The Search for Life on Mars

The Biggest Question

Is there — or was there ever — life on Mars? This is one of the most profound scientific questions humanity has ever asked. Mars once had the ingredients considered essential for life: liquid water, energy from the Sun, and the chemical building blocks found in its rocks and atmosphere. Whether life actually arose on Mars remains one of the great unsolved mysteries driving exploration of the Red Planet.

What Life Needs

On Earth, life exists almost everywhere water is found — from boiling hot springs to frozen Antarctic lakes, from deep ocean trenches to cracks in underground rock. Scientists use Earth’s life as a guide for what to look for on Mars. The basic requirements include:

  • Liquid water (past or present)
  • Energy sources (sunlight, chemical reactions)
  • Organic molecules (carbon-based building blocks)
  • Stable conditions over time (enough time for life to develop)

Early Mars appears to have met all of these requirements. For hundreds of millions of years, Mars had flowing rivers, standing lakes, a thicker atmosphere, and warmer temperatures. This period, known as the Noachian era (roughly 4.1 to 3.7 billion years ago), is when scientists believe Mars had the best conditions for life to emerge.

The Viking Experiments

The first dedicated search for life on Mars came with NASA’s Viking landers in 1976. Viking 1 and Viking 2 each carried a set of biology experiments designed to detect living microorganisms in Martian soil. The experiments produced intriguing and controversial results.

One experiment, the Labeled Release experiment, detected a chemical reaction in the soil that initially looked like it could indicate biological activity. However, the other experiments did not find clear evidence of organic molecules. Most scientists concluded that the results were likely caused by chemical reactions with highly reactive compounds in the soil, not by living organisms. The debate over the Viking results continues to this day.

Organic Molecules on Mars

Decades after Viking, NASA’s Curiosity rover made a breakthrough discovery: organic molecules in Martian rocks. In 2018, Curiosity found complex organic compounds in 3.5-billion-year-old mudstone at the bottom of Gale Crater’s ancient lake. Additional organic molecules were detected in drilled rock samples at multiple locations.

It is important to note that organic molecules do not necessarily mean life exists. These molecules can form through non-biological processes, such as chemical reactions or delivery by meteorites. However, the presence of organics confirms that Mars has preserved the building blocks of life in its ancient rocks — exactly where scientists would expect to find biosignatures if life ever existed.

The Methane Mystery

As discussed in the atmosphere section, trace amounts of methane have been detected on Mars. On Earth, the majority of atmospheric methane is produced by living organisms. The seasonal fluctuations and localized nature of the Martian methane detections have fueled speculation that biological processes could be involved.

Geological explanations — such as the reaction of water with olivine-rich rock in a process called serpentinization — can also produce methane without any biology. Determining the source of Martian methane is a top priority for ongoing and future missions.

Mars Sample Return

The most ambitious effort yet to answer the life question is the Mars Sample Return campaign. NASA’s Perseverance rover is collecting carefully selected rock and soil samples from Jezero Crater’s ancient lakebed and delta — locations chosen specifically because they are the most likely to preserve signs of ancient microbial life.

These sealed sample tubes will eventually be retrieved and returned to Earth, where scientists can analyze them with the most powerful laboratory instruments available. Laboratory analysis on Earth can detect evidence of life at levels far beyond what any rover instrument could achieve on Mars. The returned samples may provide the definitive answer to whether Mars ever harbored life.

Could Life Exist on Mars Today?

While the Martian surface is extremely harsh — cold, dry, and bathed in ultraviolet radiation — life might survive below the surface. Potential refuges include:

  • Subsurface aquifers where liquid water may exist beneath the ice caps
  • Underground caves and lava tubes that could provide shelter from radiation and temperature extremes
  • Deep rock environments similar to those on Earth where microorganisms thrive in total darkness, drawing energy from chemical reactions

On Earth, extremophile organisms — life forms that thrive in extreme conditions — have been found in environments once thought to be lifeless: inside Antarctic ice, in highly acidic mine drainage, and kilometers below the surface in solid rock. These discoveries on our own planet expand the range of environments where Martian life could potentially survive.

What Would It Mean?

The discovery of life on Mars — even simple, microbial life — would be one of the most significant scientific findings in human history. If Martian life shares a common origin with Earth life (perhaps transferred between planets by meteorites, a process called panspermia), it would reveal that life can travel between worlds. If Martian life arose independently, it would demonstrate that the emergence of life is not a unique accident but a natural outcome of the right conditions — suggesting that life may be common throughout the universe.

Either way, the search for life on Mars pushes humanity to explore, to ask bold questions, and to develop the technology that will one day carry us to the Red Planet in person.