Solving a Lunar Mystery with Extrasolar Planets

Composite image of the lunar nearside taken by the Lunar Reconnaissance Orbiter in June 2009. Note the presence of dark areas of maria on this side of the moon. Image: NASA

Composite image of the lunar nearside taken by the Lunar Reconnaissance Orbiter in June 2009. Note the presence of dark areas of maria on this side of the moon. Image: NASA

In 2009, astronomers discovered the planet COROT-7b orbiting a star 489 light years from Earth. COROT-7b had some interesting things to teach astrobiologists about the nature of extrasolar planets, but recently it also helped solve a 55-year-old mystery much closer to home.

Throughout human history, the Moon has been a familiar sight in the night sky. One of the first things you notice when looking at the Moon is the pattern of large, dark regions that give it a recognizable ‘face.’ These dark areas are actually basalt seas known as maria.

For centuries, humankind has had the same view of the Moon’s face because Earth’s natural satellite is tidally locked to our planet. This means that the nearside of the Moon always faces the Earth.

In 1959, the Soviet Luna 3 spacecraft returned the first images of the Moon’s farside. Scientists were surprised to see that very few maria existed on this side of the Moon, which permanently faces away from our planet. Nobody knew why the two sides of the Moon were so different. The question became known as the Lunar Farside Highlands Problem.

Now, a team of astrophysicists from Penn State may have solved the mystery.

“I was listening to a talk about the exoplanet CoRoT-7 b during our department colloquium by Dr. Diana Valencia, an exoplanet researcher,” explained Jason Wright, co-author of the study and Assistant Professor of Astronomy at Penn State University. “She discussed how the heat of the star CoRoT-7 effected the geology of the surface of the close-in exoplanet orbiting it.  The star-facing side of the planet is so hot the rock is molten.  This got me thinking about the young Moon, which, when it was forming, was similarly close to the Earth. At that time, the Earth was as hot as a star because of the impact that formed the Moon.”

Composite image of the lunar farside taken by the Lunar Reconnaissance Orbiter in June 2009. Note the absence of dark areas. Image: NASA

Composite image of the lunar farside taken by the Lunar Reconnaissance Orbiter in June 2009. Note the absence of dark areas. Image: NASA

The leading theory for the origin of the Moon begins with a Mars-sized body slamming into the proto-Earth shortly after our planet formed. The collision created a ring of dust and debris that circled our planet, and coalesced to form the Moon.

The energy from this immense impact event would have left both the Earth and Moon as giant balls of molten rock. The Moon cooled down first, and the much larger Earth remained hotter for longer.

“Earth basically looked like a small star from the Moon’s perspective,” added lead author Arpita Roy, also of Penn State. “That much energy being radiated towards one face of the Moon had to have had noticeable consequences (maybe even long-lasting ones).”

The energy cooked the nearside of the Moon while the farside stayed dark and relatively cool. Early in its formation, the Moon had a thick atmosphere and was covered in magma oceans. As it cooled, the atmosphere began to condense… but the temperature gradient created by heat from the Earth meant that things condensed differently on the nearside and the farside.

“The farside of the Moon accumulated calcium and aluminum, two of the building blocks of the particular kinds of rock that characterize the Moon’s crust,” said Wright.

Over time the Moon cooled and the crust was formed. With more raw materials – the deposits of Calcium and Aluminum – the crust on the farside of the Moon became thicker.

Artist impression of the extrasolar planet Corot-7b. Credit: ESO/L. Calcada

Artist impression of the extrasolar planet Corot-7b. Credit: ESO/L. Calcada

“The GRAIL mission made sensitive measurements showing that [the crust] is 20–30 kilometers (km) thick on the nearside, and up to 60 km thick on the farside – but with lots of variation from mountains and impact basins,” said Roy.

Over time, impacts occurred on both sides of the Moon. The thin crust on the nearside made it easier for basaltic lava to later bubble up from underneath and fill the impact basins.

The study provides a possible explanation for the Lunar Farside Highlands Problem. In addition, it’s a wonderful example of comparative planetology and the multidisciplinary nature of astrobiology. By studying a planet 489 light years away, scientists revealed information that helped solve a longstanding question about our closest neighbor in the Solar System. Now, maybe the Moon itself can provide a nearby test-bed for how rocky worlds form around distant stars.

“The Moon is our closest celestial body and it informs a lot of our understanding of how planets and their satellites are formed,” said Roy. “Detailed modelling of the early stages of Moon formation could tell us a lot about what to expect in terms of observing other solar systems in different stages of formation.”

“Planets orbiting other stars almost certainly have moons that we have not detected yet, and the formation of the Moon was caused by a giant impact,” added Wright. “These giant impacts may have happened to other exoplanets, as well, and have implications for those planets’ ability to have liquid surface water.  The formation of the Moon tells us a lot about how planets form in general, and what they are made of.  Someday, we might even detect the afterglow of a similar giant impact in another planetary system.”

The new study provides further details about the origin of the Moon, and could improve the accuracy of moon formation models.

“We hope or work on the Moon will inspire others to consider the effects of the hot young Earth in more detailed studies of the Moon’s formation,” said Wright.

The study was published in the The Astrophysical Journal Letters, and supported by the NASA Astrobiology Institute and the Pennsylvania State Astrobiology Research Center.