Unique Process Formed a Strange Lunar Dome

We tend to think of the Moon as a boring old rock.  “We’ve been there, so we know all there is to know, onto the next one.” But the Moon still holds secrets, and has its surprises.  The formation of the Moon through collision of proto-Earth with a Mars-sized object is an idea that can give us a lot of insight into how the Earth formed and what raw materials we started with.  But of course to study it we have to see what the Moon is made of in addition to what the Earth’s rocks contain.

In recent years, orbiting satellites have revealed new and unexpected information about the Moon. Discovery of water contained in craters could give humanity future rocket fuel on a Moon base, acting as a staging point to Mars and beyond.  And very recently, a team of geologists have come up with two possible scenarios to explain a strange domed structure on the surface, describing geologic processes that have never been seen before.

A topographic view of the South Pole-Aitken Basin. Reds are high; blues are low. Mafic Mound, (the reddish area in the center) stands 800 meters above the surrounding surface. Credit: NASA/Goddard/MIT/Brown

Mafic Mound, standing 800 metres tall with a diameter of 75 kilometres, is a unique structure in  the centre of a large impact crater called the Aitken Basin at the Moon’s South pole.  Scientists are suggesting it formed through unique volcanic processes during the original collision that formed the Aitken basin.

“If the scenarios that we lay out for its formation are correct, it could represent a totally new volcanic process that’s never been seen before,” said Daniel Moriarty, a Ph.D. student in Brown’s Department of Earth, Environmental and Planetary Sciences and the study’s lead author.

In the first scenario, the impact would create a giant chasm of molten rock, around 50 km deep.  The slow cooling of the massive molten rock sheet would cause it to contract.  The contracting cooled rock on top would push the still-molten subsurface rock up through the centre of the basin, like squeezing out a tube of toothpaste.

The second scenario involves a large amount of rock being blasted away by the impact.  The lack of rock would create a region with slightly lower gravity, allowing the rock to rebound upwards, shock-heating the mantle and erupting through the centre of the impact, forming the mound.

Both theories fit data obtained by the researchers from three orbiting spacecraft.  The Moon Mineralogy Mapper, flying aboard India’s Chandrayaan-1 spacecraft, provided minerological data, NASA’s Lunar Orbiter Laser Altimeter provided topographic data, and the GRAIL mission characterized local gravitational anomalies.

The biggest clue was the mineralogical data, which showed that the mound is rich in high-Calcium Pyroxene, while the surrounding material is largely lacking in Calcium content.

As with all foreign worlds, the data we can get from orbiting spacecraft has a limit, and to truly understand how Mafic Mound formed, we have to bring back rock samples from the site.  This will give a precise measure of the mineralogical make-up of the region and allow researchers to pinpoint the formation time-line and understand which of the unique processes was responsible for the strange mound.

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