It seems almost paradoxical: of all places, there is water ice on the planet Mercury heated by the nearby sun. This ice remains in the deep permanent shadow of some polar craters. Previously, meteorite strikes were primarily considered to be a possible source of these ice deposits, but researchers have now found another explanation. Accordingly, the solar wind, combined with the heat on the Mercury day side, could trigger chemical reactions in the regolith that lead to the formation of water molecules. These water molecules could then drift around the planet as water vapor and deposit themselves in the cold craters of the poles as ice.
Mercury is a planet of extremes: while on its day side there is a heat of more than 400 degrees Celsius, the 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 – similar to that on the moon. And similar to our satellite, the existence of water or water ice on Mercury was long considered impossible. However, as early as 1991, researchers discovered some highly reflective areas near the poles during radar-based observations of the surface of Mercury, which could indicate water ice occurrences. In 2012, NASA’s Messenger spacecraft provided confirmation: in some craters in the planet’s polar region, there is water ice that is partially covered by a thin layer of dust. These ice deposits can be found in places that are never illuminated by the sun and where the temperatures are therefore constantly well below zero.
Was the Mercury ice formed after all?
However, this discovery raises the question of how this water ice was created on Mercury. “It is generally accepted that water and other volatile organic materials have reached the moon or Mercury due to meteorite impact,” said Brant Jones of the Georgia Institute of Technology in Atlanta and his colleagues. But as you have now found out, the water ice of Mercury could have been formed at least in part on site. “The chemical mechanism underlying this idea has been observed dozens of times in studies since the 1960s,” says Jones. However, these reactions only took place in the laboratory and on special surfaces. It is a chemical reaction, also known as recombinant absorption (RD). The 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.
If sufficient energy is now supplied to these compounds, a rearrangement can occur between adjacent hydroxyl groups, during which metal oxides and H2O molecules are formed. Theoretically, water and thus water ice could be created in this way. However, there is a catch: “Typically, the activation energies for the formation of H2O through recombinant absorption are high,” report Jones and his colleagues. “This energy barrier reduces the importance of this reaction on the moon.” Because there the energy input from the sun is usually not sufficient to start this reaction.
How heat and solar wind produce water
However, this is different on Mercury, as Jones and his team now report. For one thing, the surface of the planet is hit by an intense solar wind. Through this, many high-energy protons patter on the Mercury regolith, which favor the attachment of hydroxyl groups to the minerals. On the other hand, the temperatures on the day side of the planet reach temperatures of 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, around three times ten to 30 water molecules per Mercury day can be generated by recombinant absorption,” said Jones and his colleagues. These H2O molecules are created on the day side of the planet. While many of them decay again due to the radiation, some of them end up in the cold shadow zones of the Mercury poles, where they freeze out and are deposited in the craters.
According to the researchers’ calculations, this mechanism could have deposited around eleven trillion tons of water ice on Mercury in the past three million years. “This process could easily be responsible for up to ten percent of all ice on Mercury,” said Jones colleague Thomas Orlando. The scientists also suspect that molecular water could have been and is still being produced by this chemical reaction on other celestial bodies in the solar system.
Source: Brant Jones (Georgia Institute of Technology, Atlanta) et al., Astrophysical Journal Letters, doi: 10.3847 / 2041-8213 / ab6bda