Much of the history and structure of the Moon remain poorly understood — including why the near and far sides of the Moon are so different to one another. Now four papers in the journal Science from the Japanese SELENE (Kaguya) mission shed new light on this mystery.

Scientists believe that the Moon was formed about 4.5bn years ago when a Mars-sized body collided with the young Earth, ejecting huge quantities of material that bound together in orbit. Much of the young Moon is thought to have been in a molten state, with the resulting magma ocean then partially crystallizing and differentiating and forming a distinct mantle and crust. This crust hardened but was then extensively cratered by meteorites during a period of “late heavy bombardment”, which ended about 3.8bn years ago.

The difference between near and far sides is more consistent with processes originating inside the Moon, rather than outside mechanisms Gregory Neumann, NASA

SELENE was launched in September 2007 and has produced a wide range of new data telling us about different stages of the Moon’s evolution, and in particular about the different conditions that existed on the lunar near and far sides at the time of the late heavy bombardment. On Earth we are only able to see just over one half of the surface of the Moon because the Moon’s period of rotation is almost equal to the time it takes to orbit the Earth.

Very different side

The far side first came into view in 1959, when the Soviet spacecraft Luna 3 flew around the Moon — and we now know that it is very different to the side we see in the night sky. The near side is covered with large dark patches that are low-lying pools of solidified lava — the so-called lunar seas or maria. By contrast, the far side consists mainly of bright and more heavily cratered highland material.

Probing the different historical conditions in the near and far sides has been made possible through radio tracking of the Moon’s gravitational field. This involves monitoring the Doppler shift of radio signals from a satellite — which is affected by changes in the velocity of the craft as it travels over regions of greater or lesser gravitational pull.

Previous missions have revealed a number of areas of high gravity, known as “mascons” (short for mass concentrations), on the Moon’s near side. These mascons coincide with maria and it has been proposed that the extra gravitational tug in these regions is due to the dense volcanic deposits inside the maria. However, one such region on the edge of the far side, beneath Mare Orientale, consists of a core of high gravity surrounded by lower than normal gravity. This feature cannot be readily explained by volcanic activity and scientists believe that this mascon might instead be caused by part of the denser mantle being thrust up into the crust during a meteorite impact.

Mascons found

Scientists have speculated whether there might be other mascons on the far side that exist under craters not filled with maria, and now SELENE has found such features.

Noriyuki Namiki of Kyushu University and colleagues at the National Astronomical Observatory of Japan were able to make a far improved gravitational map of the Moon's far side using data from SELENE, which relays its signals to and from Earth via a small companion satellite located in a higher elliptical orbit (previous satellites could not carry out radio tracking when the Moon blocked their line of sight with the Earth). The data show a number of farside mascons, each with a central region of high gravity surrounded by a ring of low gravity (Science 323 900).

Gregory Neumann of the NASA Goddard Space Flight Center in Maryland, who has written an article in Science to accompany the four papers, says that this new gravity map tells us that the farside was cooler than the nearside during the late heavy bombardment. This, he explains, is because only relatively cool conditions would have kept the crust solid and capable of preserving some of the energy of the impacts as gravitational potential, in the form of mascons, whereas a hotter mantle on the nearside would have partially melted, causing magma to rise to the surface and erupt. “This means that the difference between near and far sides is more consistent with processes originating inside the Moon, rather than outside mechanisms such as a giant meteorite impact on the nearside,” he says.

Unlikely to contain water

Meanwhile, Hiroshi Araki of the National Astronomical Observatory of Japan in Tokyo and colleagues have used a laser altimeter onboard SELENE to make a very detailed topographic map of the Moon. Spectral analysis of the map reveals that the Moon’s mantle is likely to contain little water, and this is a key factor in the large-scale structure of the lunar surface, given that the water inside the Earth is instrumental in continent formation and plate tectonics (Science 323 897).

In the two other papers, Takayuki Ono of Tohoku University in Sendai and co-workers report how SELENE’s radar instrument has revealed previously unseen reflective layers beneath the maria on the Moon’s near side, suggesting that a lunar-wide cooling was largely responsible for geological formation within the last 2.8bn years (Science 323 909), while a team led by Junichi Haruyama at the Japan Aerospace Exploration Agency in Sagamihara describes the images obtained using SELENE’s Terrain Camera, and how these images indicate volcanic activity on the far side lasted until at least 2.5bn years ago.