Science & technology | Mapping the Moon with gravity

GRAIL hunt

NASA’s latest Moon mission ends with two bangs

IF EBB and Flow do not sound like the sorts of names that scientists at NASA would give to their space probes, that is because they are not. The two probes started off as “GRAIL A” and “GRAIL B”, but were renamed by a class of schoolchildren in Bozeman, Montana, who thus won a competition run by America’s space agency to give the craft more resonant monikers than the original.

Sadly, Ebb and Flow are no more. Ebb expired at 22:28:51 Greenwich Mean Time on December 17th and Flow at 22:29:21. Their mission over, NASA deliberately crashed them into a lunar mountainside. But the first results of that mission, the Gravity Recovery and Interior Laboratory to give its name in full, were published a few days before the crafts’ demise.

GRAIL was designed to study the Moon’s interior by looking for anomalies in its gravitational field caused by variations in its topography and the density of its rocks. Such anomalies alter the orbits of satellites, and the instruments on board Ebb and Flow were exquisitely sensitive to them.

The idea of using satellites’ orbits to study gravitational anomalies goes back to the 1960s. In that decade, as part of preparations for the Apollo manned landings, NASA sent a series of probes called Lunar Orbiters to scan the Moon’s surface for landing sites. This they did successfully, as the subsequent history of Apollo shows. They also, though, gave the first hints of the world below the surface because their orbits were thrown slightly out of kilter by unexpected features that were dubbed “mascons”, or concentrations of mass.

The mascons corresponded to several of the lunar maria, which are dark patches of the Moon’s surface identified by early astronomers as possible oceans. In a way, the maria are indeed seas. But they are seas of frozen lava, not liquid water. The mascons, which are approximately circular, mark the sites of huge asteroid impacts early in the Moon’s existence that melted its primitive crust, generating this lava.

Ebb and Flow have improved understanding of the pounding that created the mascons and the maria, and a series of papers published in last week’s Science by GRAIL’s chief scientist, the appropriately named Maria Zuber of the Massachusetts Institute of Technology, and her team reveal the preliminary results. The left-hand and right-hand sides of the gravitational map of the Moon that the probes have drawn (see picture above) are the Moon’s far and near faces. The difference in their topographies (the far side is mostly mountainous, and almost all the maria are on the near side) is evident from their gravitational fields, too. The three biggest mascons, corresponding to Mare Imbrium, Mare Serenitatis and Mare Crisium, are visible on the upper right-hand side. Myriad smaller craters, each creating its own gravitational imprint, pepper the whole surface.

This was all expected, of course. But Ebb and Flow have revealed new features, too. Using on-board rocket motors to vary both the distance between the probes and their altitude above the surface (which was 55km when mapping started, and was then gradually brought down to as low as 2km), the mission’s controllers were able to tease out fine detail unavailable from previous lunar satellites.

Holey GRAIL

The first combing of these data by Dr Zuber and her colleagues has yielded two surprises. One is the low density of the lunar crust. The other is that it is traversed by huge cracks.

The crust’s chemical composition is known from the Apollo missions, and also from spectroscopic data. Similar rocks are found on Earth, so their properties are well understood. But Ebb and Flow suggest the Moon’s crust is 12% lighter than it should be, based on this composition. The best guess is that this is because, about 4 billion years ago when the Moon was pummelled by the swarm of asteroid impacts that created the maria (known as the late heavy bombardment), the effects were even more violent than previously suspected. These impacts shattered the crust into rock that is full of holes: Swiss cheese, then, rather than the traditional green variety.

The cracks, however, suggest something stranger still. Most bodies shrink as they cool. That causes their surfaces to wrinkle. Cracks are the opposite of wrinkling. They are caused by tension, rather than compression, indicating the Moon actually expanded during its adolescence.

Most of these cracks—of which there are dozens, some more than 500km long—are straight. That is exactly what would be expected if the surface were being pulled apart because the Moon was expanding. Also, they are filled with solidified magma that rose from the interior to fill the gaps, forming features geologists call dykes. From these observations, Dr Zuber and her colleagues reckon the Moon may have got as much as 10km wider in its youth.

Why this happened is obscure. It suggests the Moon’s formation was a complex process and that different layers inside it have heated and cooled at different rates.

To gather all this information, Ebb and Flow worked in tandem. Though it is possible, as the initial observations of the Lunar Orbiters showed, to measure from Earth the anomalies in the path of a satellite going around the Moon, you can do that only when you can see the satellite in question. Once a probe’s orbit takes it behind the Moon, terrestrial instruments are blind. To study the far side thus requires two satellites. These can observe each other, then return their stored data to Earth when the home planet is visible.

Though the destruction of Ebb and Flow might seem a wanton act, there was method in it for it enabled the mission’s scientists (or, more strictly, its engineers) to carry out one last experiment. This was to fire the two probes’ rocket motors until they stopped working, to find out whether their calculations about how much fuel was left in the tanks were correct. Those calculations suggested the tanks were almost empty, and without fuel the satellites could not manoeuvre in orbit in order to take new readings. If the calculations were wrong, then suitable corrections can be applied to future missions. But if it turns out that the tanks were fuller than expected, then Ebb and Flow may have gone to their graves prematurely.

This article appeared in the Science & technology section of the print edition under the headline "GRAIL hunt"

The gift that goes on giving

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