Red Planet recon
At Mars, NASA is conducting a series of missions over several decades to address the question of whether life ever arose there. The search began with determining whether the Martian environment was ever suitable for life.
Newly Deployed Sojourner Mars Rover (1997-07-05) by NASA/JPL-CaltechNASA
In the 1990s, NASA turned increasing focus on missions to the Red Planet. The Pathfinder mission (in 1997) and Mars Exploration Rovers (Spirit and Opportunity, which landed in 2004) were sent to Mars with the mantra to "follow the water," recognizing that liquid water is necessary for life to exist on Earth.
Curiosity Approaching Mars (Artist's Concept) (2011-10-13) by NASA/JPL-CaltechNASA
After these and other missions established that Mars once had significant amounts of water on its surface, NASA sent the Curiosity rover to Mars (landing there in 2012) to determine whether the planet had the right ingredients in the rocks to host life, signaling a shift to the next theme of "explore habitability."
Curiosity set out to answer the question: Did Mars ever have the right environmental conditions to support small life forms (microbes)?
Early in its mission, Curiosity's scientific tools found chemical and mineral evidence of past habitable environments on Mars.
It continues to explore the rock record from a time when Mars could have been home to microbial life.
Curiosity Mars Rover Drilling Sample (2013-02-20)NASA
The history of Martian climate and geology is written in the chemistry and structure of the rocks and soil. Curiosity explores a site called Gale Crater and acquires rock, soil and air samples for onboard analysis. It hunts for special rocks that formed in water and/or have signs of carbon-rich organic chemistry.
The rover reads this record by analyzing powdered samples drilled from rocks. It also measures the chemical fingerprints present in different rocks and soils to determine their composition and history, especially their past interactions with water.
NASA's Mars 2020 Rover Artist's Concept (2017-11-17) by NASA/JPL-CaltechNASA
NASA's next rover, slated for launch in 2020, will seek to determine whether life may have left telltale signatures in the rocks on Mars' surface -- a further shift to the current science theme "seek the signs of life."
The rover will have a drill that can collect samples of the most promising rocks and soils and set them aside in a "cache" on the surface of Mars.
A future mission could potentially return these samples to Earth. That would help scientists study the samples in laboratories with special equipment that would be too large to take to Mars.
Fossil River Delta on Mars (2003-11-13) by NASA/JPL-Caltech/MSSSNASA
Finding fossils preserved from early Mars might tell us that life once flourished on the Red Planet.
We can search for evidence of cells preserved in rocks, or at a much smaller scale: compounds called biosignatures are molecular fossils, specific compounds that give some indication of the organisms that created them.
Mars Ocean (2015-03-05) by NASA's Goddard Space Flight CenterNASA
On Earth, all forms of life need water to survive. It is likely, though not certain, that if life ever evolved on Mars, it did so in the presence of a long-standing supply of water.
On Mars, we will therefore search for evidence of life in areas where liquid water was once stable, and below the surface where it still might exist today.
Mars-like Environment on Earth (2017-04-20) by NASA/JPL-CaltechNASA
So far, however, the kinds of biosignatures we know how to identify are those found on Earth. It's possible that life on another planet might be very different.
The challenge is to be able to differentiate life from non-life no matter where one finds it, no matter what its varying chemistry, structure and other characteristics might be.
Life detection technologies under development will help us define life in non-Earth-centric terms so that we are able to detect it in all the forms it might take.
For more information about NASA's Mars Program, visit https://mars.jpl.nasa.gov.