Near-Earth objects (NEOs) include rocky asteroids, icy comets, and other celestial bodies whose orbits intersect with Earth’s. Within their interiors, some of these objects are believed to harbor deposits of ice, which is of keen scientific interest, as it may offer insight into the origin of our planet’s life-giving water. This ice, however, diffuses very slowly into the vacuum of space, and the length of time that an airless body can retain its deposits has been a topic of spirited debate.
Schörghofer and Hsieh now address this controversy using a new, two-dimensional spherical model that analyzes the depths at which both NEOs and main-belt asteroids can retain ice under a variety of environmental conditions. The findings from this tool, which calculates the bodies’ interior temperature fields and rates of ice retreat across a wide range of scales, have important implications for understanding the nature of some of Earth’s closest neighbors.
The results indicate that in many asteroids—as well as in a subset of NEOs—water ice could be retained for the entire age of the solar system. This is due in part to the frequent development of hourglass-shaped temperature fields, which would allow many celestial objects’ polar regions to harbor subsurface ice long after it had completely retreated from their centers.
The team also calculated that bodies in the middle asteroid belt with a radius greater than 10 kilometers, as well as objects in the outer belt with a radius of just a few kilometers, could still host ice that formed around the time of their accretion. By contrast, the team’s findings indicate that ice on NEOs, which are much smaller, on average, may still be present under select circumstances, such as when their spin axis has a low tilt or the bodies are very young. Altogether, these findings are another step toward a better understanding of the origins of the water that makes Earth habitable. (Journal of Geophysical Research: Planets, https://doi.org/10.1029/2018JE005568, 2018)
—Terri Cook, Freelance Writer