Forget the thousands of unreachably far-away planets we keep finding. When it comes to extraterrestrial life, each year new scientific discoveries make our very own Solar System look soggier, more hospitable, and more ripe for it. Chris McKay, a senior planetary scientist at NASA’s Ames Research Center, explained his top four candidates to hunt for life outside Earth.
If you’re hunting for life outside Earth, then Enceladus—Saturn’s 6th largest moon—is without doubt your first stop, McKay says. The reasons are abundant: Last year, scientists discovered that this ice-ball of a moon has an enormous subsurface ocean deep under its southern pole. That ocean connects to the moon’s surface with active, spurting geysers. And just last month, astronomers also revealed that the ocean’s waters are at least partially warm—and studded with hydrothermal vents, like Earth’s own life-friendly Lost City Hydrothermal Field in the Atlantic. All those discoveries together point a hopeful finger toward life.
But the argument for life on Enceladus gets even better. McKay says scientists have also investigated the makeup of the moon’s ocean with NASA’s Cassini probe—by examining the geyser water that had been sprayed into space. To scientists’ delight, they found almost everything we imagine life would need. “We’ve identified organic molecules and we’ve identified molecules that could provide energy and nutrients,” McKay says. “It’s practically chicken soup!”
Some scientists have argued that perhaps Enceladus’s oceans are fairly recent phenomena in our solar system, and haven’t been around long enough for life to generate. But McKay says that new research continually suggests the ocean may actually be much older than we originally thought, “and, frankly, we don’t know if life takes 5 minutes or 5 million years to get started,” he says. McKay, who is pushing for life-hunting missions throughout the solar system, says, “Enceladus is almost too good to be true.” A satellite sent with today’s technology could comb through the geyser spray to detect life, and a lander could slip down through a geyser to the uncharted oceans below.
Finding living organisms on Mars is a pretty grim prospect, says McKay—our best bet is still an extraordinarily slim one. Although we already know that Mars’ surface is a desolate wasteland, there’s a working theory that microbial life could live in puddles of hyper-salty mud buried under the red planet’s crust. Those microbes could, theoretically, eat the atmospheric carbon monoxide (a gas that’s deadly to humans) that seeps in.
But these hypothetical, hardy microbes are not why McKay has Mars at the number 2 spot. “I don’t think we have a good chance of finding anything alive,” says McKay. “What we’d likely find is something dead. And that’s just as exciting.”
We think we have a great shot at some astro-paleontology on Mars because “our understanding now is that early Mars, a few billion years ago, was the most Earth-like environment in our solar system. It had flowing water, a thick atmosphere, [and] a [protective] magnetic field” to shield any potential life from space radiation. Sadly, over time, Mars lost its atmosphere and magnetic field—and with those went the surface water we think life needs. Nonetheless, “our best bet might be finding leftover, frozen remains of microscopic life” under the planet’s icy poles, McKay says.
So why exactly is dead life just as exciting as something alive? “Even if we found something living on Mars … we’d have to kill it to analyze it anyway,” McKay says. And even finding dead microbes could answer many questions about how, when, and if life spread throughout the solar system.
Europa—Jupiter’s 4th largest moon— is McKay’s number three choice for one big, wet reason: “It’s got liquid water, and lots of it,” he says. But Europa’s apparent habitability starts to waver after that point. For one, the moon’s water is locked tight in an deep ocean under a thick, icy shell, and, unlike Enceladus, there doesn’t seem like any obvious way to get to it—so it’s hard to say what’s down there.
“We know water is the most important aspect for ecosystems here on Earth,” McKay says, “but we can’t say with certainty that Europa’s oceans are Earth-like by any stretch of our imaginations … [chiefly] because we don’t actually know if the ocean contains either energy sources or nutrients” for potential life-forms.
And the same barrier stemming our assessment of Europa’s deep waters will also hinder any immediate future mission to hunt for life. There are currently proposals with madcap ideas like sending down an irradiated submarine that can melt through the ice-cover, but McKay says none of these can be realistically achieved with our modern technology. So, “Europa is really interesting, but in kind of a frustrating way,” he says.
Here’s where our search for life starts to get, well, weird. Titan—Saturn’s largest moon—is our fourth best bet because, McKay says, “it’s the only place outside Earth with beaches. Meaning, it’s a place where liquid ‘oceans’ meets a shore under an atmospheric sky.”
But no astronomer would confuse Titan with Tahiti. Titan lacks liquid water—instead, its oceans are made with liquefied ethane and methane, two chemicals that form gasses on the much hotter (by several hundred degrees) temperature of Earth’s surface. To humans (or bacteria or algae, or any other life we’ve ever found) Titan is a brutal hellscape. But McKay explains that life across the galaxy may not always require exactly what Earth-life needs.
“Granted, we have a hard time imagining how life could live without liquid water,” he says. But several scientists are currently experimenting and trying to figure out if liquid methane and ethane could replace the role water plays in Earth-life for life on Titan. We don’t know for sure yet, McKay says, but we also can’t say the prospect is impossible.
And finding life on Titan could be the most exciting discovery of life anywhere in the solar system. “When we talk about life on places like Mars, Enceladus or Titan, we imagine it’s possible that they could have shared the same origin as life on Earth,” McKay says. “Perhaps meteorites spread life from one planet [or moon] to another. But there’s no way we’re finding second cousins on Titan. Finding life there would radically change how we view the potential for life to exist throughout the universe.”