Life on Mars

By: Nicole Willett

The question of whether life exists or has ever existed on Mars is one of the most compelling in all of science. For over a century, humans have speculated about life on the Red Planet, and in recent decades, robotic spacecraft have given us the tools to move from speculation to scientific investigation.

The Viking Experiments

NASA’s Viking landers, which arrived on Mars in 1976, carried the first instruments specifically designed to search for life on another planet. The biology experiments conducted by Viking I and II yielded results that remain debated to this day.

The Labeled Release (LR) experiment, designed by Dr. Gilbert Levin, added nutrients to Martian soil samples and monitored for the release of gases that would indicate biological activity. The experiment produced a positive result — the soil appeared to respond in a way consistent with biological metabolism. However, the Gas Chromatograph Mass Spectrometer (GCMS), which searched for organic molecules in the soil, failed to detect any organics, leading most scientists to conclude that the LR results were due to chemical, not biological, processes.

Dr. Levin, however, maintained until his passing that the Viking LR experiment had indeed detected life on Mars. The debate highlights the challenges of designing unambiguous life-detection experiments for an alien environment.

Following the Water

Since Viking, NASA’s Mars exploration strategy has been guided by the principle of “follow the water,” recognizing that liquid water is a prerequisite for life as we know it. This strategy has led to a series of missions that have progressively built the case that Mars was once, and may still be in limited ways, a habitable world.

The Mars Exploration Rovers Spirit and Opportunity found extensive evidence of past water, including minerals that only form in the presence of water. The Phoenix lander confirmed the presence of water ice just below the surface near the north pole. And the Curiosity rover has found evidence of ancient lakes, rivers, and the chemical building blocks of life in Gale Crater.

Where to Look

Scientists have identified several promising environments for finding evidence of life on Mars:

• Subsurface environments: Below the harsh surface conditions, liquid water may persist in underground aquifers where temperatures and pressures are more favorable.
• Ancient lake beds: Sedimentary deposits in areas like Gale Crater and Jezero Crater may preserve biosignatures from a time when Mars was warmer and wetter.
• Recurring Slope Lineae: These seasonal dark streaks on crater walls may be associated with briny water, potentially providing a transient habitat for extremophilic organisms.
• Polar regions: The ice caps and subsurface ice deposits could harbor dormant organisms or preserve ancient biosignatures.

The Search Continues

The search for life on Mars is far from over. Each new mission adds to our understanding of Mars’ habitability and brings us closer to answering the fundamental question: are we alone? Whether the answer comes from a rover drilling into Martian rock, a sample returned to Earth, or the first human geologist to set foot on Mars, the discovery of life beyond Earth would be one of the most profound in human history.