Unusual minerals found in craters on the moon may be alien, a new study suggests.
The claim is made in a paper published in Nature Geoscience that uses computer modelling to show these rare minerals may not be indigenous to the moon as previously believed.
Magnesium-rich spinel and olivine found in the central peaks of lunar craters have long been assumed by scientists to have been forced to the surface by the impact of the asteroid or meteorite that caused the crater.
It was believed any collision that big would melt or vaporize the impacting material, leaving behind only geochemical traces and tiny fragments.
However, Jay Melosh from Indiana's Purdue University and colleagues used computer simulations to show asteroids were capable of still producing these craters at lower impact speeds, giving them greater chance of survival.
"We find that for ... impact velocities below about 12 kilometres per second, the projectile may both survive the impact, and be swept back into the central peak of the final crater as it collapses," the authors write.
On Earth, spinel forms under high temperatures and pressures not seen near the moon's surface. It is also found in some asteroids and meteors.
But the detection of concentrations of spinel and olivine in central lunar crater peaks has raised speculation about the minerals' origin.
The authors performed simulations using a variety of impact speeds between six and 16 kilometres per second.
"We focused on a simulation of the 93-kilometre diameter Copernicus crater because of the reports of olivine and magnesium-spinel in its central peak," the authors write.
"But our simulation also applies to the similar-size Theophilus crater ... as well as to the smaller Tycho crater."
Their simulations revealed about a quarter of lunar impacts occur at speeds below 12 kilometres per second, which is slow enough for a significant fraction of the impacting object to remain largely intact.
"The olivine observed in the central peaks of Copernicus and other lunar craters may be a remnant of the projectile and thus does not indicate deep excavation of the lunar mantle or lower crust," the authors write.
Their conclusions about low-velocity asteroid impacts are entirely plausible, according to astronomer Michael Brown of Melbourne's Monash University.
"When a large asteroid hits Earth, because of the Earth's gravity and because of the velocity of the asteroid, you're looking at impact speeds of 20-30 kilometres per second," says Brown.
"On the moon you have lower gravity ... it's possible that near-Earth asteroids, which move relatively slowly compared to the speed of the Earth or the moon, might come in at such slow speeds that a large part of the asteroid might not melt at all."
According to Brown, asteroid impact speeds can be as low as three, four or five kilometres per second, and still leave a crater with a central peak.
"A rock 10, 20 or 50 kilometres [wide], coming in at five kilometres per second still packs a mighty punch," says Brown. "So it's certainly possible for a large asteroid to create a large crater."
However, Brown says there are some concentrations of spinel on the moon's surface that can't be associated with impact events.
"And to throw another spanner in the works, there are other examples which can't be explained by low-speed impacts," says Brown.
A commentary on the research in Nature by Erik Asphaug of Arizona State University, points to some craters with spinel that also have evidence of large volumes of molten rock.
"That's presumably a signature of a high-velocity impact," says Brown.
"So perhaps some spinel was also excavated from below the lunar surface during an impact after all."