Paper No. 1
Presentation Time: 8:05 AM
WATER ICE ON MERCURY: CONFIRMATION OF A DECADES-OLD HYPOTHESIS
LAWRENCE, David J., Space Department, Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, NEUMANN, Gregory A., Planetary Geodynamics Laboratory, NASA Goddard Space Flight Center, Mail Code 698, Greenbelt, MD 20771, PAIGE, David, Earth and Space Sciences, UCLA, 595 Charles E. Young Drive East, Los Angeles, CA 90095 and SOLOMON, Sean C., Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, David.J.Lawrence@jhuapl.edu
The surface of the planet Mercury does not initially seem a location favorable to the accumulation and retention of water ice. As the closest planet to the Sun, its surface reaches temperatures greater than 700 K, and it retains no atmosphere. Nevertheless, near Mercury’s poles are locations where water ice can persist in stable form for geologically long times. The possibility of polar water ice on a rocky planetary body was first investigated for Earth’s Moon in the early 1960s, when it was shown that craters at the Moon’s poles are oriented so that their interiors are in permanent shadow and are at surface temperatures sufficiently cold to retain water ice. By analogy with the Moon, it was hypothesized in the mid 1970s that water ice deposits might also exist at Mercury’s poles. The first experimental evidence for water ice within permanently shaded craters on a rocky planetary body came from radar images of Mercury, which showed enhancements of circularly polarized radar reflections. These radar data, combined with thermal models, strongly suggested that Mercury’s poles harbored large amounts of water ice. The possible presence of water ice at Mercury’s poles was later questioned, and it was suggested that deposits of sulfur and/or special properties of cold silicate materials might be responsible for the radar-bright regions.
One of the primary objectives of NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission has been to characterize Mercury’s polar regions and definitively address the hypothesis that Mercury’s poles contain large amounts of water ice. This objective was accomplished through independent determinations of hydrogen concentrations, surface reflectance measurements, and topography-constrained temperature models of Mercury’s north polar region. These findings indicate that the permanently shaded craters at Mercury’s north pole contain thick deposits (greater than tens of centimeters) of nearly pure water ice. Many of these deposits lie beneath a layer 10 to 30 cm thick that is volatile, contains far less hydrogen than water ice, is dark at near-infrared wavelengths, and may be enriched with complex organic material. Comets or volatile-rich asteroids could have delivered the water and organic material to Mercury’s surface some time in the last 100 million years.
