The hugely successful GRAIL twins impacted the Moon on Monday (December 17^th), ending their extended mission to map the gravitational field and interior of the Moon. Meanwhile, the LADEE orbiter is getting through various environmental tests ahead its launch in summer 2013.

Ebb and Flow, the two spacecrafts of the GRAIL mission came to rest at a 2.5-km mountain on a crater rim near the lunar North Pole. NASA named the impact site after the late astronaut Sally Ride, who led the MoonKAM educational program of the mission. GRAIL mapped the lunar gravitational field with exquisite detail, and the first results already revealed surprises. I just quickly recall a few, but I advise all of you to read Emily Lakdawalla's wonderful post at the Planetary Society to get the background and the details.

The calculated crustal density of the lunar highlands.

The average crustal density of the Moon was found to be 2550 kg/m³, 12% lower than previously assumed. This means that the crust must be fractured and porous, caused by the constant hammering and shattering of impacts. It is even lower around relatively young mare regions that deposited a thick layer of even more porous and fractured ejecta around them but higher at the South Pole-Aitken basin which exposes denser, iron-rich material.

The lunar gravity gradient map, dikes (linear features) are highlighted.

Anything we see on the surface is at least 700 million years younger than the Moon itself. Any feature before that age was obliterated by the impacts. GRAIL, however, was able to gather information even beyond that limit. The lunar gravity gradient map shows how steeply the gravity changes from one spot of the surface to the next. Crater and mare borders are obvious, but some other, interesting linear features show up too. Those are the fingerprints of really old dikes: long, straight features filled with denser material. Dikes are like stretch marks, they mean that the body beneath the crust was expanding and thus the crust fractured. We know that the Moon shrunk as the interior cooled but what could have possibly caused any expansion? The answer lies in the unique way the Moon formed: after the huge impact, the debris ring around the Earth first formed a relatively cold nucleus that was eventually surrounded with a mantle, heated from the energy of impacts. This heat slowly propagated to the nucleus, and the gradual heating and expansion fractured the crust.

LADEE during construction

We lost two great spacecrafts, but fear not: soon another will arrive to the Moon. LADEE (Lunar Atmosphere and Dust Environment Explorer) will investigate the extremely thin exosphere and the dust environment around the Moon, and find out if the diffuse emissions above the surface, sighted by the Apollo astronauts is dust or glowing sodium gas. LADEE will be equipped with three scientific instruments: a Neutral Mass Spectrometer, a UV-Visual Spectrometer and the Lunar Dust Experiment. All three units are based on the instruments of other missions, from LCROSS to MSL and Cassini. It will also be a technology demonstrator: the base of the spacecraft is the Modular Common Spacecraft Bus and and it will be equipped with a laser communications demo unit.

Electromagnetic interference test

LADEE is currently going through various environmental tests: electromagnetic interference, acoustic, vibration, shock and thermal-vacuum evaluations. If the spacecraft passes all of them without problems or delays, the launch will happen in August 2013, from Wallops atop a Minotaur V rocket. It will take 5 months to reach the nominal lunar altitude, a near-circular orbit at about 50 km. The primary mission will be 100 days which will be then followed by a 9-month technology-demonstration phase at a higher orbit.

László Molnár

Image credits:

1. NASA/JPL-Caltech/GSFC/ASU
2. NASA/JPL-Caltech/ IPGP
3. NASA/JPL-Caltech/CSM
4. 5. NASA/Ames RC