Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.
Detection of Molecular H2O on Nominally Anhydrous Asteroids
Published February 2024
Main takeaway:
Using the Stratospheric Observatory for Infrared Astronomy (SOFIA) — a now-decommissioned Boeing 747 that toted an infrared telescope to 40,000 feet for more than a decade — Anicia Arredondo (Southwest Research Institute) and collaborators investigated the surface compositions of four asteroids. The spectra of two asteroids in the sample contained a feature unambiguously attributed to water, marking the first time water has been detected on the surface of an asteroid.
Why it’s interesting:
Asteroids represent material left over from the formation of the solar system. By studying asteroids, researchers hope to learn about the solar nebula from which the planets coalesced. Of particular interest is the distribution of water in the early solar system, which can tell us about how Earth got its water, as well as how planets in other star systems might develop. The four asteroids examined in this study are S-type asteroids, which are thought to form in the inner solar system, where volatile materials like water are scarce. The discovery of water molecules on these supposedly dry asteroids provides an important constraint on theories of solar system and planet formation.
Spectra of the four asteroids examined in this work. Two of the asteroids, (7) Iris and (20) Massalia, clearly show the 6-micron water feature, while the other two asteroids do not. Click to enlarge. [Arredondo et al. 2024]
On the definitive spectral feature:
Detecting a particular molecule in space can be difficult because the spectral features from different molecules often overlap. Many asteroids exhibit a spectral feature at 3 microns (1 micron = 10-6 meter) that arises from stretching of the chemical bond between an oxygen atom and a hydrogen atom. This bond is present in a water molecule — meaning that this feature could be due to water — but it’s also present in many other molecules. To make their definitive detection of water, Arredondo’s team searched instead for a spectral feature at 6 microns, which is solely due to water and has been detected previously on the Moon. The water molecules discovered in this study might be trapped in glass beads on the asteroids’ surfaces, adsorbed onto silicon, or bound up in minerals. The team plans to use JWST to continue their search for water on asteroid surfaces.
Citation
Anicia Arredondo et al 2024 Planet. Sci. J. 5 37. doi:10.3847/PSJ/ad18b8