NASA's GRAIL lunar orbiters produce most detailed gravity map of the Moon
Variations in the lunar gravity field as measured by NASA's GRAIL mission (Image: NASA/JPL-Caltech/MIT/GSFC)
Image Gallery (12 images)
Data from NASA’s twin
Gravity Recovery and Interior Laboratory (GRAIL)
probes has been used to create the highest resolution gravity map yet
of any body in the Solar System. The two washing machine-sized
spacecraft acted as a 225-kilometer (140 mi) long gravity detector for
studying the interior composition of the Moon.
The GRAIL spacecraft were launched in September 2011 and were
originally designated “A” and “B,” but were renamed “Ebb” and “Flow” in
January by elementary students in Bozeman, Montana as part of a
nationwide contest. They are orbiting the Moon in near-polar,
near-circular orbits 50 kilometers (31 mi) above the surface. They are
in precise formation to one another at a distance ranging from 175 to
225 kilometers (109 to 140 mi).
Artist’s concept of NASA’s GRAIL mission (Image: NASA/JPL)
Their main mission is to map gravitational anomalies on the Moon.
This is important because the Moon isn't homogeneous. There are many
areas of greater or lesser density and this causes an uneven
gravitational field. As the two spacecrafts orbit, they constantly
measure the changing distance between them to within a few tenths of a
micron per second by means of precisely timed radio signals. Flying over
a gravitational anomaly alters this distance and after a series of
orbits, a map can be made of these anomalies. These measurements were
enhanced by laser topographical measurements taken by NASA’s Lunar
Reconnaissance Orbiter, which allowed the gravitational anomalies to be
mapped against topographical features.
Scientists at MIT, NASA, the Jet Propulsion Laboratory and elsewhere
are using the map as a way of learning more about the interior of the
Moon and the early history of the Solar System. "What this map tells us
is that more than any other celestial body we know of, the Moon wears
its gravity field on its sleeve," said GRAIL principal investigator
Maria Zuber of the Massachusetts Institute of Technology. "When we see a
notable change in the gravity field, we can sync up this change with
surface topography features such as craters, rilles or mountains."
Mercator projection of lunar gravity map (Image: NASA/ARC/MIT)
Geologically, the Moon is relatively inactive and lacks any weather,
so it's very close to the state it was in billions of years ago, whereas
the Earth’s tectonic plate movements and robust weather has destroyed
most traces of its early days. When the Solar System formed, all the
planets underwent a barrage of impacts by objects, some as big as
planets themselves, and the Moon still shows signs of those times. The
most dramatic evidence is the fact that the Moon’s crust is completely
pulverized with cracks that may extend into the mantle, as discovered by
the GRAIL gravity survey. This indicates that the early Solar System
underwent a much more severe and violent origin than previously thought.
The new information also provides a much clearer picture of the
nature of the lunar crust. "With our new crustal bulk density
determination, we find that the average thickness of the moon's crust is
between 21 and 27 miles (34 and 43 km), which is about 6 to 12 miles
(10 to 20 km) thinner than previously thought." said GRAIL
co-investigator Mark Wieczorek of the Institut de Physique du Globe de
Paris. "With this crustal thickness, the bulk composition of the moon is
similar to that of Earth. This supports models where the moon is
derived from Earth materials that were ejected during a giant impact
event early in Solar System history."
Map of lunar dikes mapped by NASA's GRAIL mission (Image: NASA/JPL-Caltech/CSM)
Another aspect of gravity mapping is that it revealed internal
geological structures, such as the fact that beneath the crust, the Moon
is extremely smooth and marked by linear structures formed by magma
seeping into cracks. “We used gradients of the gravity field in order to
highlight smaller and narrower structures than could be seen in
previous datasets," said Jeff Andrews-Hanna, a GRAIL guest scientist
with the Colorado School of Mines in Golden. "This data revealed a
population of long, linear, gravity anomalies, with lengths of hundreds
of kilometers, crisscrossing the surface. These linear gravity anomalies
indicate the presence of dikes, or long, thin, vertical bodies of
solidified magma in the subsurface. The dikes are among the oldest
features on the moon, and understanding them will tell us about its
early history."
These dikes are more than just curiosities. If they were present on
early earth-like planets, it would have affected how they lost heat and
how fluids would move from the interior. Similarly, the degree that the
Moon’s crust is pulverized tells a lot about its composition, which
helps us understand the early evolution of the planets.
The GRAIL mission will continue until December 17. The first results have been published in three papers in the journal
Science.
The video below is an animation of the lunar gravity map.
Sources:
NASA,
MIT
