1. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, T6G 2R3, Canada
2. School of Natural and Environmental Sciences, Newcastle University, Newcastle Upon Tyne NE1 7RU, U.K.
3. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, United States
4. School of Natural Resources, University of Nebraska-Lincoln, Lincoln, Nebraska 68583, United States