The Moon Through LRO's Eyes

It's a banner week for all things lunar.

First, don't forget that Saturday is the first-ever International Observe the Moon Night, for which a bunch of different organizations in and out of NASA have partnered to get your looking up at that big, bright, round thing in the sky.

Then, yesterday, a panel of lunar scientists described new global observations of the Moon acquired over the past year by NASA's Lunar Reconnaissance Orbiter. LRO isn't the only spacecraft to have orbited the Moon recently — the Japanese launched Kaguya, the Chinese sent Chang-e 1, and space exploration in India took a big step with Chandrayaan 1. But over the past year the spotlight has been on LRO and, in particular, the ability of its high-resolution camera to pick out long-ago (and sometimes long-lost) spacecraft from the early decades of lunar exploration.

One LRO instrument that's been quietly amassing observations is the Lunar Orbiter Laser Altimeter. By firing 140 ultrashort pulses of laser light at the surface per second and precisely timing their reflections, LOLA has amassed 2½ billion altitude measurements in its first year of operation. That's enough coverage to create a topographic map of the entire globe. Combine that with a little 3D shading, and the result is a mind-blowingly detailed "relief map" that reveals every nook and cranny on the lunar surface — without having to worry about the vagaries of lighting geometry, surface brightness, image artifacts, and the like.

A team led by James Head III (Brown University) has used the ever-improving LOLA database to identify every crater on the Moon, near- and farside, at least 12 miles (20 km) across. The total, in case you're curious, is 5,185 craters. When Head's team mapped the distribution of these craters, they found that some areas of the lunar highlands are completely saturated with impacts. In other words, craters are packed so closely together that every new one destroys an old one.

Most of those hits came during the Moon's first billion years of existence. But planetary scientists have long debated whether the impacting asteroids and comets come in an assortment of sizes — what's termed a size-frequency distribution (SFD) — that's always been the same. Some geologists, most notably Robert Strom (University of Arizona), have hypothesized that there weren't nearly as many really big boppers in the mix later on as there were initially. Others, most notably Gerhard Neukum (Free University, Berlin) have argued that the SFD has remained the same even though the impact rate diminished over time.

What the LOLA data reveal is that Strom had it right: the older, heavily cratered highlands have more big craters, compared to small ones, than do the younger surfaces covering the maria. The transition, Head's team suggests, probably came about when the Moon's last giant basin, a 600-mile-wide multiringed bull's eye called Orientale, became forever etched in the lunar landscape about 3.8 billion years ago.

The other two Science papers describe LRO's mapping of the Moon in a whole new light. An instrument called Diviner has been taking the Moon's temperature by mapping the thermal (heat) radiation coming from its day and night sides. For example, about this time last year, the Diviner science team announced that temperatures at and near the lunar poles are among the coldest ever measured in the solar system.

Now attention has turned to Diviner's ability to record the infrared "fingerprints" of different minerals in the lunar surface. A team led by Benjamin Greenhagen (Jet Propulsion Laboratory) reports that overall the Moon's composition is much as geochemists have believed. There's old, heavily cratered "crust" consisting of anorthosite, a mix of low-density silicate minerals largely lacking dense metals like iron and magnesium; and there's younger, metal-enriched basalt covering the maria.

But Diviner has also turned up some oddities here and there. For example, geochemists had hoped that an enormous basin on the lunar farside, kludgily named "South Pole-Aitken," had punched down deep enough to dredge up chunks of the lunar mantle. If some of that lay atop the lunar surface, waiting to be plucked by some future lander, it could clear up many unknowns about how the Moon came to be. But no such luck, say Greenhagen and his team: Diviner doesn't see any evidence for pristine mantle material anywhere on the lunar surface.

Timothy Glotch (State University of New York) and others report that a smattering of surface patches appear to be highly enriched in silica (SiO₂). Some of these exposures probably result from volcanoes spewing the last-to-solidify dregs of subterranean magma reservoirs. Others, notably a silicic spot near the already-weird Aristarchus, might be telling us that a near-surface hotspot caused the anorthositic crust to melt and erupt onto the surface.

(As an aside, "silicic" is a great word — one that seems to be all consonants.)

None of these LRO results are turning lunar science upside down. But they will alter how planetary scientists approach questions of the Moon's formation and history. Moreover, this "renaissance of understanding," Head notes, "opens up the whole Moon to future human and robotic exploration."

Learn more about yesterday's announcements by clicking on the NASA press releases here and here. The new results appear as a trio of reports in the September 17th issue of Science.