The first alien life may be found close to home

The Short Version

Three scientists who search for life in different ways all spend a good part of their attention on worlds we can already reach: Mars, Venus, Europa and Titan [1][2][3]. Nathalie Cabrol explains what a trace of ancient Martian life would look like and why it may survive underground, David Kipping weighs the disputed hints on Venus and Mars and the ethics of touching Europa's ocean, and Michelle Thaller describes the mission planned for Titan and the asteroid sample that came back carrying the building blocks of DNA [1][2][3]. None of them promises that the first discovery will be local, and two of them say outright that a planet around another star could get there first [2][3]. What the nearby worlds offer is something no telescope can: the chance to land, sample, repeat a measurement and rule out the non-biological explanations one by one [1][2][3].

Table of contents

What They Said

Nathalie Cabrol: what Mars could still be hiding

Cabrol, who directs the Carl Sagan Center at the SETI Institute, built her career on Martian water [1]. Her PhD was on water on Mars, and she later proposed that it had pooled as lakes inside impact craters, at a time when talk of standing water there made people wave their arms [1]. As missions began to ask whether Mars had been habitable, she says, "lakes are particularly good places to look for those questions" [1].

She is specific about what could be left [1]. Mars is smaller than Earth and lost its magnetic field and atmosphere quickly, so conditions at the surface began to degrade less than a billion years after the planet formed [1]. On Earth it took 82 percent of geological history to go from simple microbial life to complex animals, so if life appeared on Mars, "It would have been simple life" [1]. Everything at the surface would have disappeared except, perhaps, two kinds of trace [1]. One is morphological: structures like stromatolites, which she describes as "rock formations that are built by microbes" and which scientists know how to recognize [1]. The other is chemical [1]. Rovers have found an abundance of carbon-12, the form of carbon life on Earth favors, and methane, but she notes that non-biological processes can produce both, so they are intriguing and not proof [1]. That is why rovers are sent to ancient lakes, impact craters and very old rocks [1].

Cabrol also thinks the story may not be over [1]. She believes there was as much chance of life originating on Mars as on Earth, perhaps even earlier since Mars may have had water sooner [1]. If it did arise, it may persist underground, possibly not far below the surface, where ice thaws during climate shifts [1]. To find it, she says, "I have to become the microbe": think about shelter, water and energy at the microbe's scale, like the endoliths on Earth that live inside translucent rocks that block ultraviolet light [1]. She proposes a network of environmental stations on Mars to map habitability at that scale, help with planetary protection and support future astronauts [1].

Her method for learning where to look is to visit the closest thing to early Mars on Earth [1]. Since 2002 she has climbed Andean volcanoes around 20,000 feet, where thin atmosphere, intense ultraviolet light, volcanic activity and evaporation recreate Mars as it was 3.5 billion years ago; she calls it "a time machine" [1]. She expected to find nothing alive there, and 20 years later her team is still studying what they found: the same kinds of microorganisms that made Earth's first fossils [1]. Her conclusion from the field is short: "life finds ways" [1].

She takes the idea of looking close to home one step further than anyone else in these conversations [1]. She likes the "shadow biosphere" proposal, that life may have arisen more than once on Earth itself in forms so different that our tests cannot detect them, because it forces her to rethink how to search, though she does not claim it is true [1].

David Kipping: hints on Venus and Mars, and a pristine ocean on Europa

Kipping, an astronomer at Columbia University whose own research is on planets and moons around other stars, gives a careful tour of the local candidates [2]. He starts with what counts as a sign of life [2]. Oxygen was long treated as the key signal, since without photosynthesis Earth would lose its oxygen within about a million years [2]. But most of Earth's history had life without oxygen, and oxygen can appear without life, for example when ultraviolet light splits water vapor and the hydrogen escapes to space [2]. Researchers now also consider nitrous oxide, methane combined with other gases, and phosphine [2].

Phosphine brings him to Venus [2]. He discusses the claim, made about two years before the interview, that phosphine had been found in Venus's atmosphere, and says the signal may instead have been sulfur dioxide [2]. Venus's surface is hellish, but its cloud layer has Earth-like temperature and pressure, and he credits the claim with helping to spur NASA's VERITAS and DAVINCI missions, along with possible European and Chinese ones [2].

On Mars, he says the methane that has been seen could be biological or could be leaking seasonally from an underground reservoir [2]. He is in awe of the landings themselves: because of the light-travel delay no human can steer the descent, so the whole multi-stage sequence of parachute and sky crane runs on its own while engineers wait to learn whether it worked [2].

Europa raises a problem the others do not mention [2]. No one knows how thick its ice is, and Kipping sees a real ethical question about contamination: it is almost impossible to sterilize every microbe on a lander, and the hidden ocean is probably pristine [2]. His suggestion is in-place micro-laboratories that analyse on the spot and send data home instead of returning samples [2].

Michelle Thaller: Titan, Venus and what fell from Bennu

Thaller, an astrophysicist and retired NASA executive, describes the neighborhood with obvious affection [3]. Saturn's moon Titan has a thick atmosphere, organic molecules, evidence of water beneath its surface, and lakes, rivers and rain made of liquid methane and ethane; she speaks of "great lake-sized lakes on Titan" [3]. Humans have already put a probe on its surface as part of the Cassini mission, and she says some of the readings it took before it froze were intriguing about the possibility of life there [3]. NASA hopes to launch Dragonfly, an octocopter drone that would fly from site to site on Titan, around 2028 [3].

Venus is the hard case [3]. She notes that only the Soviet Union has landed there, more than once, on a surface of about 1,000 degrees under pressure like being a mile underwater, beneath clouds of sulfuric acid [3].

Her strongest evidence that the raw materials are close by came back in a capsule [3]. NASA's OSIRIS-REx probe reached the asteroid Bennu, found it covered in boulders, and had to be reprogrammed in flight to navigate on its own and collect dust from small craters [3]. The sample, parachuted into Utah, contained all the nucleobases of both DNA and RNA, plus others life on Earth does not use, and signs that the asteroid was once soaked in water [3]. She takes it as evidence that the building blocks of life rain down from space [3].

Thaller is frank about her own expectations and how they have shifted [3]. Everyone she worked with at NASA thinks there must be life out there, and she says, "I always keep a bottle of champagne chilling" for clear evidence [3]. Asked what would make her open it, she answers "Definitely bacteria," and adds that pond scum or "some little microbe on Mars" would do [3]. But describing a disputed James Webb Space Telescope result that hinted at molecules resembling what plankton produce, she says she would never have thought the first hard chemical evidence might come from a planet around another star: "I thought we'd maybe find it on Mars or on some of the moons of Jupiter and Saturn" [3].

Where They Agree, and Where They Don't

The pattern across the three is a shared list and a shared caution [1][2][3]. Mars appears in all three conversations, Venus in two, and each scientist adds an icy or hazy moon: Europa for Kipping, Titan for Thaller [1][2][3]. All three treat current hints as unproven [1][2][3]. Cabrol says the carbon and methane on Mars are not proof, Kipping says the Venus phosphine may have been sulfur dioxide and Mars methane may be geological, and Thaller reports that other scientists judged the exoplanet result by saying "the data is not good enough yet" [1][2][3].

They also agree on what kind of life to expect nearby: microbes [1][3]. Cabrol's reasoning is that Mars did not stay habitable long enough for anything more, and Thaller's champagne is reserved for bacteria [1][3].

Where they part is on whether the first detection will in fact be local, and this cuts against the simple version of the headline [2][3]. Kipping predicts that any near-term biosignature detection will come from a planet around a red dwarf star, not from a true Earth twin, and the sources do not record him ranking that against a discovery in our own solar system [2]. Thaller now thinks an exoplanet atmosphere could plausibly deliver the first evidence, where she once assumed Mars or the moons of Jupiter and Saturn [3]. Cabrol is the one most invested in Mars, and even she says her interest has moved from whether there is life on Mars to the origin and nature of life in general [1].

The reason the views differ is the kind of evidence each trusts [1][2][3]. Thaller is impressed by what spectroscopy can now do: starlight passing through a planet's atmosphere can be split into a rainbow in which every element and molecule leaves its own fingerprint [3]. Kipping, who works with that data, stresses its cost: detecting a marginal biosignature on the planet TRAPPIST-1e with the Webb telescope would take on the order of 200 transits, roughly four years of staring at one star, which he says will never happen on a shared telescope [2]. Cabrol works with rocks and living microbes and thinks in terms of what a rover's instruments can resolve [1].

There is a smaller difference over Venus [2][3]. Thaller presents it only as a brutal surface, while Kipping points to the cloud layer as the place with Earth-like temperature and pressure [2][3]. These are two altitudes of the same planet and not a conflict [2][3].

Why It Works

The case for looking nearby rests on being able to check [1][2]. Cabrol explains that Viking's life-detection experiments in 1976 produced a strange signal that some people still read as life, though most of the community thinks it can be explained by chemistry [1]. The landers arrived without the geological context needed to interpret what they saw, and the lesson NASA drew was to understand the environment first, because "life and environment co-evolved" [1]. Decades of return visits have followed, each able to choose its landing site in the light of the last [1].

That return access is what makes her "ladder of life detection" possible: a sequence of tests that makes non-biological explanations progressively less likely [1]. She jokes that "the only way we would know that there is life on Mars if there was a rabbit jumping in front of the rover," which is her way of saying that a single observation will not settle it and that the work is cumulative [1].

Kipping supplies the contrasting mechanism for distant worlds [2]. Besides the telescope time required, a future image of an Earth-like planet would blend planet and moon into a single pixel, so methane from a Titan-like moon combined with oxygen from water-splitting on a lifeless planet could fake a biosignature [2]. A lander on Mars or a drone on Titan does not have that problem [2][3].

The ingredients argument supports all of it [1][3]. Cabrol says asteroids and comets carry carbon, hydrogen, oxygen, nitrogen and phosphorus and deliver energy when they strike, which makes impact craters creative as well as destructive [1]. Thaller's Bennu sample is a direct measurement of that delivery [3]. And Cabrol's Andean microbes show that organisms of the earliest known kind can live in conditions resembling early Mars, including bacteria that keep their ultraviolet protection switched on constantly at altitude [1].

Where It Applies, and Where It Doesn't

The sources support a narrower claim than a prediction that the first alien life will be found close to home [1][2][3]. They show that serious scientists are looking nearby and why that search has advantages; they do not show that it will win the race [1][2][3]. Readers should hold the headline as a possibility [2][3].

On Mars, the most that is expected is traces of simple life or microbes below the surface, and Cabrol says proof will be hard because we do not know what life is [1]. On Venus, the leading hint is disputed [2]. On Europa, the obstacle is partly ethical, since exploring the ocean risks contaminating it [2]. On Titan, the liquid is methane and ethane, not water, and the intriguing readings Thaller mentions are not described in detail in the sources [3].

Finding ingredients is not finding life: Cabrol stresses that the delivery of building blocks by comets and asteroids distributes ingredients and does not explain life's origin [1]. She also cautions that our tests are built to recognize life as we know it [1].

Mission plans and findings change [1][2][3]. Dragonfly's launch date, the status of the Venus missions, the phosphine debate, and the Mars carbon and methane results are all given as they stood when these conversations were recorded, and readers should check the current position [1][2][3]. Cabrol adds a practical warning for anyone tempted to treat nearby worlds as a refuge: "the Moon or Mars is not going to save your butt" [1].

What To Do With It

1. When a possible sign of life on Mars is reported, ask which rung of Cabrol's ladder it reaches and which non-biological explanations remain [1].
2. Do not treat one gas as decisive; Kipping notes oxygen can arise without life, and Cabrol says the same of carbon-12 and methane [1][2].
3. Expect any Martian discovery to be microbial, a rock structure or a chemical trace, and look for missions that go to ancient lakes, craters and old rocks or below the surface [1].
4. Follow the missions named in these conversations: Dragonfly to Titan, and VERITAS and DAVINCI to Venus, checking their current schedules [2][3].
5. Watch how Europa is approached, and whether missions use in-place analysis of the kind Kipping suggests to avoid contaminating its ocean [2].
6. Keep an eye on returned samples such as Bennu's, which Thaller cites for nucleobases and signs of past water [3].
7. Read exoplanet biosignature headlines with both sides in mind: Thaller's excitement about what Webb can now see, and Kipping's point about how much telescope time a firm detection would need [2][3].
8. Learn from Earth's extreme environments, as Cabrol does, since they show what kind of life survives Mars-like conditions and what instruments are needed to detect it [1].
9. Apply Thaller's standard to every claim, local or distant: "You have to be honest about what you don't know" [3].

Sources

1. Nathalie Cabrol on the Search for Alien Life, Diving in Volcanic Lakes and Surviving an Earthquake at 20,000 Feet · Lex Fridman Podcast
2. David Kipping on Exomoons, Habitable Worlds and Why We Must Stay Agnostic About Aliens · Lex Fridman Podcast
3. Michelle Thaller on Black Holes, Time, Quantum Weirdness and the Limits of Science · The Joe Rogan Experience