On Thursday afternoon I didn't go to very many talks, but I made sure to get into the astrobiology session for one talk with the very provocative title "Meteoroid Transfer to Europa and Titan," given by Brett Gladman, who is not an astrobiologer (he's a solar system formation guy, somebody who spends lots of time crunching numbers and differential equations to figure out how you form solar systems and how big and small bodies migrate around inside them).
Gladman began, "Obviously we know that one can transfer meteoroids between planetary-scale objects" because of the discovery of lunar and Martian meteorites on Earth. Meteoroid transfer occurs when a very big impact happens on some body, big enough that it can toss ejecta out of the impact zone at a speed faster than the escape velocity of the object. It's clearly easier to launch stuff from objects with lower gravity, where the escape velocity is lower; smaller impacts will do the job. It's reasonably easy to launch stuff from the Moon, less so but still quite likely for Mars; Earth is a much tougher proposition because of its much larger mass. (The atmosphere isn't that important because impacts big enough to launch things into space on Earth happen when bodies hit that are big even on the scale of the thickness of Earth's atmosphere, 10 or 20 kilometers or more.)
So, Gladman said, he didn't expect to see much Earth material moving around when he tried to simulate Earth impact ejecta moving through space: "When I set out to do this project, I didn't think I would get the result I ended up with. Yet we are looking at the transfer of terrestrial impact ejecta throughout the solar system." He showed some graphs representing the result of a simulation of an impact of the scale of the Cretaceous-Tertiary (K-T) impact event. "For the K-T impact scale, you are looking at 6 Â· 108 meteoroids launched," or 600 million rocks that leave Earth's surface and go into space, at least briefly. These rocks would be a few centimeters to 10 meters in diameter, and could contain some interesting stuff in their interiors -- Earth rocks are just infested with microorganisms. "Where do these fragments go? How long does it take? Could transfer [to another solid body] be fast enough to avoid the destructive effects of space like cosmic ray damage and dessication?"
Gladman's study involved huge simulations and years worth of computer time. "We looked at 20,000 particles at 3 different launch speeds over 5 million years." The launch speeds he considered were 5, 8 and 10 kilometers per second. For low launch speeds, "You just barely get away from the Earth, and you're stuck in Earth-crossing orbits." These kinds of things don't go very far away, and usually re-impact Earth. "But 10 kilometers per second is much more likely for terrestrial impacts." What happens to this fast-moving stuff?
"Ejected material proceeds to scatter off of the four terrestrial planets." By "scatter," Gladman meant that a reasonably close encounter with another terrestrial planet -- Mercury, Venus, Earth, or Mars -- can radically alter the shape of its orbit. "There's nothing for it to do but to hit a terrestrial planet or scatter off the terrestrial planets. Some material works its way to Jupiter- and Saturn-crossing orbits. Saturn and Jupiter are big gorillas and they are efficient at ejecting stuff out of the solar system," never to return. "But during that scattering some material will fly close to the [giant planet] satellites and have the potential to hit them."
This is where it gets interesting. We've been looking very hard for "habitats" elsewhere in the solar system -- places where the environmental conditions could support life -- and have found possibilities at Europa, Titan, and Enceladus; other icy satellites could also be possibilities, especially in warmer and more active geologic pasts. Looking at the Jupiter system, Gladman said, "You get on the order of 100 objects striking each Galilean satellite." So for each and every large Earth impact, you get roughly 100 rocks hitting each of the four moons of Jupiter. Interesting, no?
"So you can hit Europa. Do you have to hit a crack if you want to get to subsurface water?" In other words, supposing an Earth ocean-dwelling microorganism survived the trip through space to Europa, could it land in a place where it could get to that habitat? I should note here that it's pretty much been conclusively demonstrated that microbes can survive being launched into space by an impact through a process called spalling, where the shock waves of the impact can launch material at very high speeds without heating it. Frighteningly, the research to answer this question is often funded by the Department of Defense, who care very much about the answer to the question of whether microbes can survive being launched at high speeds -- i.e. on the tip of a rocket launched from an unfriendly country. And other research has demonstrated that such microorganisms can survive at least some time spent in space, particularly if they are well sheltered within the mass of a large rock. But unfortunately for our Earth bugs launched into space toward Europa, that's not the biggest problem they face, according to Gladman.
"The problem turns out to be that the impact speeds are uncomfortably high. Typically 25 kilometers per second with a minimum near 8 and a maximum near 40. This would be very frustrating, if you were a bacterium that survived launch from Earth, only to perish once you hit the surface of Europa." (That comment got a chuckle from the audience.) "Survival of intact microbes or even proteins is unlikely" at such high speeds -- the stuff would just vaporize on impact. "So you can get there but you arrive lickety-split and that's not very promising" for the transfer of life. Looking at the Saturn system, "Enceladus has similar problems to Europa" in terms of impact speed "but it's rather hard to hit, but you only get about 1 impact onto Enceladus" for each Earth impact event.
"But Titan is a different story. It's very large, and it's astrobiologically interesting all over, not just in cracks in the ice. You get on the order of 30 meteorites delivered to Titan in a few million years. And Titan is out far enough from Saturn that the gravitational focusing isn't as bad -- Saturn isn't as big a bully as Jupiter -- and you get to the top of Titan's atmosphere with lower velocity, 5 to 20 kilometers per second. And Titan's atmosphere is an aerobrake. Arriving terrestrial rocks decelerate and fragment, and the fragments free-fall to the surface. As long as the atmosphere is persistently there, it's a nice safety net for the asteroids to reach the surface. One question: Can Titan's surface 'use' terrestrial material, integrated over the entire early history of the solar system? Titan, we have reason to believe, hasn't been exactly like it is now over the last 3 billion years."
That's food for thought -- could Earth have seeded Titan with microbial life? If Gladman's simulations are correct, the material has definitely gotten there in the past. Gladman added, in conclusion, that "if you ever had atmospheres on any of the [presently] airless satellites, they could have acted as aerobrakes" just like Titan's would today."
The astrobiologists in the session clearly enjoyed this talk. One stood up to ask, "Earth is tropical compared to Titan. What do the cool temperatures on Titan imply" for the survivability of microorganisms that came from Earth's tropics to Titan's arctic conditions? Gladman laughed and said, "That's for you guys to figure out; I'm just the pizza delivery boy." He has shown that Earth rocks and everything they contain can feasibly get to Titan -- what happens when they arrive there is another research project altogether.