the surface of a dusty snowpack or glacier.
On Earth, dust within ice can create what are called cryoconite holes — small cavities that form in ice when particles of windblown dust (called cryoconite) land there, absorb sunlight, and melt farther into the ice each summer.
Eventually, as these dust particles travel farther from the Sun’s rays, they stop sinking, but they still generate enough warmth to create a pocket of meltwater around them.
The pockets can nourish a thriving ecosystem for simple lifeforms.
“This is a common phenomenon on Earth,” said Dr. Phil Christensen, a researcher at Arizona State University.
“Dense snow and ice can melt from the inside out, letting in sunlight that warms it like a greenhouse, rather than melting from the top down.”
In 2021, the authors discovered dusty water ice exposed within gullies on Mars, proposing that many Martian gullies form by erosion caused by the ice melting to form liquid water.
Their new paper suggests that dusty ice lets in enough light for photosynthesis to occur as deep as 3 m (9 feet) 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, because unlike Earth, Mars lacks a protective magnetic field to shield it from both the Sun and radioactive cosmic ray particles zipping around space.
“The water ice that would be most likely to form subsurface pools would exist in Mars’ tropics, between 30 degrees and 60 degrees latitude, in both the northern and southern hemispheres,” the researchers said.
The paper appears in the journal Communications Earth & Environment.
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A.R. Khuller et al. 2024. Potential for photosynthesis on Mars within snow and ice. Commun Earth Environ 5, 583; doi: 10.1038/s43247-024-01730-y
This article is a version of a press-release provided by NASA.
