Mercury: full‐disk radar images and the detection and stability of ice at the North Pole

Abstract

The first full-disk radar images of Mercury were obtained on August 8 and 23, 1991. These images were constructed using the Very Large Array (VLA) in Socorro, New Mexico, to receive and map radar flux at 3.5 cm (X band) which was continuously transmitted from the 70-m Jet Propulsion Laboratory/Deep Space Network antenna at Goldstone, California. Approximately 77% of the surface was imaged, at resolutions as good as 150 km. About half of the hemisphere photographed by Mariner 10 was imaged, as well as most of the hemisphere which has not previously been photographed. At the time of the observations, the north pole was visible, and the feature with the highest same sense (SS) circular reflectivity in the images is near the nominal polar position. The peak SS reflectivity of this feature was 7.9%, and the circular polarization ratio throughout much of the feature is > 1. Our best estimate of the size of the feature is that its diameter is •<350 km. We interpret the feature to be indicative of the presence of ices because of the signal strength and polarization characteristics. The ices must be very clean and were thus probably deposited in a relatively short time period. The most likely place to find these ices is in any permanently shadowed areas in the polar regions, which would be very cold. In comparison to absolute refiectivities of other icy bodies and regions in the solar system, the reflectivity of the north polar feature is slightly depressed. This is probably because the ices do not fill an observational resolution cell (i.e., there is incomplete areal coverage), or are covered by a layer of dust or soil, which absorbs some of the incoming and outgoing radar energy, or both. A covering layer would also protect any ices from erosion by energetic sources. Other prominent features on the unphotographed hemisphere correspond to positions where atmospheric sodium enhancements have been measured from Earth. This may indicate that these locations are large basins similar to the Caloris basin, where an atmospheric potassium enhancement has been measured, which may be the result of increased degas sing in the disrupted surface and near surface. Direct comparison of the radar echoes from our features with those from the Caloris basin are hard to make, as Caloris was not imaged by us in a favorable geometry. Features on the photographed hemisphere are smaller and mostly associated with craters and crater complexes, most notably the Kuiper crater and its environs. 15,003

Evidence for water ice on the Moon: Results for anomalous polar craters from the LRO Mini‐RF imaging radar

“…Yet all three icy Galilean satellites show CBOE, with mean CPR values of 1.2 or greater [Ostro, 2002], increasing with increasing optical albedo. In addition, the polar deposits of Mercury exhibit a muted CBOE, thought to be caused by admixture of silicate particles into the ice regolith [Butler et al, 1993]. As the fraction of rock fragments increases, the CBOE decreases, but both Mercury and the icy Galilean satellites show that even significant amounts of silicates cannot totally remove the CBOE effect [Nozette et al, 1996].…”

“…Radar backscatter from the lunar surface is modeled as a mixture of these specular and diffuse components. Diffuse scattering from rocky areas associated with fresh craters is assumed to have CPR of about 1.0; and ice is assumed to have CPR of 2.0, as observed in the polar features of Mercury and Mars and the surfaces of the icy Galilean satellites of Jupiter [Muhleman et al, 1991;Harmon and Slade, 1992;Butler et al, 1993;Ostro, 2002]. The differences in appearance between the lunar and mercurian polar craters suggest that more ice is present on Mercury than on the Moon, probably a result of the higher cometary flux near Mercury compared with the Moon [e.g., Hartmann et al, 1981] and the fact that the higher surface gravity on Mercury (~0.3 g) means that body will retain more water on its surface.…”

The neutron signature of Mercury's volatile polar deposits

“…The volatiles producing the anomalously high same-sense radar backscatter could be pristine (outgassed from the interior) or brought in by cometary and meteoritic material [Butler et al, 1993] The importance of a definitive chemical identification of the anomolous radar-scattering deposits near the poles of Mercury has led us to explore their range of leakage neutron flux signatures. This study is motivated by previous simulations of neutron transport after production by galactic cosmic ray interactions in the Moon [Feldman et al, 1991] and in Mars [Feldman et al, 1993a].…”

“…These are based in part on the similarity of the backscatter signal to that from the icy Galilean satellites [Campbell et al, 1978] and from the polar caps of Mars [Muhleman et al, 1991]. Thermal modeling indicates that if regions within craters are permanently shielded from sunlight, the temperatures will be cold enough for water ice to be stably stored within them for billions of years [Paige et al, 1992;Butler et al, 1993]. Frozen CO2 was ruled out, however, on the basis of the very low temperature required for long-term stability.…”

Regolith properties in the south polar region of the Moon from 70-cm radar polarimetry

“…Earth-based 12.6-cm radar observations show enhanced backscatter and circular polarization ratio only in association with the walls and proximal ejecta of impact craters or likely craters smaller than can be resolved by the imagery. No contiguous, high-backscatter regions covering crater floors, similar to polar features on Mercury attributed to ice (Butler et al, 1993;Harmon et al, 1994;Black et al, 2002), are evident (Stacy et al, 1997;Campbell et al, 2003). The Clementine spacecraft radio transmitter and an Earth-based receiver were used to acquire bistatic radar echoes from the Moon.…”