The upcoming suicide plunge of NASA's Cassini Saturn probe is an act of noble self-sacrifice.
Cassini is running out of fuel, so the end of its historic mission was looming regardless. But controllers are steering Cassini to a fiery demise in Saturn's cloud tops early Friday morning (Sept. 15), chiefly to protect two of the ringed planet's most intriguing moons — Titan and Enceladus.
"Protect" in this case means to keep pristine. Cassini's handlers want to avoid a scenario in which the spacecraft, drifting through the Saturn system beyond their control, ends up spiraling down to the surface of either moon. [Cassini's Saturn Crash 2017: How to Watch Its 'Grand Finale']
The Cassini spacecraft has been in space for 20 years, but microbes are tough, and some may still be alive aboard the probe. And if any of these little Earth creatures managed to reach Titan or Enceladus, they might end up surviving and proliferating there: Cassini has revealed that both moons may be capable of supporting life.
Here's a closer look at each of these faraway worlds, which scientists are eager to explore in much greater depth.
With a diameter of 3,200 miles (5,150 kilometers), Titan is the second-largest moon in the solar system. (The Jovian satellite Ganymede is a smidge bigger.)
Titan looks like a fuzzy orange ball from space, because it has a thick, nitrogen-dominated atmosphere that blocks views of the surface. However, Cassini peered through this Titanic haze with radar and found something remarkable: seas and lakes of methane and ethane, as well as associated channels that carry hydrocarbon rain into these bodies.
Titan is the only world besides Earth known to harbor stable liquid on its surface. Cassini also spotted lots of organic chemicals — the carbon-containing building blocks of life as we know it — in Titan's air.
Some astrobiologists therefore believe that life may teem on Titan's surface, though if it does, it will be very different than the liquid-water-dependent creatures that populate Earth.
Speaking of liquid water: Titan likely has a great deal of that stuff, too, in an ocean buried deep beneath the moon's frigid crust. So it's conceivable that Titan could host two different types of life — one on the surface that depends on liquid hydrocarbons as a solvent, and another underground that needs water.
Researchers hope to learn more about this bizarre world before too much longer. Mission concepts under development by various teams include a Titan orbiter, a drone that would study the moon from both land and air, and a robotic submarine that would ply the moon's exotic seas. (The first two ideas are among 12 proposals that NASA is considering for its next New Frontiers mission — the same class of project that includes the New Horizons Pluto probe and Juno Jupiter orbiter. The agency is expected to announce two or three finalists by the end of the year and the selected mission in 2019.)
The Cassini orbiter also carried a piggyback European lander called Huygens, which dropped onto Titan in January 2005, pulling off the first-ever soft touchdown on any object in the outer solar system. (The $3.2 billion mission's official name is Cassini-Huygens, and it's a joint effort of NASA, the European Space Agency and the Italian Space Agency.) Huygens gave humanity its first, and so far only, up-close images of Titan's otherworldly surface. [Landing on Titan: Pictures from Huygens Probe on Saturn Moon]
You may be wondering why the Cassini team is bothering to protect Titan, since the mission already landed a spacecraft there. Well, the Cassini orbiter has shown, over the course of 127 close Titan flybys, that the moon is more habitable than scientists had previously thought. So, better safe than sorry. And there's also Enceladus to worry about.
Bright, shiny Enceladus is just 313 miles (504 km) across — about one-tenth as wide as Titan. But Enceladus packs a lot of action and intrigue into that small package.
In 2005, Cassini spied geysers of water vapor, organic compounds and other material blasting from "tiger stripe" fractures near Enceladus' south pole. Later observations by the Saturn orbiter strongly suggested that this water is coming from a big ocean of liquid water beneath the moon's icy shell — and that this ocean likely has hydrothermal "hotspots" that could provide energy for life, if any exist in the dark depths.
"Enceladus has no business existing," Cassini program scientist Curt Niebur, who's based at NASA headquarters in Washington, D.C., said during a news conference late last month. "And yet there it is — practically screaming at us, 'Look at me! I completely invalidate all of your assumptions about the solar system!'"
Niebur meant that a moon so small and so far from the sun seemingly has no business harboring a possibly habitable ocean — it should have frozen through long ago. But strong gravitational tugs from Saturn and some of Enceladus' fellow satellites create enough internal heat (from friction) to keep this water sloshing.
So Enceladus is in astrobiologists' crosshairs as well. Indeed, two of the 12 New Frontiers candidates target the icy moon. One of these mission concepts, known as Enceladus Life Finder (ELF), would fly through the icy moon's plumes, then look for complex organic molecules — some of which could theoretically even be signs of life — among the captured particles. (Cassini did some such analyses, but ELF would sport more advanced and specialized instruments). Not much has been publicly revealed about the other concept, which is called Enceladus Life Signatures and Habitability.
"These two new worlds, Titan and Enceladus, which were so completely revealed to us by Cassini, have changed the idea that ocean worlds like Earth and [Jupiter's moon] Europa are rare in the universe," Niebur said during last month's news conference. "And this in turn is changing our views about finding [habitable worlds] and about how prevalent and common habitable environments and even life beyond Earth might truly be."
Visit Space.com for complete coverage of Cassini's crash into Saturn on Friday, Sept. 15.