A new study supported in part by the Emerging Worlds Program uses simulations to examine the amount of mass from impactors that could have been retained in the lunar crust and mantle during the later stages of the Moon’s accretion.

Planets and many other large objects in space coalesce from dust and debris in a process called accretion. Understanding accretion can help astrobiologists determine how and where terrestrial worlds might form around stars. This information is important in determining the initial conditions that can ultimately lead to habitable planets similar to the Earth. Certain elements can help scientists track the later stages of this accretion process, namely a class known as highly siderophile elements (HSEs). HSEs include gold, iridium, osmium, palladium, platinum, rhenium, rhodium and ruthenium.

The Earth and the Moon are thought to have originated from a cache of similar materials. One of the most popular theories is that a large object was well into its accretion process, and then was struck by a second, large object. The resulting debris from this massive impact then accreted into the objects that became the Earth and the Moon. However, the HSE budget of the Earth and the Moon do not match up. The Earth appears to have accreted a much larger mass of HSEs.

The team of scientists turned to impactor simulations to better understand why this is the case. Their work shows that the amount of mass retained by the Moon from impacts could be three times lower than previously thought. In order to reach the amount of HSEs estimated to be in the lunar crust and mantle today, the Moon may have only started retaining mass from impacts around 4.35 billion years ago. At this time, most of the magma ocean that covered the Moon early in its formation would have been solidified. The team believes that any HSEs that arrived on the Moon prior to this time would have been lost to the lunar core. If HSEs were lost to the core early in the Moon’s accretion, and only HSEs arriving later were retained in the mantle, this could explain why the late-accreted mass of the Moon is much lower than the Earth’s.

The study, “Reconstructing the late-accretion history of the Moon,” was published in the journal Nature. This work was supported by the Emerging Worlds Program. The NASA Astrobiology Program provides resources for Emerging Worlds and other Research and Analysis programs within the NASA Science Mission Directorate (SMD) that solicit proposals relevant to astrobiology research. This research is a critical part of NASA’s work to understand the Universe, advance human exploration, and inspire the next generation. As NASA’s Artemis program moves forward with human exploration of the Moon, the search for life on other worlds remains a top priority for the agency.