NASA's Mars 2020 Perseverance rover has a key role in the search for ancient life

As NASA's latest Mars-bound rover readies for launch, it's time to reflect on what we know about life on Mars. 

The search for life on the Red Planet presses on as NASA's Perseverance rover prepares to lift off this Thursday (July 30) to study the planet and search for signs of ancient life. But this isn't the first investigation into the habitability of ancient Mars, as previous orbiters, landers and rovers have taken significant strides in the search for life.

So, what do we know about (possible) life on Mars? Let's explore what previous missions and studies have uncovered about Mars and the creatures that may have once lived there. 

In photos: NASA's Mars Perseverance rover mission to the Red Planet

Early observations

There are records of people studying Mars dating all the way back to the second century BCE, when ancient Egyptian astronomers were observing planets, stars and constellations in the night sky. Later on, through the centuries, more and more detailed descriptions of the Red Planet began popping up, leading up to the first telescopic observation of the planet in 1610 by the Italian astronomer Galileo Galilei. 

With more advanced telescopic and orbital observations of the planets many years later, scientists learned that, up until about 3.7 billion years ago, Mars had a much thicker carbon-dioxide dominant atmosphere, which kept the planet warm and supported liquid surface water. 

While the planet's atmosphere was stripped away (scientists think that powerful solar storms could have been a major factor in the loss of atmosphere), researchers still think that it's very possible that microscopic life could have existed in the time that Mars was more "Earth-like."

However, they think that if life existed on Mars, it was most likely microscopic — not the green and gray aliens from science fiction. Researchers believe that if life evolved on Mars similarly to how it did on Earth, it could have happened more than 3.7 billion years ago, as microbial life is thought to have possibly existed on Earth as early as 4.1 billion years ago. 

Related: Ancient Mars lake could have supported life (photos)

Up-close revelations

After a couple of failed Mars missions from the Soviet Union, in 1976, NASA's Viking project landed the first spacecraft safely on another planet when Viking 1 and Viking 2 (both orbiter/lander combination missions) landed on Mars to search for signs of life. The missions ended up discovering unexpected chemical activity in the Mars soil but did not reveal any direct signs of past or present life of any kind. 

Subsequent landers and rovers continued to probe Mars' surface and uncover clues about the planet's history. For example, NASA's Spirit rover, which launched in 2003, found rocks unusually rich in magnesium and iron carbonates that had formed when the planet had a thick atmosphere and was still warm and wet. 

Spirit also found 90% pure silica, which usually exists on Earth in hot springs that are home to heat-loving microbes, and evidence of ancient volcanic eruptions of steam from underground water, which could have paralleled with extreme conditions on Earth.

NASA's Opportunity rover, which also launched to the Red Planet in 2003, made a number of discoveries related to the search for life, like clay minerals on Mars. Because the minerals must have formed in neutral-pH water, this strengthened the notion that the planet was once wet. Opportunity also found hematite, which usually forms in water. 

Findings like those made by Opportunity, which identify past or present water on Mars, are important because water is a key ingredient to life as we know it here on Earth. 

Following a slew of boundary-pushing probes, NASA's Curiosity rover took major steps forward in the search for life. In 2018, the vehicle discovered that methane on Mars changes seasonally. This is significant because here on Earth, the primary source of methane is life. 

In another major breakthrough, Curiosity also discovered organic molecules, or the carbon-based building blocks of life, on Mars. While the finding is not direct evidence of life, "there is a possibility that [the organics] are from an ancient life source; we just don't know,"  Jennifer Eigenbrode, a scientist at the Solar System Exploration Division at NASA's Goddard Space Flight Center in Greenbelt, Maryland told Space.com at the time of the discovery. "And even if life was never around, they [the molecules] tell us there was at least something around for organisms to eat." 

Related: The search for life on Mars: a photo timeline

Percy pushing boundaries

NASA and a variety of international space agencies have evolved our understanding of Mars and the possibility that life once existed on the Red Planet. Now, NASA hopes to expand that understanding even further with the new Perseverance rover, previously known as the Mars 2020 rover. 

Perseverance, set to launch to the Red Planet July 30 and arrive at Mars Feb. 18, 2021, has three major objectives: "The first is to seek the signs of life. The second is to collect and cache a suite of samples that a future mission could bring back to Earth. And the third is to test technologies," Ken Farley, a project scientist for Perseverance, said during a news conference Monday (July 27).

"What we are looking for is likely very primitive life. We are not looking for advanced lifeforms … we're looking by analogy to what we find in the similar time on Earth, microbial life," Farley told Space.com during the news conference. 

Among the new instruments on the rover is a tool known as SHERLOC (Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals). SHERLOC can observe simple features like color and texture but also detect what chemicals and minerals lie in front of it. SHERLOC, Farley explained, "will allow us to not only detect organic matter, but also map its distribution."

These detections will be bolstered by future studies conducted here on Earth with the samples taken by Perseverance that are set to be delivered with a future mission in 2031. 

Email Chelsea Gohd at cgohd@space.com or follow her on Twitter @chelsea_gohd. Follow us on Twitter @Spacedotcom and on Facebook.