The thermal stability of water ice at the poles of Mercury

Icarus, 1999

In order to assess the thermal stability of polar ice deposits, we present model calculated temperatures of flat surfaces and surfaces within bowl-shaped and flat-floored polar impact craters on Mercury and the Moon. Our model includes appropriate insolation cycles, realistic crater shapes, multiple scattering of sunlight and infrared radiation, and depth-and temperature-dependent regolith thermophysical properties. Unshaded water ice deposits on the surface of either body are rapidly lost to thermal sublimation. A subsurface water ice deposit is stable within 2 • latitude of the Moon's poles. Meter-thick water ice deposits require billions of years to sublime if located in the permanently shaded portions of flat-floored craters within 10 • latitude of the poles of Mercury and 13 • latitude of the poles of the Moon. Results for craters associated with radar features on Mercury are consistent with the presence of stable water ice deposits if a thin regolith layer thermally insulates deposits at lower latitudes and within smaller craters. A regolith cover would also reduce losses from diffusion, ion sputtering, impact vaporization, and H Lyα and is implied independently by the radar observations. Permanently shaded areas near the Moon's poles are generally colder than those near Mercury's poles, but the Moon's obliquity history, its orbit through Earth's magnetospheric tail, and its radar-opaque regolith may limit the volume and radar detectability of ice deposits there.

Icarus, 1997

face of the Moon by interaction of solar wind protons and oxygen-bearing minerals in the soil. Previous to the We predict the OH column that will be present in the polar regions of the mercurian exosphere for physically realistic ice Mariner 10 flyby, this process, known as chemical sputdeposits at the poles, including both surface and buried ice. tering (Roth 1983), was suggested by Thomas (1974) as a The probable rates of accretion by meteoritic, asteroidal, and source of water on Mercury. Nevertheless, the discovery cometary sources are computed and compared with loss rates. of radar-bright poles on Mercury came as a surprise. More The rate of accretion of water at the poles from the nominal recently, Potter (1995) suggested that water produced by meteoritic infall is 2.5-8 ؋ 10 8 molecules cm ؊2 sec ؊1. Including proton sputtering of silicates could be a source of ice in the uncertainty in meteoritic plus asteroidal infall rate, the the mercurian polar region. accretion rate is 1-12 ؋ 10 8 cm ؊2 sec ؊1. For T Ͻ 115 K, the Although exposed water ice was shown to be stable limiting loss process for surface ice is vaporization by micromeagainst thermal evaporation at temperatures lower than teoroids. The loss rate due to meteoritic vaporization of freshly deposited ice, with subsequent dissociation to OH, is about 112 K, the modeled temperatures for flat surfaces at lati-1-2 ؋ 10 8 cm ؊2 sec ؊1. Thus, the net accretion rate from meteortudes above 86Њ are 140-180 K (Paige et al. 1992); however, itic impact is 0-4.5 m/4 ؋ 10 9 years. Since the steady-state thermal modeling (Butler et al. 1993) showed that temperainflux of water from meteoroids equals or exceeds the loss rate tures could remain as cold as 60 K in the interiors of highfrom all processes, any additional water accreted from a comet latitude deep craters, and therefore water would be stable or extinct comet nucleus would be retained. The most probable against thermal evaporation in permanently shadowed povalue of the depth of water at the pole from comet impacts is lar craters. Subsequently, high-resolution radar maps (Har-3 m, with large uncertainties. The background zenith OH mon et al. 1994) showed that the regions of high radar 3085-Å emission from vaporized meteoritic material in the abreflectivity are confined to the interiors of craters in both sence of ice deposits is expected to be 12 R in the equatorial the north and south polar regions, lending support to the region and 0.3-0.7 R above the poles at aphelion. The backtheory that condensation and cold trapping of a volatile ground exceeds the emission from an outgassing source from buried ice deposits at Ͻ112-118 K. An OH exosphere resulting substance are responsible for the radar anomalies. All of from buried ice deposits would be difficult to observe from the source craters that have been classified are relatively ground-based or Earth-orbiting instruments, but fresh deposits pristine, and thus do not have degraded rims. The southern would be easily observable. A UV spectrograph in orbit about polar feature is associated with the crater Chao Meng Fu, Mercury that could determine both latitudinal variations and whereas the northern polar features are associated with scale heights could be used to infer buried deposits at T Ͼ the crater Desperez and a number of smaller unnamed 112 K or surface ice deposits. © 1997 Academic Press craters. Most of the larger features in the north are on the unmapped hemisphere. Other radar-bright features found at lower latitudes have been associated with a fresh Tycho 1. INTRODUCTION class impact crater, a possible shield volcano, and a linear feature possibly associated with smooth plains, respectively Radar mapping of mercurian polar regions revealed (Harmon and Slade 1995). bright polar anomalies that were attributed to the presence Rawlins et al. (1995) considered exogenic sources for of several meters of water ice (Harmon and Slade 1992, water at Mercury, and concluded that one-third of the Slade et al. 1992). The possible presence of water ice at predicted amount of water (several meters) could be acthe lunar pole has been the subject of numerous papers creted by meteoritic bombardment, assuming that the meover the past three decades, beginning with the pioneering teoroids contain 10% water; however, they did not consider work of Watson et al. (1961), who concluded that lunar polar regions were possible cold traps for water ice. Opik loss processes at the poles. We consider accretion rates by meteoroids, asteroids, and comets versus prompt and slow (1962) speculated that water could be created on the sur-195

Icarus, 1998

such as grain size and distribution, albedo, thermal conductivity (cf. Hapke 1993). To aid in the interpretation of We present and use a new rough-surface thermal model to aid in the analysis of new mid-infrared spectral measurements two-dimensional images and spectra obtained in the midof Mercury from 5-12.5 m. The model calculates spatially infrared (5 to 14-Ȑm region), it is helpful to remove the resolved thermal emission from slowly rotating, airless bodies. purely thermal continuum flux from the data. The residual The Mercury data contain the first spectral measurements of emission structure can then be examined, and if features Mercury between 5-7.5 m, a region not accessible from are present, can be interpreted in physical terms. To this ground-based telescopes, and are also the first observations end, we have developed a numerical model which simulates of Mercury to be made while flying on the Kuiper Airborne the thermal emission from the surface of slowly rotating, Observatory (KAO). They were obtained with the Highairless bodies. The effects of surface roughness are incorpoefficiency Infrared Faint Object Grating Spectrometer rated in a manner similar to that used by other authors (Hansen 1977, Spencer 1990, and Colwell et al. 1990). An KAO data has a Hapke of 20؇. A strong 5-m emission energy balance equation is solved for temperature inside feature was present during both observing periods. We suggest the 5-m excess may be a result of near surface thermal gradispherical section craters which are distributed uniformly ents in regolith materials with a 30 to 100-m grainsize, but across the surface and for smooth surface elements located cannot entirely rule out an observational artifact resulting from between these craters. From these temperatures, we can our instrumentation or the telescope on the KAO. Other feacreate a disk-integrated thermal emission model spectrum tures in the spectra are consistent with a feldspathic and feldas well as a spectrum of two dimensional model images spathoidal surface composition.

Bulletin of the AAS

The presence of meters-thick polar deposits exposed directly on the surface of Mercury provides unique science opportunities that should be prioritized in the next decade of planetary exploration. The poles of Mercury provide a natural laboratory for understanding the chemical, physical, and thermal processes that have governed the supply, retention, modification, and loss of water and other volatiles delivered to the inner solar system through time. Polar deposits on Mercury are composed primarily of water ice, and the additional and coincident presence of organic-rich frozen volatiles exposed on the surface offers a nearby opportunity to study the chemistry of extremely lowtemperature, icy environments applicable to the Moon, asteroids, comets, and icy worlds of the outer solar system. Understanding the processes responsible for the delivery and storage of volatiles on Mercury would greatly enhance our ability to understand the origin and distribution of resources across the inner solar system, including at the lunar poles.

Space Science Reviews, 2014

Mercury's regolith, derived from the crustal bedrock, has been altered by a set of space weathering processes. Before we can interpret crustal composition, it is necessary to understand the nature of these surface alterations. The processes that space weather the surface are the same as those that form Mercury's exosphere (micrometeoroid flux and solar

Icarus, 1999

Radar images have revealed the possible presence of ice deposits in Mercury's polar regions. Although thermal models indicate that water ice can be stable in permanently shaded regions near Mercury's poles, the ultimate source of the water remains unclear. We use stochastic models and other theoretical methods to investigate the role of external sources in supplying Mercury with the requisite amount of water. By extrapolating the current terrestrial influx of interplanetary dust particles to that at Mercury, we find that continual micrometeoritic bombardment of Mercury over the last 3.5 byr could have resulted in the delivery of (3-60) × 10 16 g of water ice to the permanently shaded regions at Mercury's poles (equivalent to an average ice thickness of 0.8-20 m). Erosion by micrometeoritic impact on exposed ice deposits could reduce the above value by about a half. For comparison, the current ice deposits on Mercury are believed to be somewhere between ∼2 and 20 m thick. Using a Monte Carlo model to simulate the impact history of Mercury, we find that asteroids and comets can also deliver an amount of water consistent with the observations. Impacts from Jupiter-family comets over the last 3.5 billion years can supply (0.1-200) × 10 16 g of water to Mercury's polar regions (corresponding to ice deposits 0.05-60 m thick), Halley-type comets can supply (0.2-20) × 10 16 g of water to the poles (0.07-7 m of ice), and asteroids can provide (0.4-20) × 10 16 g of water to the poles (0.1-8 m of ice). Although all these external sources are nominally sufficient to explain the estimated amount of ice currently at Mercury's poles, impacts by a few large comets and/or asteroids seem to provide the best explanation for both the amount and cleanliness of the ice deposits on Mercury. Despite their low population estimates in the inner solar system, Jupiter-family comets are particularly promising candidates for delivering water to Mercury because they have a larger volatile content than asteroids and more favorable orbital and impact characteristics than Halley-type comets.

Reports on Progress in Physics, 2002

The planet closest to the Sun, Mercury, is the subject of renewed attention among planetary scientists, as two major space missions will visit it within the next decade. These will be the first to return to Mercury, after the flybys by NASA's Mariner 10 spacecraft in 1974-5. The difficulties of observing this planet from the Earth make such missions necessary for further progress in understanding its origin, evolution and present state. This review provides an overview of what is known about Mercury and what are the major outstanding issues. Mercury's orbital and rotation periods are in a unique 2:3 resonance; an analysis of the orbital dynamics of Mercury is presented here, as well as Mercury's special role in testing theories of gravitation. These derivations provide a good insight into the complexities of planetary motion in general, and how, in the case of Mercury, its proximity to the Sun can be described and exploited in terms of general relativity. Mercury's surface, superficially similar to that of the Moon, presents intriguing differences, representing a different, and more complex history in which the role of early volcanism remains to be clarified and understood. Mercury's interior presents the most important puzzles: it has the highest uncompressed density among the terrestrial planets, implying a very large, mostly iron core. This does not appear to be the completely solidified yet, as Mariner 10 found a planetary magnetic field that is probably generated by an internal dynamo, in a liquid outer layer of the large iron core. The current state of the core, once established, will provide a constraint for its evolution from the time of the planet's formation. Mercury's environment is highly variable. There is only a tenuous exosphere around Mercury; its source is not well understood, although there are competing models for its formation and dynamics. The planetary magnetic field appears to be strong enough to form a magnetosphere around the planet, through its interaction with the solar wind. This magnetosphere may have similarities with that of the Earth, but is more likely to be dominated by global dynamics that could make it collapse at least at the time of large solar outbursts. The future understanding of the planet will now await the arrival of the new space missions. The review concludes with a brief description of these missions.

Eos, Transactions American Geophysical Union, 1978

The similarity of the airless intense ly cratered surfaces of Mercury and the moon (Figures la and 1?)) is in striking contrast to the different den sities of the two objects. The similar geologic history inferred from the sur face record is surprising, since the different size and density of these two bodies might suggest different amounts of radiogenic heating, sili cate/metal ratios, and thermal conduc tivity (Figure 2

Icarus, 2001

The recently upgraded Arecibo S-band (λ12.6-cm) radar was used to make delay-Doppler images of Mercury's north polar region, where earlier observations had shown strong echoes from putative ice deposits in craters. The image resolution of 1.5-3 km is a substantial improvement over the 15-km resolution of the older Arecibo images J. K. Harmon et al. 1994, Nature 369, 213-215). The new observations confirm all the original polar features and reveal many additional features, including several at latitudes as low as 72-75 • N and several from craters less than 10 km in diameter. All of the new features located on the Mariner-imaged side of the planet can be matched with known craters or other shaded areas. We find the north pole to be located 65 km from the original Mariner-based pole and 15 km from the new Mariner-based pole of M. S. Robinson et al. (1999, J. Geophys. . The improved resolution reveals fine structure in the radar features and their respective host craters, including radar shadowing/highlighting by central peaks and rim walls, rim terracing, and preferential concentration of radar-bright deposits in shaded southern floor areas. The radar features' high brightness, circular polarization inversion (µ c = 1.25), and confinement to regions permanently shaded from direct sunlight are all consistent with volume scattering from a coldtrapped volatile such as clean water ice. The sizes and locations of most of the features show good agreement with the thermal model of A. R. Vasavada, D. A. Paige, and S. E. Wood (1999, Icarus 141, 179-193) for insulated (buried) water ice, although the problems of explaining radar features in small craters and the rapid burial required at lower latitudes suggest that other factors may be suppressing ice loss after emplacement.

Space Science Reviews, 2007

Mercury’s surface is thought to be covered with highly space-weathered silicate material. The regolith is composed of material accumulated during the time of planetary formation, and subsequently from comets, meteorites, and the Sun. Ground-based observations indicate a heterogeneous surface composition with SiO2 content ranging from 39 to 57 wt%. Visible and near-infrared spectra, multi-spectral imaging, and modeling indicate expanses of feldspathic, well-comminuted surface with some smooth regions that are likely to be magmatic in origin with many widely distributed crystalline impact ejecta rays and blocky deposits. Pyroxene spectral signatures have been recorded at four locations. Although highly space weathered, there is little evidence for the conversion of FeO to nanophase metallic iron particles (npFe0), or “iron blebs,” as at the Moon. Near- and mid-infrared spectroscopy indicate clino- and ortho-pyroxene are present at different locations. There is some evidence for no- or low-iron alkali basalts and feldspathoids. All evidence, including microwave studies, point to a low iron and low titanium surface. There may be a link between the surface and the exosphere that may be diagnostic of the true crustal composition of Mercury. A structural global dichotomy exists with a huge basin on the side not imaged by Mariner 10. This paper briefly describes the implications for this dichotomy on the magnetic field and the 3 : 2 spin : orbit coupling. All other points made above are detailed here with an account of the observations, the analysis of the observations, and theoretical modeling, where appropriate, that supports the stated conclusions.