Ebb and Flow, the twin Gravity Recovery And Interior Laboratory (GRAIL) spacecraft launched by NASA in September 2011, have revealed unexpected details about the Moon's interior.
|Update: Having completed their extended missions, the GRAIL probes were intentionally crashed into the Moon on December 17th. They hit the lunar surface 30 seconds apart, at 5:28:51 p.m. EST and 5:29:21 p.m. EST, striking the southern face of a tall, unnamed mountain at 75° 37′ north, 26° 38′ east (northwest of the crater Goldschmidt). NASA also announced that the collision sites would be named to honor the late astronaut Sally Ride.|
Just as a rock pile on Earth is full of gaps and spaces, so too is the upper lunar crust. According to investigator Mark Wieczorek (University of Paris), on average this pulverized outer layer has a porosity of 12% — though he excluded the maria, because their high density would bias the measurements. The porosity is even greater (up to 20%) around relatively fresh mega-impacts like the far side's Orientale and Moscoviense basins.
Also, although the lunar crust was presumed to be 40 miles (60 km) thick in post-Apollo days, and somewhat less than that based on more recent spacecraft results, it now seems that nowhere is the Moon's rigid outer skin thicker than about 27 miles (43 km) — and quite a bit less in many spots. Moreover, its density is much lower than thought, averaging only 2.55 g/cm3. This thinner skin implies that the Moon's bulk allotment of aluminum (an element common in low-density silicate rocks) must be very comparable to Earth's, notes Wieczorek — yet another line of evidence reinforcing a "big splat" lunar origin.
So what's the source of these revelations? They are courtesy of identical spacecraft named Ebb and Flow, the business end of NASA's Gravity Recovery And Interior Laboratory mission. GRAIL's objective is to map the Moon's gravity field. Those of you "of a certain age" might recall that this has been done before, beginning with the first orbiters in the 1960s. Back then NASA flight controllers noticed that their spacecraft were periodically accelerating and decelerating very slightly while orbiting the Moon. Apparently specific lunar features, most notably some of the lava-topped maria, harbored concentrations of mass (mascons) that weren't obvious to the telescopic eye. Since then other orbiters, notably NASA's Lunar Prospector and Japan's Kaguya, have produced ever-better gravity maps.
But GRAIL's new gravity maps are an eye-popping 100 to 10,000 times more detailed than previous efforts. During its prime mapping phase, from March 1st to May 30th, Ebb and Flow were never separated by more than 135 miles (218 km) as they chased each around around the Moon at a mean altitude of just 35 miles (55 miles).
Here's how it works: When the lead craft is drawn toward a mass down below — be that a high-standing crater rim or a buried mascon — it accelerates slightly farther ahead of its sibling and vice versa. The key to GRAIL's success is that the craft kept track of each other's whereabouts five times per second — and they do so very, very accurately, detecting velocity changes as small as 50 nanometers per second. "That is one twenty-thousandth the velocity that a snail moves," comments Maria Zuber, the MIT geophysicist who serves as the mission's principal investigator.
In fact, the just-released gravity maps are so good, she notes, that they resolve the tiny gravitational effects from the high rims and low floors of every lunar crater at least 20 miles (30 km) across and a great many smaller ones.
The new results appear online in December 5th's Science Express. Those three articles are accessible by subscription only, but you can freely view and download a series of terrific global views and animations of GRAIL results prepared by the Science Visualization Studio at NASA's Goddard Space Flight Center.
At NASA's briefing, investigator Jeff Andrews-Hanna (Colorado School of Mines) described the discovery of narrow high-density structures up to 200 miles long that crisscross the surface. These appear to be dikes, thin veins of solidified magma buried globally beneath the lunar surface. They appear to be very ancient, because they're frequently interrupted by craters. "There'll be a lot of work to understand what these dikes mean for lunar evolution," notes Andrews-Hanna.
In order for the dikes to form, the young lunar crust must have become fractured when the interior expanded and pushed outward. All told, the Moon's diameter appears to have increased by up 6 miles (10 km). By implication, the lunar interior must have have started out cooler than the exterior; then the interior got hotter and expanded — but only after the crust had solidified. (Theorists think this outside-in heating scenario is precisely what would have occurred if the Moon built itself up from collisional debris.)
The GRAIL mission isn't over, at least not quite yet. A few months ago, flight controllers cut Ebb and Flow's altitude by half, to an average of just 14 miles (23 km), and that should yield another fourfold improvement in the gravity map's resolution.
But there's a penalty for skimming so close to the crater-tops: "We need to do three maneuvers a week just to avoid crashing on the Moon," Zuber cautions. In just a few weeks, the craft will be allowed to crash into the Moon — most likely targeted and timed so that the eagle-eyed Lunar Reconnaissance Orbiter can watch.