Download A global geological map of Ganymede

News • Mission Update
Space Shorts
Mars in the morning
The longest-serving Mars
orbiter, NASA’s Mars Odyssey,
has changed track a little,
moving towards a morningdaylight orbit, to be achieved
by November 2015. The goal
is data on changing ground
temperatures after sunrise
and sunset across the planet.
No Mars orbiter since the
1970s has flown on a path that
allows imaging of the ground
in morning daylight, preferring
instead less hazy afternoon
lighting. But now detailed Mars
images suggest that warm
season flows exist and that
carbon dioxide ice may thaw and
lead to geysers.
http://mars.jpl.gov/odyssey
Lunar impact spotted
The largest observed meteorite
impact on the Moon took place
on 11 September 2013, on the
Mare Nubium. The impact
produced a bright flash detected
by an automated telescope
system, MIDAS (the Moon
Impacts Detection and Analysis
System) led by Prof. Jose Maria
Madiedo from the University of
Huelva, and Dr Jose L Ortiz from
the Institute of Astrophysics
of Andalusia. The researchers
estimate the impactor as a
400 kg body between 0.6 and
1.4 m across, making a 40 m
crater; the results are published
in Monthly Notices of the RAS.
A global geological map of Ganymede
Ganymede has played a part in astronomy since Galileo observed it orbiting Jupiter more than 400 years ago.
Now the best imagery from NASA’s Voyager 1 and 2 spacecraft in 1979 and the Galileo orbiter between 1995
and 2003 have been combined to produce a global mosaic of the icy surface (right) and a geological map (left).
The map is the work of a group led by Geoffrey Collins of Wheaton College, USA, who used the relative ages
of features such as impact craters and grooves and ridges arising from tectonic activity to set up a geological
timescale. Broadly, darker areas are older, lighter regions younger; in detail, the team identified three major
periods in the history of the moon. Initially, impact cratering dominated, then tectonic activity was significant.
More recently, geological activity as a whole decreased. The synthesis of relative age data over the moon as a
whole allowed researchers to compare hypotheses about the development of the icy surface. Cryovolcanism
– eruption of water and ice from volcanoes – turns out to be very rare on Ganymede, for example. (USGS
Astrogeology Science Center/Wheaton/NASA/JPL-Caltech)
http://1.usa.gov/1mLj53x
suggests that magnetic reconnection
may lead to auroral storms, as it does
at Earth, and sheds light on the heating mechanisms of the atmospheres
of the giant planets.
http://1.usa.gov/1gGEtmm
Trojan Hektor’s
exotic orbit
http://bit.ly/1eJxWET
Jade Rabbit not roving
The status of China’s lunar rover
Yutu – “jade rabbit” – remains
in doubt, although engineers
restored normal signal reception
functions after a second lunar
night. China was only the
third nation to achieve a soft
landing on the Moon when the
Chang’e-3 lander and Yutu rover
touched down in December
2013. Yutu uses solar power and
shuts down during the lunar
night. After the first fortnight
of dormancy, problems with
movement and communication
arose; many assumed that the
rover had failed completely.
After the second night it carried
out only fixed point observations,
according to the Chinese State
Administration of Science,
Technology and Industry for
National Defence. The rover
was intended to roam the lunar
surface for three months.
http://www.cast.cn
2.8
1: This Cassini image shows a bright curtain aurora above Saturn on 29
November 2010, over a depth of about 1500 km, together with faint star
trails arising from the spacecraft’s movement. (NASA/JPL-Caltech/SSI)
a 3D view of Saturn’s aurora in April
and May of 2013, at the same time
as Hubble Space Telescope observations of the northern aurora. Cassini, orbiting Saturn at close range
(about six Saturn radii) collected
data at infrared, visible and ultraviolet wavelengths, while the HST
data were ultraviolet. Together, they
showed how the aurora changed as
the solar wind fluctuated.
Cassini’s visible wavelength data
has also shown curtain-like auroras
on Saturn (figure 1), very like those
seen on Earth. But while those on
Earth are red at the top and green
below and arise from excited nitro-
gen and oxygen atoms and molecules, Saturn’s are purple at high
levels and red lower in the atmosphere and the light comes dominantly from excited hydrogen.
The Cassini ultraviolet imaging spectrometer (UVIS) data were
especially interesting because they
showed changing patterns of emissions on a scale of a few hundred kilo­
metres, including persistent bright
spots, one in particular rotating
in lockstep with the moon Mimas.
UVIS data had previously shown an
intermittent bright spot rotating with
the moon Enceladus. The evolution
of the aurora with solar wind also
The largest Trojan asteroid, Hektor,
and its moon have a complex orbit
that suggests that they are Kuiper
belt objects captured during reshuffling of the giant planets.
Trojans orbit 60° ahead or behind
Jupiter; Hektor’s unnamed moon
was discovered in 2006 by a team
led by Franck Marchis of the SETI
Institute. It took eight years of
observations with the W M Keck
Observatory to determine the orbit.
The 12 km moon orbits the 250 km
asteroid every three days at a distance of 600 km in an ellipse inclined
almost 45° with respect to the asteroid’s equator.
The elliptical inclined orbit, which
appears to be stable yet differs
from other Trojans with satellites,
together with Hektor’s fast spin and
elongated shape, suggests an origin
in some sort of slow encounter. The
results are reported in Astrophysical
Journal Letters.
http://bit.ly/1lqsM9n
A&G • April 2014 • Vol. 55