Polar Ice Deposits on Mercury

Despite the blazing heat from the nearby Sun, water ice exists at Mercury’s polar regions. Ice from comets that crashed into the planet has been cached in deep craters near the poles, where sunlight never reaches. Just as on the Moon, ice is an invaluable resource for humans and their machines.

- Ben Bova, Mercury

Figure 1: Artist’s rendering of Mercury.

Mercury is the closest planet to the Sun and the time it takes to complete one orbit around the Sun is 88 Earth days. There is no atmosphere on Mercury; as a result, Mercury’s surface experiences great temperature variations. During the day, temperatures at some equatorial regions on Mercury can reach over 700 K (427 °C), hot enough to melt zinc. Without an atmosphere to retain the day’s heat, night time temperatures can sink below 100 K (-173 °C).

On 18 March 2011, NASA’s MESSENGER spacecraft entered orbit around Mercury and began a mission to study the closest planet to the Sun. Around the planet’s north pole, in areas permanently shielded from the Sun’s rays, NASA’s MESSENGER spacecraft has found vast deposits of water ice and possible organic materials. Previous Earth-based radar observations have already hinted at the existence of water ice deposits at Mercury’s poles. These radar observations revealed areas of high radar backscatter near Mercury’s north and south poles. These areas are postulated to be near-surface deposits of water ice up to several metres thick.

Figure 2: Perspective view of Mercury’s north polar region with areas of high radar backscatter shown in yellow. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

Figure 3: A depiction of the MESSENGER spacecraft at Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

Figure 4: Schematic of an impact crater located at high latitude on Mercury. The crater rim blocks the angled rays of the Sun, resulting in extremely cold temperatures in regions of permanent shadow. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

MESSENGER’s discovery of polar deposits of water ice and possible organic materials on Mercury comes from three independent lines of evidence: (1) measurements of excess hydrogen at Mercury’s north pole with MESSENGER’s Neutron Spectrometer (Lawrence et al., 2013); (2) measurements of the reflectance of Mercury’s polar deposits at near-infrared wavelengths with the Mercury Laser Altimeter (MLA) on MESSENGER (Neumann et al., 2013); (3) and the first detailed models of the surface and near-surface temperatures of Mercury’s north polar regions that utilize the actual topography of Mercury’s surface measured by the MLA (Paige et al., 2013).

When cosmic ray particles strike the surface of Mercury, they liberate neutrons from atomic nuclei in the near-surface material. The neutrons travel up to the surface and escape into space. These neutrons can be detected by the Neutron Spectrometer on the MESSENGER spacecraft. If a layer of water ice happens to be present on the surface, the abundant hydrogen atoms within the ice will stop the neutrons from escaping into space. Hydrogen has a unique ability to stop neutrons because hydrogen atoms and neutrons have the same mass, which allows very efficient momentum transfer between the two.

Measurements by MESSENGER’s Neutron Spectrometer show a decrease in the flux of neutrons from Mercury’s north polar region that is consistent with the presence of water ice in permanently shadowed regions. The decrease in neutron flux towards the north pole is smaller than expected and this indicates that most of the water ice is buried beneath a 10 to 20 cm thick layer of material that is less rich in hydrogen. Based on the measurements by MESSENGER’s Neutron Spectrometer, the total mass of water at Mercury’s poles is estimated to be as much as 1 trillion metric tons. To put it into perspective, that is equal in volume to a cube of liquid water with edges 10 km in length.

Figure 5: Hydrogen atoms within the layer of water ice stop the neutrons from escaping into space. A decrease in the measured flux of neutrons indicates enhanced hydrogen concentrations and, by inference, the presence of water ice. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

Figure 6: Neutron Spectrometer measurements of the flux of high-speed neutrons versus latitude (red). Simulated count rates are shown for the cases of no hydrogen (black) and for a thick layer of pure water ice (blue) located at the surface in all regions of high radar backscatter. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

The Mercury Laser Altimeter (MLA) made measurements of the surface reflectance of permanently shadowed areas near Mercury’s north pole and found regions of anomalously dark and bright deposits. MLA measurements show 3 types of reflectance: (1) typical Mercury reflectivity; (2) a much darker subset corresponding to the MLA-dark deposits; (3) and a smaller subset that is substantially brighter, corresponding to the MLA-bright deposits. The MLA-bright regions are consistant with surface water ice deposits, while the MLA-dark regions are consistant with a surface layer of complex organic material. Since both the bright and dark deposits are spatially correlated with regions of high radar backscatter, it indicates that the dark deposits are a radio-transparent surface layer of complex organic material that partly overlies buried water ice (Figure 7).

Figure 7: Schematic showing the distribution of water ice and complex organic material within a permanently shadowed region. In the coldest areas, water ice remains exposed on the surface (white) and corresponds to the MLA-bright regions. The surface layer of organics (black) partly covers and partly surrounds the water ice deposits, and corresponds to the MLA-dark regions. Credit: NASA/UCLA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

When comets or volatile-rich asteroids impact the surface of Mercury, a mixture of water and organic compounds become spread over a wide region. A small fraction migrates to the permanently shadowed areas around Mercury’s poles where they can become cold-trapped. Over time, water ice in warmer regions sublimates and leaves behind a surface layer that is rich in organic material. Exposure to Mercury’s space environment darkens the layer of organic material to produce the dark deposits observed by MLA.

Thermal models for the north polar region of Mercury were created using the topographic measurements made by MLA on MESSENGER (Figure 8). The models show that the regions of high radar backscatter is well matched by the predicted distribution of thermally stable water ice. Within some permanently shadowed regions, the maximum annual temperature never exceeds ~50 K (-223 °C). In fact, large areas of permanently shadowed regions on Mercury’s north polar region never exceed 100 K (-173 °C) and these are the areas where water ice deposits will remain stable over billion-year timescales. The temperature at which a water ice deposit can be considered thermally stable depends on the timescale under consideration. For instance, at a temperature of 102 K (-171 °C) a 1 m thick layer of pure water ice would sublimate in 1 billion years, while at a temperature of 210 K (-63 °C) a 1 m thick layer of pure water ice would sublimate in 35 days.

Figure 8: Maps of calculated surface and subsurface temperatures in the north polar region of Mercury, superposed on a shaded relief map derived from MLA topographic measurements. (A) Biannual maximum surface temperatures. (B) Biannual average temperatures at 2 cm depth. Credit: NASA/UCLA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

Sean Solomon of the Columbia University’s Lamont-Doherty Earth Observatory, principal investigator of the MESSENGER mission mentions: “For more than 20 years the jury has been deliberating on whether the planet closest to the Sun hosts abundant water ice in its permanently shadowed polar regions. MESSENGER has now supplied a unanimous affirmative verdict.” Soloman also adds: “But the new observations have also raised new questions. Do the dark materials in the polar deposits consist mostly of organic compounds? What kind of chemical reactions has that material experienced? Are there any regions on or within Mercury that might have both liquid water and organic compounds? Only with the continued exploration of Mercury can we hope to make progress on these new questions.”

References:

- Lawrence et al., “Evidence for Water Ice Near Mercury’s North Pole from MESSENGER Neutron Spectrometer Measurements”, Science 18 January 2013: Vol. 339 no. 6117 pp. 292-296 DOI: 10.1126/science.1229953

- Neumann et al., “Bright and Dark Polar Deposits on Mercury: Evidence for Surface Volatiles”, Science 18 January 2013: Vol. 339 no. 6117 pp. 296-300 DOI: 10.1126/science.1229764

- Paige et al., “Thermal Stability of Volatiles in the North Polar Region of Mercury”, Science 18 January 2013: Vol. 339 no. 6117 pp. 300-303 DOI: 10.1126/science.1231106