Comet impacts over the past 100 million years can account for many of the features in the mysterious lunar swirls – wispy bright regions scattered on the lunar surface, says a group of planetary researchers from the Lawrence Livermore National Laboratory and Brown University.
The Reiner Gamma formation is a well known lunar swirl region, due to its nearside location. Swirls are described by curving, diffuse bright patterns, which are interspersed with darker lanes. These anomalous albedo patterns are associated with local magnetic anomalies of varying intensities; swirl complexes are often extensive and may stretch across thousands of kilometers, well beyond localized magnetic anomalies. Image credit: NASA / LPI.
Twisty swirls of bright soil on the lunar surface have long fascinated scientists.
In some cases, they stretch for thousands of miles across the Moon’s surface. Most are found on the unseen far side of the Moon, but one famous swirl called Reiner Gamma can be seen by telescope on the southwestern corner of the Moon’s near side.
An early theory said the swirls were caused by anomalies in the lunar crust’s magnetic field. But a new study, published in the journal Icarus, suggests a different cause – crashing comets.
“We think this makes a pretty strong case that the swirls represent remnants of cometary collisions,” said Dr Peter Schultz of Brown University, senior author on the study.
“At first glance, the swirls do not appear to be related to large impact craters or any other topography. They simply look as if someone had finger-painted the surface. There has been an intense debate about what causes these features.”
In the 1970s, scientists discovered that many of the swirls were associated with anomalies of the Moon’s crustal magnetic field. That revelation led to one hypothesis for how the swirls may have formed.
But Dr Schultz had a different idea for how the swirls may form – one that has its roots in watching the lunar modules land on the Moon during the Apollo program.
“You could see that the whole area around the lunar modules was smooth and bright because of the gas from the engines scoured the surface. That was part of what got me started thinking comet impacts could cause the swirls,” Dr Schultz said.
Comets carry their own gaseous atmosphere called a coma. When small comets slam into the Moon’s surface, the coma may scour away loose soil from the surface, not unlike the gas from the lunar modules. That scouring may produce the bright swirls.
Dr Schultz first published a paper outlining the idea in the journal Nature in 1980. In the new study, he and his colleague, Dr Megan Bruck-Syal from the Lawrence Livermore National Laboratory, analytically and numerically model the effects of a comet impact at the Moon. The simulations showed that the impact of a comet coma plus its icy core would indeed have the effect of blowing away the smallest grains that sit atop the lunar soil.
They showed that the scoured area would stretch for perhaps thousands of kilometers from the impact point, consistent with the swirling streaks that extend across the Moon’s surface. Eddies and vortices created by the gaseous impact would explain the swirls’ twisty, sinuous appearance.
The comet impact hypothesis could also explain the presence of magnetic anomalies near the swirls. The simulations showed that a comet impact would melt some of the tiny particles near the surface. When small, iron-rich particles are melted and then cooled, they record the presence of any magnetic field that may be present at the time.
Taken together, the results offer a more complete picture of how the lunar swirls form.
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Megan Bruck Syal & Peter H. Schultz. 2015. Cometary impact effects at the Moon: Implications for lunar swirl formation. Icarus, vol. 257, pp. 194-206; doi: 10.1016/j.icarus.2015.05.005
