Although celestial bodies of the inner solar system are predominantly composed of silicates and metals, surprisingly there are places where temperatures become sufficiently low enough that ice can exist on the surface.
Perhaps the most unlikely place to find ice is on the surface of Mercury, where the side that faces the sun can reach temperatures of up to 700K (400 degrees C). The temperatures on Mercury change from day to night. Before sunrise the temperature is as low as 100 K (-170 degrees C) and by noon it will rise to about 700 K (400 degrees C). These wide variations are due to Mercury's rotation and lack of atmosphere. During the day the temperature is so high that it could melt some metals, but during the night the temperature drops well below freezing.
This being said, the areas that are the coldest on Mercury are near the poles at the bottoms of craters. It is here that Earth-based radar imaging of Mercury has found about 20 circular areas of high radar reflectivity. The strength and polarization of these radar echoes-very different from the rest of Mercury's rocky surface-are similar to the radar characteristics of the south polar cap of Mars and the icy Galilean satellites, prompting researchers to suggest that Mercury's radar-reflective areas may be deposits of water ice or other volatile material.
So how, one might ask, given its proximity to the sun, low gravity and high surface temperatures, might ice survive on a planet with temperatures as wide-ranging as Mercury?
The answer lies in Mercury's lack of atmosphere. Water ice on the surface is directly exposed to a vacuum, leading to rapid escape unless it is extremely cold at all times, and never exposed to sunlight.. The only places on Mercury where such conditions might exist are within craters near the poles. Unlike the earth, where the 23.5 degree tilt of our spin axis gives us the seasons, Mercury's spin axis is barely tilted at all-only 0.1degree. Therefore, the strength of solar illumination on the surface does not vary, regardless of where Mercury is in its orbit.
Theoretical studies predict that typical craters at Mercury's poles may contain areas that never get warmer than 100 K and that water ice in the polar craters could have remained stable since the creation of the solar system. If this is the case, then ice may have originated from the insides of falling comets and meteorites that became trapped at the poles over billions of years.
An alternative theory to water ice is that the polar deposits may not be water ice at all, but rather some other material such as sulfur, which could have sublimated from minerals in surface rocks over the millennia to become trapped at the poles.
These hypotheses, among many others will be tested as part of the Messenger (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission. The Messenger spacecraft, designed and built by the Applied Physics Laboratory (APL) at Johns Hopkins University, in conjunction with NASA, is en route to Mercury and will rely on several flybys of Venus before reaching the inner solar system and Mercury's orbit. Messenger's payload includes a Gamma Ray and Neutron Spectrometer (GRNS), which will help determine if hydrogen exists in the polar deposits, which would indicate the presence of water ice. Another instrument on the spacecraft, called the Energetic Particle and Plasma Spectrometer (EPPS), will help detect if there is sulfur ice in these polar deposits.
We are just beginning to understand how ices contribute to the formation and evolution of planets, moons and small bodies; however, we still have much to learn about the unique and fascinating role of ice in the solar system.
Understanding if and what types of ice may exist on Mercury is the beginning of a profound source of study about the possible origins of the inner solar system, and what it might mean for scientific discovery in the future.
Source: Prockter, Louise. Ice in the Solar System. Johns Hopkins APL Technical Digest, Volume 26, number 2 (2005).
Last Updated: 7 February 2011