Twelve years ago, a small spacecraft parachuted through a thick, extraterrestrial haze and drifted down toward an alien surface. It punched a small hole in the icy ground, bounced, slid, and wobbled.
After a few seconds, it came to rest in a damp floodplain on Titan, Saturn’s largest moon.
On January 14, 2005, the European Space Agency’s Huygens probe became the first robot explorer to touch the surface of this smoggy, orange moon and beam back detailed pictures. Huygens frantically gathered data for about an hour before its batteries died and its mothership, NASA’s Cassini spacecraft, disappeared over the horizon.
During its descent and those moments on the surface, the probe gathered enough data to give scientists a glimpse of an alien world that looks deceptively Earth-like.
Nearly 900 million miles from the sun, temperatures on Titan are so low that ice is as hard as rock and organic compounds like ethane and methane—which are normally gases on Earth—are chilled into liquids that pool and flow into enormous lakes and seas.
But with its mountains, rain, winds, and waves, 3,200-mile-wide Titan is really more like a planet than just another dead, cratered moon. It not only has those oily seas on its surface, but it also harbors a buried ocean of liquid water, making it among the best places to search for life beyond Earth.
“Titan is kind of a double ocean world,” said planetary scientist Sarah Hörst of the Johns Hopkins University. “In principle, there’s the possibility that it has both life as we know it and life as we don’t know it.”
Piercing the Haze
The Huygens probe hitchhiked to the Saturn system on board NASA’s Cassini spacecraft, which dropped off the lander not long after pulling into orbit around the ringed planet.
Though Cassini had been assigned the task of exploring Saturn and its many moons, that little probe—named after the Dutch astronomer Christiaan Huygens—had just one target in sight: Titan, which is shrouded in a puffy atmosphere that mostly obscures the moon’s surface.
After a 20-day journey to Titan, Huygens sank through the moon's atmosphere for more than two hours before landing on a frigid plain, where temperatures are nearly 300 degrees below zero, and gathering data.
“Huygens revealed Titan’s environment directly, whereas most of what was 'known' before was rather indirectly determined,” says the University of Arizona’s Ralph Lorenz. “It showed us the surface close up.”
Data showed us highlands, ravines, and channels carved by liquids, as well as winds whipping through Titan’s nitrogen atmosphere. Measurements from the surface showed the probe didn’t land in a dry desert, and that some kind of liquid was moistening the sand beneath it.
Huygens’ atmospheric measurements have since helped scientists reconstruct the composition of Titan’s ancient atmosphere (probably a combination of ammonia and methane) and study how organic molecules can behave in a predominantly nitrogen atmosphere, as Earth’s once was before life evolved and oxygen became abundant.
“It’s the only other substantial nitrogen atmosphere besides ours,” Hörst says. “Presumably, a lot of the chemistry in Titan’s atmosphere could have been happening on the early Earth, before oxygen.”
The Huygens mission marks the first and—so far—last time humans set a spacecraft on a moon other than our own.
A mission called the Titan Mare Explorer was once among NASA’s top picks for a future interplanetary adventure. Had it been selected, that mission would have sent a space boat to explore Ligeia Mare, one of Titan’s largest northern seas. Made of runny, liquid hydrocarbons, the moon’s surface seas could be home to life with a completely different chemistry than ours.
“That’s a test for a diversity of life: Could you also find life that uses a different liquid than life on Earth?” Hörst says.
In addition, humanity will soon lose its ambassador to the whole Saturn system: Cassini’s mission ends later this year, with a dramatic plunge into the giant planet. Still, even after they’re gone, data gathered by spacecraft like Huygens and Cassini can continue to offer new clues to our solar system and help scientists build the case for future missions.
With Titan, we’ve found a world that is both totally alien and yet somewhat familiar. Seasonal rains darken its plains, it’s rich in organic molecules, and a wintry vortex decorates its poles (complete with hydrogen cyanide clouds).
“Titan's appeal is its mix of the familiar and exotic,” Lorenz says. “Ultra-low temperatures, ices, organics, liquid methane—but making familiar rain, rivers, dunes, seas.”
Simply put, it’s a world that scientists would dearly love to get to know a little bit better, not only because of the excitement of exploring extraterrestrial terrains, but because it can teach us more about this planet we call home.
“Titan is so active and has so many Earth-like processes, it’s such a good test for our fundamental understanding of how planets work,” Hörst says.