My first glimpse of the surface of Saturn's giant moon, Titan, sent from the Cassini mission's Huygens lander in 2004, blew my mind-- not because it looked so exotic, but because it looked so much like Earth. There they were: channels and debris flows, on a moon nearly a billion miles away.
This Earthly verisimilitude has an incredibly cool twist, of course. Titan's climate generally hovers around 93 Kelvin, so frozen water ice assumes the roles of silica-based rocks, pebbles, and boulders on Earth, while liquid methane and ethane act like water.
And that raises an interesting question. When rocks and stream beds are made up of water ice, how quickly do they erode? On Earth, or even Mars, the rate of erosion—the impacts as particles whack against a larger surface--is a function of rock tensile strength and elasticity. But ice is not your ordinary solid.
A few weeks ago, on a tip from my geomorphologist pal Jill Marshall, I headed out to UC Berkeley's Richmond Field Station in Richmond, Calif., where researchers were addressing that question in a phone-booth-sized freezer, by pelting a two-foot-diameter ice disk with ice rocks, under Titanesque conditions.
Peter Polito, a master's student in the lab of San Francisco State University geology professor Leonard Sklar, was gathering data in a mad rush before defending his thesis. This involved dropping an object on the ice thousands of times to simulate the erosion of bedrock. They were recording the volume of ice knocked off during the impacts using a laser scanning technique that measures small changes in the ice disk topography.
Peter's colleague Beth Zygielbaum, also a master's student in the Sklar lab, is examining how this tensile strength changes with temperature.
Creating a bit of Titan on Earth takes some preparation. The group had already discovered they couldn't just freeze a giant ice cube. In order to create a realistic sort of polycrystalline ice “sandstone,” the group ground up ice in a Sno Kone machine, and added nearly freezing water. To keep the ice at 93 Kelvin, the disk is surrounded by dry ice and iquid nitrogen snaking around the perimeter of the ice through copper tubing.
The enclosed setup can be treacherous, since oxygen is in short supply, what with all that dry ice and liquid nitrogen. There's a reason Peter makes sure to have assistants.
The day I showed up, unfortunately, the data-taking was on hold while Peter and his lab partners for the day–his wife Elizabeth Polito, and Kimberly L. Litwin, also master's students in Sklar's lab who will continue work on the project--were awaiting the delivery of 1000 liters of liquid nitrogen.
Peter tossed me a parka, saying “You’ll need it.” We stepped in the freezer to take a look at the frozen disk, which was already showing signs of erosion after over 5,000 impacts. After five minutes, my glasses were frozen.
Preliminary results from the NASA-funded project are showing that ice on Titan erodes at a slower pace than rocks on Earth, all else being equal. These first-of-their-kind experiments “suggest the physical modeling of extra-planetary surfaces is not only possible using terrestrial-based models, but can yield scientifically relevant results,” notes Peter. The group plans to submit their results later this year, possibly to Icarus.