When Carolyn Porco started exploring the outer solar system, it was all about the rings. Her 1983 doctoral thesis at Caltech focused on shifting spokes in Saturn's rings discovered by the Voyager spacecraft. As Voyager sailed past Uranus and Neptune, Porco led a group dedicated to their rings, too. So when she became the head of the imaging team for the Cassini satellite mission to Saturn, the gas giant's majestic rings were expected to be the highlight.
But in Cassini's seven years orbiting the great ringed planet, a new part of the system turned Porco's head. One of Saturn's icy moons, Enceladus, is spurting salty water full of compounds like propane, benzene, hydrogen sulfide and formaldehyde. The discovery vaulted Enceladus to the top of many astrobiologists' wish lists for the next place to look for life in the solar system.
Wired.com caught up with Porco to talk about microbe-filled snow and bringing pictures to the public.
Wired.com: What's coming up next for Cassini?
Carolyn Porco: We have lots of plans to monitor all the exciting things we've found so far: hydrocarbon lakes on Titan, activity that we've discovered at the south pole of Enceladus, looking for changes in the rings, storm activity.
We'll be going out to 2017, if we're lucky. That will be the height of northern summer. We arrived at the height of southern summer, so between the two hemispheres we’ll be able to see the effects of a full seasonal cycle at Saturn. That's just so scientifically fruitful. It will show us how this alien planetary system, very far from the sun, responds to seasonal forcing.
Wired.com: What's been the highlight so far?
Porco: Our most significant discovery is the activity at south pole of Enceladus, which points to a subsurface regional salty sea, laced with organic matter and emitting excess heat. It is the best, most accessible habitable zone we have in our solar system.
As far as I'm concerned, Enceladus has become the go-to place in our solar system for issues bearing on extraterrestrial life. It's a great place to examine extraterrestrial organic chemistry that is water-based, and therefore like biotic chemistry on Earth.
Wired.com: So if you had a lineup of Enceladus, Titan and Europa, which are always brought up as good targets for astrobiology, which would you choose?
Porco: Oh, Enceladus wins hands down. Titan has no liquid water on its surface and any liquid water beneath its surface is inaccessible to us, as far as we know. It has hydrocarbon lakes, but we don't know of any organisms that could live in those, not at the temperatures that we find on Titan. Any reference to possible life in lakes on the surface of Titan is pure speculation.
We do know of subsurface Earth ecologies that could thrive in the subsurface environment on Enceladus. Now that we know there's salty water there, that shows there's liquid water in contact with rock. Biotic chemistry could occur that we know exists in volcanic environments miles underneath Earth's surface, where liquid water percolates through hot rocks.
As far as Europa goes, Europa very likely has an ocean under its surface. In that regard, Europa and Enceladus are on equal par. But on Europa, the ocean is at least several kilometers under the surface and the moon is bathed in an intense radiation field. We can't go there and just drill several kilometers down because the intense radiation field would fry a properly equipped spacecraft in several months.
So while there could indeed be life within the ocean of Europa, it is presently inaccessible. The beauty of Enceladus is all you have to do is land on the surface, look up and stick your tongue out. It could be snowing microbes at the south pole. We would be foolish not to head back there immediately.
>"The beauty of Enceladus is all you have to do is land on the surface, look up and stick your tongue out."
Wired.com: What are the actual plans for going back?
Porco: This issue was examined recently, and NASA chose Europa. There’s a lot of political momentum behind a Europa mission. But, because of budgeting restrictions, it’s not clear we’ll be going there any time soon. I think Enceladus could be done for less expense, and is more promising. It's very possible to go back to Enceladus. I just hope we do it soon.
Wired.com: Aside from Enceladus, what are the big questions left for Cassini?
Porco: How does the surface environment on Titan change with the seasons? Titan is the one place in our solar system whose geographical and atmospheric diversity and complexity are rivaled only by the Earth's. It’s very Earth-like, but it’s also very different, which means we have a lot to learn as well as a prayer of understanding what we find.
Until Cassini got there, Titan was the largest single expanse of unexplored terrain that we had remaining in our solar system. Now we've basically seen all of it.
I can't help but think of Cassini in its historical context. It was the first orbiter we put around Saturn, the first time humans have touched something in the outer solar system with something of our own making. I think it's an opportunity for people all over the world to be high-fiving each other, saying "Yay for the humans!"