On September 10, 2025, NASA announced (again) that it had found potential evidence for life on Mars. A few people asked me about it, so I’m dusting off (and updating) this Quora post from March 2014 on the subject.
First, let’s discuss the most recent find, from a rock outcrop investigated by Perseverance in July 2024, last year. Speculation about this data, which is uploaded daily, has been rife in the community. Now the paper in Nature outlines the extraordinary efforts taken by the Mars science community to attempt to explain these features without reference to biology.
The image below, taken from NASA’s website (scroll to sol 1188), shows tiny millimeter scale features on a sedimentary mudstone. Detailed spectral analysis finds metabolic gradients and a variety of iron sulfide minerals consistent with biological processes. For more details, check out the paper!
Regular readers will not find me shocked by this discovery. The fashion since the Viking landers has been to treat any evidence of Mars life with extreme (and not entirely unjustified) skepticism, but I Want To Believe.
So what are the other lines of evidence?
Despite the problem of extreme inaccessibility and arguments over what life even is, I think there are a few lines of evidence suggesting life on Mars, at least in the distant past.
Habitability
We know that Mars in the past, and probably even today, has environments that could support life, and that gave birth to life on Earth. Now, no-one is sure how life got started, but it seems to have been some sort of self-complicating electro-chemical metabolic reaction. There is no compelling reason why it could not have formed on Mars. In fact, we now know that Mars had liquid water about 100 million years before Earth. Its lack of magnetic field, low gravity and high levels of volcanism meant that its tallest mountains were relatively exposed to solar radiation, creating the sorts of chemical gradients (eg Manganese 6+) in volcanically heated liquid water on the slopes of the peaks. Mars, unlike Earth, had the chemical complexity back then to support the metabolism-first theory of abiogenesis.
Allan Hills 84001 meteorite
A famous meteorite found in Antarctica, which originated on Mars. In August 1996, NASA announced evidence for life in this meteorite, following four lines of evidence. Three of these (nanostructures resembling bacteria, putative nanofilms, and polycyclic aromatic hydrocarbons) have since found adequate non-biological explanations.
The fourth line of evidence consists of large numbers of exceptionally regular single domain truncated hexa-octahedral magnetite crystals, of an isotopic composition and regularity that has never been observed or recreated abiotically in the lab, but which occur naturally in every magnetotactic bacteria found in every puddle on Earth.
Bacteria that live in water have to swim around and navigate in 3D – a challenging task even for humans and fish. Magnetotactic bacteria precipitate tiny magnets within their cell that cause them to align with Earth’s magnetic field, and reducing their 3D navigation problem to a 1D one. They swim using flagella (tiny electric motors) and can reverse direction using an idler gear (yes really!) depending on the local chemical or light gradient.
This ability to produce biological magnets for navigation persisted into animals and has been confirmed in fish, birds, various mammals, and humans.
Here are the 50 nm magnetite crystals from MV-1, a magnetotactic bacteria found on Earth.
Here are the 50 nm magnetite crystals from ALH84001, a chunk of Mars.
You be the judge!
Yamato 000593 meteorite
Another meteorite which originated on Mars. This Nakhlite was also found in Antarctica and has micro-tunnels cutting into faces of crystal grains. Identical features on Earth are associated with bacterial weathering, and the meteorite’s tunnels are full of carbon residue. Like ALH84001, there is speculation that such features could form abiotically, but it seems statistically unlikely.
Complexity of life on Earth
If life began on Earth, it would seem likely that the simplest lifeforms would still exist somewhere. Instead, we find that the last universal common ancestor (LUCA) of all extant life on Earth was already a rather sophisticated organism, with DNA, ribosomes, and surprisingly complete biochemistry.
This chart shows a summary of all the biochemical metabolic pathways in lifeforms, with green unique to plants, red to fungi, and black common to all organisms. What are the odds that the most primitive ancestral form of life has all this stuff, and that all this stuff is *just enough* to survive desiccation and a flight on a rock through space, say, four billion years ago.
What we see is a precipitous decline in diversity before the LUCA, somewhat like, say, penguins in Antarctica. If the only birds we’d ever seen were penguins in Antarctica, we could infer that they had somehow evolved from local fish, or that they were descended from a related species that was able to fly there from somewhere else.
Either all antecedent forms of life became extinct or were eaten, possibly because their metabolisms were exponentially less able, or else we’re seeing a founder radiation effect. That is, there is no evidence of simpler bacteria and other lifeforms on Earth because they never formed here, lived here, or could get here from Mars (or some other place).
It seems Mars had stable surface water about a hundred million years before Earth did, and life might have arisen there first, then been sprayed throughout the solar system during the late heavy bombardment.
Methane plumes on Mars
Mars has seasonal plumes of methane. We’re not certain where they originate but methane is not stable in the atmosphere, so they must be regenerating. Maybe Mars has oil? Methane can form when warm water reacts with certain minerals in a process called serpentinization. But usually methane is associated with biological processes.
Conclusion
None of these six lines of evidence constitutes a smoking gun. With a bit of luck and a lot of work, evidence may continue to strengthen in future. OTOH, life may still survive today on Mars but buried so deep in the crust we can never find it.
Mars Sample Return was intended to bring back some of Perseverance’s samples, but is currently in limbo. See my related post on how it might be done.