It seems almost paradoxical: there is water ice on Mercury, which is heated by the nearby sun. This ice remains in the deep permanent shadow of some polar craters. Until now, meteorite impacts were considered possible suppliers of these ice deposits, but now researchers have found another explanation. Thereby, solar wind, combined with the heat of the Mercury day side, could trigger chemical reactions in the regolith, which lead to the formation of water molecules. These water molecules could then drift around the planet as water vapor and be reflected in the cold craters of the poles as ice.
Mercury is a planet of extremes: while there is a heat of more than 400 degrees Celsius on its day side, temperatures on its night side drop to an icy minus 180 degrees. Because there is no balancing atmosphere, the conditions change drastically depending on the radiation incidence – similar to the moon. And, like on our satellite, the existence of water or water ice on Mercury was considered impossible for a long time. But as early as 1991, researchers discovered on radar-based observations of Mercury’s surface certain areas that were highly reflective near the poles – signatures that might indicate water ice deposits. In 2012, NASA’s Messenger spacecraft provided confirmation: In some craters of the planet’s polar region, there is water ice, partially covered by a thin layer of dust. These ice deposits can be found in places that are never lit by the sun and where the temperatures are therefore constantly well below zero degrees Celsius.
Was the Mercury ice formed on site?
This discovery, however, raises the question of how this water ice originated on Mercury. “It is generally accepted that water and other volatile organic materials have been brought in by meteorite impacts on the moon or Mercury,” explains Brant Jones of the Georgia Institute of Technology in Atlanta and his colleagues. But as they have now found out, Mercury’s water ice may have been formed at least in part on the spot. “The chemical mechanism behind this idea has been observed dozens of times in studies since the 1960s,” Jones says. However, these reactions only took place in the laboratory and on special surfaces. It is a chemical reaction, also known as recombinant absorption (RD). Prerequisite for this is the presence of hydroxyl groups (-OH), which are chemically bound to the metallic components of minerals. Such hydroxyl groups have already been detected in the lunar regolith, but also on Mercury. Regolith is a blanket of loose material formed on rocky planets in the solar system through various processes above a underlying source material.
If sufficient energy is now supplied to these compounds, a rearrangement can occur between adjacent hydroxyl groups, in the course of which metal oxides and H2O molecules are formed. In purely theoretical terms, water and thus water ice could therefore be produced in this way. However, there is a catch: “Typically, the activation energies for the formation of H2O through recombinant absorption are high,” Jones and his colleagues report. “This energy barrier reduces the importance of this reaction on the moon.” Because there, the energy supply from the solar radiation is usually not sufficient to set this reaction in motion.
How heat and solar wind produce water
But this is different on Mercury, as Jones and his team are now reporting. For one thing, the surface of the planet is hit by an intense solar wind. Therefore, many high-energy protons slam into the Mercury regolith, which favors the accumulation of hydroxyl groups to the minerals. Additionally, temperatures on the day side of the planet reach more than 400 degrees Celsius – and this is enough to overcome the energy barrier for recombinant absorption, as the researchers determined using a simulation. “According to our model, recombinant absorption can produce around three times ten high 30 water molecules per Mercury day,” Jones and his colleagues said. These H2O molecules are formed on the day side of the planet. While many of them disintegrate due to the radiation, some of them enter the cold shadow zones of the Mercury poles, where they freeze and deposit in the craters.
According to the researchers’ calculations, this mechanism could have deposited about 11 trillion tons of water ice on Mercury over the last three million years or so. “This process could easily account for up to 10 percent of the total ice on Mercury,” says Jones colleague Thomas Orlando. The scientists also suspect that molecular water may have been created and still arises on other celestial bodies in the solar system as a result of this chemical reaction.
Heiner Kubny, PolarJournal
