Shooting the moon

By Samantha Weinberg

The Moon is typically seen bathed in the white light of the sun. Like Earth viewed from above, in its full state it looks like a perfect sphere, as smooth as a snooker ball. In fact, as scientists from Arizona State University in Tempe have just revealed in a high-resolution Moon map, it has a topography that is as varied and in places extreme as anything on our planet.

In the course of its monthly elliptical orbit, the Moon moves around Earth at an average distance of roughly a quarter of a million miles. Space is a harsh environment, and the Moon has virtually no atmosphere, extremes of temperature (from -233°C at night to +123°C in the day), and no human presence—which makes mapping it tricky.

To do it, the multinational team behind this image first designed a new Wide Angled Camera (WAC) capable of withstanding the vicissitudes of outer space. They then hitched a lift for it on a spacecraft: the Lunar Reconnaissance Orbiter (LRO), which was launched by NASA in 2009 and has already gone beyond its initial mission to spend a year in close orbit of the Moon. The palm-sized WAC was mounted on the LRO, which flies at an average altitude of 50km (30 miles) above the Moon’s surface. Each picture it takes covers a swathe 70km wide.

As the LRO is constantly circumnavigating the Moon, with each orbit 30km (in ground distance) from the last, the pictures it takes overlap, giving a stereo view of almost every patch of lunar ground, every month. About 69,000 of these images were sent back and fed into a computer, which used them to build an accurate model of almost all the terrain. The bits at extreme latitudes that the WAC couldn’t survey—the holes at the poles—were covered by another laser instrument on board, known as LOLA (Lunar Orbiter Laser Altimeter).

But it was only when the models were shaded, to give a sense of the relief, and coloured in with different altitudes shown by different colours, that the topography was transformed into a living landscape—of craters, mountains and the vast basalt plains that early astronomers mistook for seas and lakes. “I could not be more pleased with the quality of the map,” said Mark Robinson, who heads the Arizona team. “It’s phenomenal. The richness of detail should inspire lunar geologists around the world for years to come.”

Seen as a whole, the Moon in these images resembles a pitted sphere of pumice more than a cue ball (the picture here shows the far side of the Moon). But each lump and dip gives scientists a clue to how it was formed. The Buys-Ballot crater, for instance, is a pear-shaped depression (near the centre of this image), measuring 47km by 62km at its longest and widest points, with a flat floor that lies 4,600 metres below the western rim—making it wider and deeper than the Grand Canyon.

The images show clearly that the crater floor is very dark, which suggests that it could be formed from iron-rich basalt, typically originating as very hot lava—possibly the output from a volcanic eruption. The Orientale Basin (the blue smudge on the rim of this picture, between three and four o’clock) is one of the largest lunar landmarks. At 900km across, ringed with concentric ranges of Moon mountains, it is believed to be the result of the most recent giant impact in lunar history.

The LRO Moon map backs up the theory that the Moon started out as a giant ball of molten magma, formed about 4.5 billion years ago, the unlikely love-child of a violent encounter between Earth and a Mars-sized object. The magma cooled into the Moon’s crust, before, soon after, being hit by the first of a series of large objects which left craters in their wake, forming the pockmarks you see here.

The map should shed light forwards as well as back, Robinson says. “We can now determine slopes of all major geologic terrains on the Moon…determine how the crust has deformed, better understand impact crater mechanics, investigate the nature of volcanic features, and better plan future robotic and human missions.”

Man has not walked on the Moon since 1972. At the end of last year China announced its aim to send a manned mission some time in the next decade and a half. If it comes to fruition, the astronauts will now, thanks to NASA and some scientists in Arizona, have a good map to guide them around the place. ~ Samantha Weinberg

For more information: lroc.sese.asu.edu