Background Image Credit: NASA/SOFIA/Lynette Cook
For the last decades, spectroscopic signatures of water (H2O) and hydroxyl radicals (OH) have been observed on surfaces of asteroids such as 1 Ceres, 24 Themis, and 433 Eros. However, the lifetime of exposed water ice, even at distances of the inner asteroid belt of 2.2 to 3.2 AU from the Sun, should be relatively short on the order of only 104 to 106 years in the absence of recent ice-exposing processes. This timescale is much shorter than the typical lifetime of asteroids of a few 109 years, which indicates that no spectroscopic signatures of water should survive on these objects. Therefore, there must be mechanisms to replenish the water on asteroids. However, such regenerative water-ice sources are still elusive.
As the surfaces of asteroids and airless bodies are chemically modified by space weathering processes through interaction of the surfaces with galactic cosmic rays, solar wind particles, and bombardment by micrometeorites, we investigated the effects of these processes on asteroid surficial water signatures in laboratory simulation experiments. We exposed samples of the Murchison meteorite – a typical surrogate of C-type asteroids - with infrared laser and energetic electrons to mimic the thermal effect of micrometeorite impact and secondary electrons generated by penetrating charged particles from the solar wind and the galactic cosmic rays, respectively. The results revealed that surface water can be generated by exposure of the samples to both simulated space-weathering components, but not single component, at low temperatures at the desired timescales. Further studies exposed that water is generated in two distinct processes by low temperature oxidation of organics and dehydration of phyllosilicates in the Murchison meteorite. Our findings help to rationalize the spectroscopic signatures of water and hydroxyl radicals as detected on airless bodies and to decipher the contemporary distribution of water in our Solar System.
In the future, we plan to investigate if solar wind and galactic cosmic ray protons can enhance the water formation efficiency, untangle the details of the underlying reaction mechanisms on the microscopic level, and quantify the contribution of these mechanisms to the surface water on asteroids compared to other proposed sources such as subsurface reservoirs.
The paper in Nature Astronomy is here: https://www.nature.com/articles/s41550-019-0900-2