Of Ancient Rivers and Rusting Robots: The Unceasing Search for Life on Mars

It’s been nearly 60 years since NASA’s Mariner 4 journeyed to Mars, becoming the first spacecraft to fly by the red planet.

Much has happened since.

Generations, even family groups, of robotic probes have made the months-long voyage to Mars, shared epic adventures with us, and retired to pass the torch to younger explorers to follow.

Decades of investigation mapping Mars’ surface, probing its interior, analyzing its atmosphere and geology, tracking its seasonal weather patterns and even digging up soil and grinding into rock to unlock its chemical secrets with microscopes and other specialized instruments has generated volumes of knowledge about our neighbor planet.

Forty successful missions of Martian exploration have been accomplished by multiple countries and agencies, including the United States, Russia, China, the European Space Agency, India and the United Arab Emirates. These include seven flyby spacecraft, 18 orbiters, eight stationary landers, six rovers and a helicopter. Even today, 11 of these robotic explorers are still in operation.

Mars is by far the most explored planet in the solar system, other than Earth.

What are we looking for?

The short answer: Martians.

A more nuanced response is: any sign of biological activity or its fossil residues, and past or present environments that may have life-friendly elements, including liquid water.

NASA’s twin rovers, Spirit and Opportunity, prospected the soil and rock of Gusev Crater and Meridiani Planum looking for the chemical signatures of past water, while the larger, next-gen robot, Curiosity, is currently scaling a mountain of sediments (Mount Sharp) to read the geologic and climate history of Mars.

Those efforts, along with the work of other landers and orbiters, have revealed the ancient Martian past as a wet one, with liquid water rivers, lakes and seas; precipitation; and a potentially life-hospitable environment.

Curiosity’s younger “sibling,” Perseverance, along with its experimental helicopter companion, Ingenuity, has just entered its third year exploring the ancient lake bed and river delta complex in Jezero crater. Since landing in February 2021, the nuclear-powered rover has driven over 9 miles, analyzing rock, soil and atmospheric gasses, and leaving behind a cache of sealed sample tubes for the proposed Mars Sample Return mission to collect and bring back to Earth.

Perseverance’s mission is to seek evidence of past microbial life — those long-sought Martians. Last September, the results from an analysis of samples of delta sediment were announced as the strongest evidence yet for past life on Mars. On Earth, river deltas are environments where life thrives, and their organic residues are often concentrated and preserved in deposited sediment.

Perseverance found very strong evidence of potential biosignatures — signs of past life — in delta sediments in Jezero. Definitive proof that the organic compounds it detected came from ancient Martian life will probably have to wait until the specimens are brought back to Earth for analysis — but the promise is exciting.

Martian robot retirement

Mars has become the final rusting place of many prospecting landers, a modern Martian version of abandoned Gold Rush equipment in Western ghost towns.

Of NASA’s arsenal, the twin Viking landers fell silent in 1980, Pathfinder/Sojourner in 1997, Phoenix in 2008, Spirit in 2010 and Opportunity in 2018.

Most recent to go off the air is NASA’s InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander, which arrived at Mars in 2018, even as the Opportunity rover’s batteries were running out of power under a sunlight-choking dust storm.

InSight performed an unprecedented four-year study of Mars’ interior to explore what the structure of its rock layers and core can reveal about the planet’s early formation and, by extension, the formation of other rocky planets, like Earth.

NASA said goodnight to InSight last December, when it was determined that the accumulation of dust on its solar panels had reduced the spacecraft’s capacity to generate electricity.

Next up?

And the adventure continues! Japan is scheduled to launch the Martian Moons eXploration (MMX) mission and India the Mars Orbiter Mission 2 in 2024. China has plans for the Tianwen-3 and NASA/ESA the Mars Sample Return missions in 2028 — and a much longer list of proposed expeditions extending decades ahead.

The evidence of actual Martian life, living today or only in the distant past, is yet to be found. But it’s probably safe to say that we’ve never been closer to that world-changing discovery.