Topic: Late May Astronomy Bulletin

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JUPITER MOON HOLDS MAGMA OCEAN
BBC News

Jupiter's innermost satellite Io, which is slightly larger than our
Moon, is the most actively volcanic body in the Solar System.  It
erupts about 100 times more lava each year than the Earth does.  In
principle Io's rotation is locked to its orbital revolution, so that
it keeps one face permanently towards Jupiter, as the Moon does
towards the Earth.  Interaction between the orbits of Io and the other
Galilean satellites, however, causes the orbits to be slightly
off-circular, so Io would appear to an observer on Jupiter to librate
slightly in longitude.  Although the actual rotation of the satellite
with respect to the Jupiter-Io line is very small, owing to the great
mass and proximity of Jupiter it results in heavy tidal squeezing that
dissipates enough energy to keep much of Io's interior molten.  Data
from the Galileo probe suggest that under a 50-km crust there is a
magma ocean at at least 50 km thick and probably much thicker.
Readings from a magnetometer on Galileo indicated that Io was
dramatically distorting Jupiter's magnetic field -- but it has taken
several years to conclude that the reason is related to the nature of
Io's rocks and how they behave when molten.  Experiments found that
when certain types of rock are melted, their electrical conductivity
increases by orders or magnitude.  It is the very high conductivity
that can create the type of signature seen by Galileo, which is
consistent with Io's being made of rocks like lherzolite, an igneous
rock rich in silicates of magnesium and iron.

The magma layer, whose temperature probably exceeds 1,200C, is thought
to make up at least 10% of Io's mantle by volume.  The mantle -- the
moon's interior mid-layer -- probably extends down a further 700-800
kilometres.  Gravity measurements suggest that the core is made of
iron and may be liquid, just like that of the Earth.  Although Io is
so much smaller than the Earth, tidal heating seems to have given it
an internal structure quite similar to the Earth's.


HALE-BOPP PHOTOGRAPHED AT 30.7 AU
Astropublishing

It is 14 years since Comet Hale-Bopp (C/1995 01) lit up the skies
during the first months of 1997, becoming one of the most spectacular
comets of the last century.  As always happens, once the comet was no
longer visible to the naked eye it slipped out of the attention of the
media, and the general public heard no more of it.  However, some
researchers continued to follow it in its 2,500-year orbit.  They
imaged the comet at red wavelengths last December, when it was 30.7
astronomical units away (about the same distance as Neptune) with the
ESO 2.2-m telescope at La Silla.  The comet appeared star-like, with
an angular diameter less than 1.9 seconds,and had a magnitude of 23.3.
There was no clear evidence of any coma or tail.  It seems as though
there is no longer any cometary activity, as is not surprising for an
object so far from the Sun.  However, some doubt remains, because,
according to an estimate of the size of the nucleus, the comet is
about three times as bright as would be expected if it had the
albedo of 4% that is thought to be typical of an inactive nucleus.


DWARF PLANET HAUMEA
ScienceDaily

Haumea, whose existence was confirmed in 2005, is the fifth dwarf
planet in the Solar System after Pluto, Ceres, Eris and Makemake.  It
moves beyond the orbit of Neptune and has two satellites, Hi'iaka and
Namaka, has the shape of a flattened rugby ball, is around 2,000 km
long, and rotates in less than four hours.  Now an international
research team has confirmed that 75% of the surface of Haumea and 100%
of that ofHi'iaka (which is about 400 km in diameter) are covered with
crystallised water-ice (with an ordered structure), and not, as would
have been expected, with amorphous ice disorganised as a result of
solar radiation.  The study suggests that the planet is made up of a
frozen outer layer and an internal volume made up of between 88% and
97% rock with a density of 3.5 g/cm3.  Since solar radiation
constantly destroys the crystalline structure of ice on the surface,
energy sources are required to keep it organised.  Possible sources
are internal heat from radioactive elements, and tidal forces between
Haumea and its satellites.  The research also highlights other
peculiarities of Haumea.  Its orbital plane is inclined at 28º to the
orbital planes of most of the other planets in the Solar System, and
the orbits of its satellites are not in the same plane either.
According to the scientists, the two satellites might have been
created by the collision of another object with Haumea, which could
also have given it its rapid rotation and moulded it into its
rugby-ball shape.


OUTFLOWING WINDS SWEEP AWAY GALACTIC GAS AND INHIBIT STAR-FORMATION
ESA

Scientists from the Max Planck Institute for Extraterrestrial Physics
in Germany, using the Herschel infrared space observatory, have
detected winds of molecular gas streaming away from galaxies.
Suspected for years, such outflows may be able to strip galaxies of
gas and halt star formation in its tracks.  The winds that Herschel
has detected are extraordinary: the fastest is blowing at more than
1,000 km/s.  They could be generated by the intense emission of light
and particles from young stars or by shock waves from the explosion of
old stars.  Alternatively, they might be triggered by the radiation
given off as matter swirls around a black hole at the centre of the
galaxy.  The discovery is important because stars form from molecular
gas, and the outflows are robbing the galaxies concerned of the raw
material needed to make new stars.  If the outflows are powerful
enough, they could even halt star-formation altogether.  The research
team studied 50 galaxies, and inferred that 1,200 times the mass of
the Sun is being lost each year from the galaxies with the most
vigorous outflows.  That is enough to strip them of their entire
reserves of star-forming gas within 1 million to 100 million years.
The fastest winds appear to be coming from the galaxies that contain
the brightest 'active galactic nuclei', in which massive black holes
are thought to be feeding from their surroundings.  The class of red
and almost structureless 'elliptical gaalxies' might owe its existence
to the absence of star-formation, owing to the removal of molecular
gas fron the original galaxies by powerful outflowing winds.


DAWN SPACECRAFT SEES ASTEROID
ScienceDaily

The Dawn spacecraft has obtained its first image of its destination,
the asteroid Vesta.  The image was taken on May 3 when the spacecraft
was approximately 1.2 million kilometres from Vesta.  Vesta is 530 km
in diameter and is the second-most-massive object in the asteroid
belt.  It has been known for about 200 years, but its angular diameter
is too small for appreciable surface detail to be seen on it from the
Earth.  Dawn is expected to enter orbit around Vesta on July 16, when
the asteroid is about 188 million kilometres from the Earth.  To enter
orbit, Dawn must match the asteroid's path around the Sun, which
requires very precise knowledge of the body's location and speed.  By
analyzing where Vesta appears relative to stars in images taken by the
spacecraft, navigators will determine its location and refine the
spacecraft's trajectory.  Dawn is intended to start collecting
scientific data in early August from approximately 2,700 km above the
asteroid's surface.  As the spacecraft gets closer, it will take
multi-angle images, allowing scientists to produce topographic maps;
it is intended ultimately to lower its orbit to about 200 km.  Dawn
will remain in orbit around Vesta for one year; then, after another
long cruise phase, it should arrive in 2015 at its second destination,
Ceres.