WHY GANYMEDE AND CALLISTO ARE DIFFERENT
SwRI
Differences in the number and speed of cometary impacts on Jupiter's
large moons Ganymede and Callisto some 3.8 billion years ago can
explain their vastly different surfaces and interior states, according
to research by scientists at the Southwest [US] Research Institute.
Like the Earth and Venus, Ganymede and Callisto are twins, and
understanding how, although they were born the same, they grew to be
so different is of great interest to planetary scientists. Ganymede
and Callisto are similar in size and are made of a similar mixture of
ice and rock, but data from the Galileo and Voyager spacecraft show
that they look different both at the surface and on the inside. In an
effort to find an explanation for the differences, researchers created
a model of melting by cometary impacts and the formation of a rocky
core to show that Ganymede's and Callisto's evolutionary paths
diverged about 3.8 billion years ago during the 'late heavy
bombardment', the phase in lunar history dominated by large impact
events.
In the model, Jupiter's gravity focuses cometary impactors onto
Ganymede and Callisto. Each impact onto Ganymede or Callisto's mixed
ice and rock surface creates a pool of liquid water, allowing rock in
the melt pool to sink to the moon's centre. Ganymede is closer to
Jupiter and that causes it to be hit by twice as many icy impactors as
Callisto, and the impactors hitting Ganymede have a higher average
velocity because they have fallen further into Jupiter's gravity well.
Core-formation begun during the 'late heavy bombardment' becomes
energetically self-sustaining in Ganymede but not Callisto.
X-RAY GAS TAILS
Michigan State University
Three years ago the Chandra X-ray telescope observed a tail of gas
extending out from a galaxy called ESO 137-001 in the cluster Abell
3627 in the southern constellation Triangulum Australe. The tail is
believed to be forming stars where few have been formed before. Now
it has been found that the tail, which is more than 200,000 light-
years in length and extends well outside the galaxy, is actually
double, and that the adjacent galaxy ESO 137-002 also has a tail of
hot X-ray-emitting gas. It is within such objects that new stars are
formed, but usually within the confines of a galaxy; stars are formed
from clouds of molecular gas, which normally are found only within
galaxies.
MAGNETIC LOOP BETWEEN STELLAR PAIR
NRAO
Radio astronomers have found a magnetic loop attached to one of the
stars making up the famous triple-star system Algol, 93 light-years
from the Earth in the constellation Perseus. Algol includes a close
pair of stars that show eclipses that are easily observable with the
naked eye every 2.86 days. The Italian astronomer Montanari
discovered the variability in 1667 and the eclipsing-binary
explanation was confirmed in 1889. The system includes a star about
three times as massive as the Sun, with a less-massive companion
orbiting it at a distance of 5.8 million miles. The newly discovered
magnetic loop emerges from the poles of the less-massive star and
stretches outward in the direction of the primary star. As the
secondary star orbits its companion, one side, the one with the
magnetic loop, constantly faces the more-massive star, just as the
same side of our Moon always faces the Earth.
DISTANT STELLAR-MASS BLACK HOLE
RAS
In 2007 an X-ray instrument on the Swift observatory scrutinised the
surroundings of the brightest X-ray source in the spiral galaxy NGC
300, about six million light-years away, and recorded periodic,
extremely intense X-ray emission, a clue that a black hole might exist
there. New observations with the Very Large Telescope indicate that
the hole has a mass of about twenty times that of the Sun, and it is
associated with a Wolf-Rayet star, which also has a mass of about
twenty Suns. The stellar-mass black holes found in the Milky Way are
up to ten times the mass of the Sun, but three with masses more than
fifteen Suns have been identified in galaxies outside our own. The
hole and the Wolf-Rayet star orbit each other with a period of about
32 hours, and the hole is stripping matter away from the star. Only
one other system of the same type has previously been seen.
Wolf-Rayet stars are very near the ends of their lives and expel most
of their outer layers into their surroundings before exploding as
supernovae, with their cores imploding to form black holes.
WISE STARTS ALL-SKY SURVEY
Science Daily
The 'Wide-field Infrared Survey Explorer' (WISE) has begun its survey
of the infrared sky. The mission is intended to spend nine months
scanning the sky one-and-a-half times in infrared light, and to
uncover hundreds of thousands of asteroids and hundreds of millions of
stars and galaxies. Its catalogue will provide astronomers and other
missions with data to mine for decades to come. It has already
discovered a previously unkonwn near-Earth asteroid designated 2010
AB78. The asteroid is currently about 158 million kilometres away.
It is estimated to be roughly 1 km in diameter and is in an orbit
tilted with respect to the plane of the Solar System. The object
comes as close to the Sun as the Earth, but because of its tilted
orbit it will not pass very close for many centuries.
SPIRIT IS NOW STATIONARY
NASA
The rover 'Spirit', which has been exploring Mars for six years, is no
longer mobile, after several months' efforts to free it from a sand
trap have proved unsuccessful. The venerable robot's primary task in
the next few weeks will be to position itself to combat the severe
Martian winter. If Spirit survives, it will continue conducting
significant new science from its present location. Ten months ago,
Spirit's wheels broke through a crusty surface and churned into soft
sand hidden underneath. The rover team tried to get the six-wheeled
vehicle free by use of its five functioning wheels (the sixth wheel
seized in 2006). In November another wheel seized, exacerbating an
already difficult situation. One stationary experiment Spirit has
begun studies tiny wobbles in the rotation of Mars to gain insight
about the planet's core. That requires months of radio-tracking the
motion of a point on the surface of Mars to calculate long-term motion
with an accuracy of a few inches. Tools on Spirit's robotic arm can
study variations in the composition of nearby soil, which has been
affected by water. Stationary science also includes watching how wind
moves soil particles and monitoring the Martian atmosphere.
Topic: Early February Astronomy Bulletin (Read 904 times)