A Portal to Mars in the Desert
In the hyper-arid core of Chile's Atacama Desert is a geological stand-in for Mars. The region's mineralogy is so similar to the Martian surface that NASA uses it to test planetary rovers. The clay deposits here mirror those in a 600-kilometer-long, 2-kilometer-deep Martian outflow channel called Mawrth Vallis. This valley on Mars, whose name is Welsh for "Mars," has the most widespread clay deposits on the planet, some over 200 meters thick.
These clays, known as phyllosilicates, require water to form, and their presence on Mars points to a watery past around 3.6 billion years ago. The clay-bearing rocks at Mawrth Vallis date to the Noachian Period (roughly 4.1 to 3.7 billion years ago), a time that coincides with the earliest evidence for life on Earth. Orbiters like the Mars Reconnaissance Orbiter and Mars Express have used spectrometers—specifically CRISM and OMEGA—to identify several types of clays at Mawrth Vallis. These include iron/magnesium (Fe/Mg)-smectites like nontronite and saponite, as well as aluminum-rich clays like montmorillonite and kaolinite. The exact same types of minerals are found in the Atacama.
A Subsurface Haven for Life
The parallel is mineralogical. In the Yungay region of the Atacama, scientists discovered a wet layer rich in smectite clay just 30 centimeters (about a foot) below the bone-dry surface. This subsurface layer, a previously unknown habitat, maintains a constant relative humidity of 78%. Within this wet clay, researchers found at least 30 species of salt-loving (halophilic) bacteria and archaea. This discovery shows that even in the driest non-polar desert on Earth, life can persist in protected niches just beneath the surface.
This finding matters for the search for life on Mars. Smectite clays are particularly good at trapping and preserving organic molecules between their sheet-like crystal layers. The discovery of a living microbial community within these clays in the Atacama suggests that similar shallow deposits on Mars could contain biosignatures. Dark cap rock of volcanic origin covers many of the clay deposits at Mawrth Vallis, which may have shielded potential ancient microbes from radiation and erosion. Future missions, like the ExoMars Rosalind Franklin rover, were specifically designed to investigate these subsurface clay deposits, making Mawrth Vallis and the nearby Oxia Planum prime targets in the search for past life.