Remains of meteorites that hit the Moon at low velocities may be preserved within lunar craters, researchers in the US report. The team used computer simulations to show that nearly a quarter of craters may contain significant remnants of the projectiles that formed them, left behind as deposits in the craters' central peaks.

The lunar surface is mainly made of the igneous rocks basalt and anorthosite. Recent spectroscopic observations of the Moon by lunar orbiters, however, have revealed the presence of deposits of unexpected compositions – such as magnesium-rich spinels and olivines – within a number of the larger lunar craters. One such crater containing these deposits is known as Copernicus and has a diameter of around 100 km.

Impacting projectiles

On the Earth, spinel is often associated with both intense metamorphism – formed in conditions of extreme temperature and pressure – and the rock peridotite, which dominates the make-up of the upper mantle. Given this, the spinel seen in impact craters on the Moon is often considered to have had its origins in the lunar mantle – having been brought up to the surface during crater formation. These minerals, however, are also common in many asteroids and meteorites, suggesting the possibility that rather than being vaporized on impact as previously assumed, significant deposits of impactors may be left in the craters they create.

The researchers tested this theory by running 2D simulations of meteorite impacts. A two-layer model was used to represent the Moon – with a dunite mantle overlain by a 30 km layer of granite, which represents a minimum estimate for the thickness of the lunar crust. In order to reproduce a crater similar to Copernicus, a 7 km diameter dunite projectile was selected – with impact velocities ranging from 6–16 km/s, based on previous estimates of velocities of lunar impactors originating from the main asteroid belt.

Simulated collisions

Even with its minimal estimate of the Moon's crustal thickness, the team observed that mantle material was not unearthed in any of the simulated collisions. The maximum excavation depth was seen to be only 7 km – less than a quarter of the modelled crustal depth. With the projectiles, however, while the impactors at velocities above 14 km/s were seen to vaporize, those below 12 km/s left significant deposits behind. In small craters, such remnants were found dispersed in the impact ejecta and across the crater floor, ultimately forming the broken rocks that fill the final hole. Larger craters, however, underwent crater collapse, sweeping the majority of the projectile fragments back together.

"Much to our surprise, we discovered from [our] modelling...that much of the impacting projectile might not only survive the impact, but that its broken remnants become concentrated in the central peaks of craters large enough to produce such," says the lead author of the paper published in Nature, Jay Melosh from Purdue University in the US. With around 25% of lunar craters predicted to be caused by impacts occurring at below 12 km/s, this result suggests that there could be a significant amount of projectile remnants preserved on the Moon's surface – possibly even including remains from ejecta from the early Earth.

"Although the idea is interesting and appears to be physically plausible, I am not sure how significant it really is in creating the mineralogical signatures observed in lunar craters," says Marc Norman, a research fellow at the Australian National University, who was not involved in the study. Norman comments that the preservation of meteorite debris on the Moon is quite rare, with only a few grains having been recognized so far. He also notes that large exposures of olivine have been observed around the rim of Copernicus, the crater that the team was emulating. Such geological evidence, he says, is more consistent with olivine excavated from the Moon's crust than the impact deposit patterns predicted by this study.

A further issue with the reality at Copernicus is also presented by Erik Asphaug of Arizona State University in his "News and Views" letter associated with the paper in Nature. Asphaug suggests that the large volumes of melted rock in the crater indicate formation resulting from a high-velocity impactor – an origin that would, according to the team's modelling, result in vaporization of the projectile, rather than the spinel deposits observed.

Melosh, however, states that there is still plenty of melt production from the anorthositic crust of the Moon with impactors colliding at 10 km/sec. "The olivine projectile is much more difficult to melt than the lunar crustal rocks," he told physicsworld.com – and therefore Copernicus did not need a high-velocity impactor to explain its creation.

The work is published in Nature Geoscience.