The key to understanding the past, present or future potential for life on Mars can be found in NASA’s four broad, overarching goals for Mars Exploration.
This animated artist’s concept depicts a scene of water breaking through the rim of Mars’ Jezero Crater, which NASA’s Perseverance rover is now exploring. Water entered the crater billions of years ago, depositing sediments that built up into a delta. Credits NASA/JPL-Caltech
On Earth, all forms of life need water to survive. If life ever evolved on Mars, it likely did so in the presence of a long-standing water supply. That's why our search for evidence of life on Mars focuses on areas where liquid water was once stable, below the surface where it still might exist, or in current "hot spots" where hydrothermal pools (like those at Yellowstone) might be habitable. Data from multiple NASA Mars missions suggest the presence of liquid water just below the surface in rare places, and on water ice at the Martian poles. Mars missions also search for energy sources other than sunlight, since life on the Mars surface is unlikely because "superoxides" break down organic (carbon-based) molecules on which life is based. We find life on Earth in many places without sunlight--dark ocean depths, inside rocks, and deep below the surface. Chemical and geothermal energy, for example, are also energy sources for life forms on Earth. Perhaps tiny, subsurface microbes on Mars could use such energy sources, too.
Differentiating life from non-life is a challenge, no matter where one finds it. On Earth, we know which markers, or biosignatures, to look for, but life on another planet might be very different in chemistry, structure, and other characteristics. Life detection technologies under development will help us define life in non-Earth-centric terms, to detect it in all the forms it might take. In the meantime, NASA Mars missions look for telltale biosignatures of current and past life. Knowing the location and form on Mars of the element carbon, a fundamental building block of life, would tell us a lot about where life might have developed. The current Martian atmosphere consists mostly of carbon dioxide. Any discovery of carbonate minerals formed on the Martian surface by chemical reactions between water and the atmosphere would be a clue that water had been present for a long time--perhaps long enough for life to have developed. By studying fossils in sedimentary rock on Earth that leave a record of past life, we know that only certain environments and types of deposits preserve fossils well. We look for lakes and streams on Mars that may have left behind similar deposits.
Images like this from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter show many channels from approximately 3 feet to 33 feet (1 meter to 10 meters) wide on a scarp in the Hellas impact basin.
