The shape
of the Moon
One of my major research interests is the
long wavelength (3000-km-scale) shape of the Moon. The shapes of
many small satellites in the solar system are dominated by tidal forces
from the primary bodies they orbit. Presently, tides from the
Earth deform the Moon by less than 10 meters. However, the
observed lunar topography has permanent, kilometer-scale deformations
at tidal wavelengths. What are the origins of these deformations
and how do they control the geology of the Moon? How do they
control which side of the Moon we see in the night sky? One of my
hypotheses is that the Moon experienced strong tidal heating when it
was closer to the Earth billions of years ago. The heating
pattern may have altered the crustal structure of the Moon, and thereby
the Moon's shape. In addition, the Moon's largest basins
(craters) influence our interpretation of the lunar shape, and ignoring
them helps us understand the appearance of the Moon just after its
crust formed. The figure below shows the observed shape at left,
and a model for the shape (from tidal heating) at right.
Lunar magnetic anomalies and a cubesat
mission to determine their origin
While the Moon has no global magnetic field like the Earth's, it
does have patches of strongly magnetized rock distributed throughout
its crust. Discovered during the Apollo era, the origin of these
features is uncertain. In addition, many of these features are
associated with unusual swirl-shaped color features on the surface,
some of which can be seen with a pair of binoculars. We have
explored two different hypotheses for the origins of these swirls: 1)
Fine, electrically-charged dust is accumulating in the swirl region,
due to plasma interactions with the local magnetic field, and 2) The
underlying surface is being protected from the darkening effects of
solar wind plasma, due to deflection of the plasma around the magnetic
field. The figure below shows Reiner Gamma swirl and its magnetic
field (from Hemingway and Garrick-Bethell (2012)).
Part of the problem in determining the origins of lunar magnetic fields
and their underlying swirl patterns is that previous spacecraft have
only acquired data from above ~20 km altitude. Ideally, one would
land on the surface with a rover to make measurements of the magnetic
field and solar wind flux, but the costs of such a mission are
prohibitive. To address this problem, I have developed a low-cost
concept that sends a cubesat on an unbraked impact trajectory into the
heart of a swirl. The cubesat transmits measurements in real-time
to the Earth, up until the last milliseconds, enabling measurements at
less than 100 meters altitude. The cubesat platform would be
partly based on the 3-Unit UC Berkeley/Kyung Hee University CINEMA
cubesat (which includes a small scientific
magnetometer), now in
orbit. A cubesat capable of such a lunar mission would also have
many other uses for near-Earth space measurements. The figure
below illustrates the Lunar Impactor concept (from Garrick-Bethell et
al. 2013).
Lunar
paleomagnetism
In addition to the magnetic anomalies described above, another surprise
of the Apollo program was that many of the rocks brought back by
astronauts were magnetized. The hypotheses for the origins of
this magnetism include either an ancient dynamo that is now extinct, or
processes related to meteoroid impacts. At the paleomagnetism
laboratory at UC Santa Cruz, and in collaboration with MIT and the
University of Hawaii, we study the magnetism of Apollo samples to help
determine the history and origins of lunar magnetism. The figure
below shows lunar troctolite 76535, which we have studied extensively.