How comets paint swirls on the Moon

Swirls2

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,’ Schultz said. ‘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. Rocks below the surface in those spots might contain remanent magnetism from early in the moon’s history, when its magnetic field was much stronger than it is now. It had been proposed that those strong, locally trapped magnetic fields deflect the onslaught of the solar wind, which was thought to slowly darken the moon’s surface. The swirls would remain brighter than the surrounding soil because of those magnetic shields.

But 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,’ Schultz said. ‘That was part of what got me started thinking comet impacts could cause the swirls.’

Comets carry their own gaseous atmosphere called a coma. Schultz thought that when small comets slam into the moon’s surface – as they occasionally do – 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.

Schultz first published a paper outlining the idea in the journal Nature in 1980. That paper focused on how the scouring of the delicate upper layer of lunar soils could produce brightness consistent with the swirls. The structure of the grains in the upper layer (termed the ‘fairy castle structure’ because of the way grains stick together) scatters the sun’s rays, causing a dimmer and darker appearance. When this structure is stripped away, the remaining smoothed surface would be brighter than unaffected areas, especially when the sun’s rays strike it at certain angles. For Reiner Gamma on the lunar nearside, those areas appear brightest during the crescent moon just before sunrise.

As computer simulations of impact dynamics have gotten better, Schultz and Bruck-Syal decided it might be time to take a second look at whether comet impacts could produce that kind of scouring. Their new 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. The simulations 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. ‘Comets carry with them a magnetic field created by streaming charged particles that interact with the solar wind,’ Schultz said. ‘As the gas collides with the lunar surface, the cometary magnetic field becomes amplified and recorded in the small particles when they cool.’

Taken together, the results offer a more complete picture of how the swirls form, the researchers say.

From Crashing comets may explain mysterious lunar swirls

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American geophysicists believe the moon’s Procellarum region, a dark patch more than half the size of Australia, resulted from a magma plume rather than a massive asteroid strike, as previously thought. The finding “deals a big blow to the asteroid theory”, said Brown University, which contributed to the study. Procellarum is the only lunar “sea” big enough to be called an ocean and is one of a number of dark spots on the moon’s surface that, when seen from Earth, resemble a face. Unlike other dark areas such as the Sea of Rains and Sea of Seren­ity, Procellarum is not surrounded by signs of impact, such as mountains and scars. Scientists have long debated whether Procel­larum is so old that the impact signs have been eroded, or was formed by a different process. Now researchers say they have settled the argument, using data from twin NASA spacecraft that orbited the moon in 2012 and mapped its gravity. The team ­created a high-resolu­tion map showing Procellarum’s border composed of sharp angles that could not have been created by an asteroid. The researchers believe the angular outline was produced by giant tension cracks in the crust as it cooled around magma from deep inside the moon. Maria Zuber, co-author of a paper in the journal Nature, said tthe cracks had formed a “plumbing system” which had allowe­d magma to ­meander to the surface and created the dark spots we see.

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Among the things the astronauts left to lighten the load for the return trips were their “defecation collection devices,” also known as emesis bags (top). So decades-old containers filled with decades-old astronaut turds are still hanging out on the Moon.

In addition to the cool-gross factor, this astro-poop has some scientific and, with the other artifacts up there, cultural value. Some astrobiologists are interested in how bacteria in the abandoned feces have fared, and some anthropologists and historians would like to see the moon landing sites and all the artifacts there protected as part of a World Heritage Site. 

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Since 1996, scientists have debated about whether the Martian meteorite ALH84001 contains evidence that life once existed on Mars. The rock holds some microscopic wormy-looking structures that some scientists have suggested could be fossilized remains of life on Mars, whereas others say the weird shapes derive from normal geochemical processes.

In a new study, physicists at University of Kent tested the hypothesis with a big gun. More specifically, they took powdered diatoms (a type of microscopic algae with a hard silica shell), packed them inside a nylon bullet, added water, and froze the sample. Then, they loaded the bullets inside a light gas gun and fired them at a sack of water at speeds ranging between 0.25 and 3.1 miles per second.

When they looked in the water afterwards, the researchers analyzed the whole and partial remains of the little diatom fossils. They concluded that small fossils could survive a meteorite impact, and that if they exist, then it’s possible to find them inside meteorites.

But there are a few important caveats.  At impact speeds above 0.62 miles per second, none of the diatom fossils survived in one piece—they broke into tiny shards. And the faster they crashed into the water, the tinier the diatom bits became.  That’s a problem for any potential fossils that would fall to Earth from other planets, because meteoroids enter the Earth’s atmosphere at speeds between 6.8 and 44.7 miles per second before they hit Earth, according to the American Meteor Society.

The other important limitation is that the diatoms were shot frozen in ice, meaning they potentially behave differently during impact than they would if they were encapsulated in rock.

So the jury is definitely still out on ALH84001, and it probably will be for many years. Even if tests provide stronger evidence that fossils can travel between planetary bodies, it doesn’t necessarily mean they did.

What is pretty neat is that, because meteorite impacts tend to be slower on the Moon, it looks like fossils that have been smashed off from Earth could survive a collision with our natural satellite.  The authors conclude that the lunar surface could be a good place to scout for fossils, and those terrestrial transplants may be better preserved on the Moon than if they had remained on Earth.

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Gregory Benford’s Guide to Terraforming the Moon

fuckyeahdarkextropian:

When it comes to remaking a celestial body in Earth’s image—“terraforming” it—the moon has clear advantages: It gets twice the sunlight of Mars. It’s a three-day trip with current technology, while getting people to Mars would take six months. Furthermore, the moon is dead and it’s small, so it…

Stick with me kids and you could have a job in the Kuiper Belt firing comets into the Moon, making it rain there for ten thousand years. In your new space-hardened, posthuman body. Would you like to know more?

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