Ice on Mercury

The Pre-MESSENGER Picture

The information below gives a historical view of the knowledge of water on Mercury before the MESSENGER mission. Recent observations by MESSENGER of ice at Mercury's poles is described in a November 29, 2012 press release.

Mercury would seem to be one of the least likely places in the solar system to find ice. The closest planet to the Sun has temperatures which can reach over 700 K. The local day on the surface of Mercury is 176 earth-days, so the surface is slowly rotating under a relentless assault from the Sun. Nonetheless, Earth-based radar imaging of Mercury has revealed areas of high radar reflectivity near the north and south poles, which could be indicative of the presence of ice in these regions (1-3). There appear to be dozens of these areas with generally circular shapes. Presumably, the ice is located within permanently shadowed craters near the poles, where it may be cold enough for ice to exist over long periods of time. The discovery of ice on the Earth's moon can only serve to strengthen the arguments for ice on Mercury.

How was the evidence for ice found?

Investigations of Mercury were done from Earth using the Arecibo radio telescope, the Goldstone antenna, and the Very Large Array (VLA). The Goldstone/VLA study (1) used the NASA Deep Space Network 70-m Goldstone dish antenna to transmit 8.51 GHz, 460 kW, right circularly polarized radar waves towards Mercury. The reflections were received by the National Radio Astronomy Observatories 26 VLA antennas. Calibration and processing of the radar returns showed radar-bright (high radar reflectivity) with depolarized signatures at the north pole. The Arecibo observations (2,3) were made by transmitting an S-band (2.4 GHz), 420-kW, circularly polarized coded radar wave at Mercury. The wave reflects off Mercury back to Earth. The wave is both transmitted and received by the Arecibo radio telescope. Filtering and processing the return signal gives a radar reflectivity map of Mercury's surface with a resolution of approximately 15 km. About 20 anomalously reflective and highly depolarized features were observed at the north and south poles.

Why are these radar-bright areas thought to be ice?

Ice is highly radar reflective and the radar reflections off ice tend to be highly depolarized, unlike typical silicate rock which comprises the bulk of Mercury's surface. While not as highly reflective as other icy solar system objects, such as Europa, Ganymede, and Callisto, these areas are still significantly more reflective than silicate material. Moreover, the depolarized nature of the reflections is also an indicator of water ice. The Arecibo results show that the radar reflective areas are concentrated in crater-sized spots. At the south pole, the location of the largest area appears coincident with the large crater Chao Meng-Fu (shown at left) and the smaller areas with other identified craters. At the north pole, much of the area containing the radar bright spots was not imaged, and so cannot be correlated with known craters. However, for the imaged areas at both poles most of the areas have been loosely correlated with known craters (3). Craters near the poles could provide the permanent, or near-permanent (see 5), shading required for ice to exist on Mercury. The radar results indicate the reflective areas are probably relatively uncontaminated ice. However, the lower reflectivity compared to pure ice features indicates the ice may be covered by a thin layer of dust or soil or else does not completely cover the crater floor (6). Note that no direct unequivocal detection of ice has been made. The coincidence of the radar bright areas with large, possibly permanently shadowed, polar craters is strong circumstantial evidence for ice. However, the radar reflections could be explained by an enhancement of some other radar reflective material, such as metal sulphides or other metallic condensates, or precipitated sodium ions.

How can ice survive on Mercury?

As mentioned above, all provinces on Mercury are exposed to the Sun for almost 90 earth-days at a time, and can reach temperatures over 700 K. Additionally, Mercury has no ambient atmosphere and very low gravity. Water ice on the surface of Mercury is exposed directly to vacuum, and will rapidly sublime and escape into space unless it is kept cold at all times. This implies that the ice can never be exposed to direct sunlight. The only locations on the surface of Mercury where this is possible would seem to be near the poles, where the floors of some craters might be deep enough to afford permanent shading. Whether such permanently shadowed craters exist on Mercury is still problematic. The only close-up images we have of Mercury were taken by the Mariner 10 spacecraft on three close passes in 1974 and 1975. The same hemisphere of Mercury was sunlit on each of these passes, so nearly half the planet has never been imaged, and no determination can be made of what polar areas, if any, are permanently shadowed. However, theoretical studies assuming typical crater dimensions show that craters near the poles should have areas which never rise above about 102 K (4) and that even flat surfaces at the poles would not exceed about 167 K (5). Other studies (6-7) also indicate that water ice in polar craters on Mercury could be stable over the age of the solar system.

How did the ice get there originally?

There are only two significant sources for ice on Mercury: meteorite bombardment and planetary outgassing. Meteorites, especially in the past, potentially carried large amounts of water to Mercury's surface. Outgassing of water from the planet's interior could also provide a non-negligible flux of water to the surface, although this is speculative. The permanently shadowed regions near Mercury's poles should act as "cold-traps" so that any water which found its way to these regions would freeze on the surface and remain. (The possibility that the ice is relatively uncontaminated may indicate that each deposit was laid down in one or a small number of rapid events (6), such as a large comet impact.) Meteorites which impacted near the poles and water which outgassed in that region could have been easily trapped. Water originating away from the poles would behave as individual, randomly moving molecules, some of which could migrate to the poles and become trapped there (6). There are mechanisms for potential loss of ice, however. These include photodissociation, solar wind sputtering, and micrometeoroid gardening. The effects of these processes are not well-understood.

How can this discovery be tested?

Direct observations of Mercury from Earth are difficult because Mercury is so close to the Sun. The only effective way to study the polar regions beyond radar observations is to send a space probe equipped with an imager and spectrometry instruments. Missions to Mercury are difficult because the planet is deep in the Sun's gravitational well. The only mission to visit Mercury was Mariner 10 (shown at right) which had three flybys in 1974 and 1975. Each of these flybys occurred when the same portion of the planet was lit by the Sun, so only about half the planet was imaged. By their very nature, the interiors of shadowed craters are too dark to image, so these pictures do not shed any light on whether or not ice exists inside these craters. A mission to Mercury called MESSENGER launched in 2004 went into orbit around Mercury in 2011.

References

1) Mercury radar imaging: Evidence for polar ice, Slade et al., Science, v. 258, p. 635, 1992
2) Radar mapping of Mercury: Full-disk images and polar anomalies, Harmon and Slade, Science, v. 258, p. 640, 1992
3) Radar mapping of Mercury's polar anomalies, Harmon et al., Nature, v. 369, p. 213, 1994
4) Stability of polar frosts in spherical bowl-shaped craters on the Moon, Mercury, and Mars, Ingersoll et al., Icarus, v. 100, p. 40, 1992
5) The thermal stability of water ice at the poles of Mercury, Paige et al., Science, v. 258, p. 643, 1992
6) Mercury: Full-disk radar images and the detection and stability of ice at the north pole, Butler et al., Journal of Geophysical Research, v. 98, p. 15,003, 1993
7) Near-surface ice on Mercury and the Moon: A topographic thermal model, Salvail and Finale, Icarus, v. 111, p. 441, 1994

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Author/Curator:
Dr. David R. Williams, dave.williams@nasa.gov
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