NEW MAP HINTS AT VENUS' WET, VOLCANIC PAST
ESA
Venus Express has made the first map of Venus' southern hemisphere at
infrared wavelengths. It hints that Venus may once have been more
Earth-like, with both plate tectonics and a water ocean. The map
comprises over a thousand individual images, recorded in 2006 and
2007. Because Venus is covered in clouds, normal cameras cannot see
the surface, but Venus Express used a particular infrared wavelength
at which it can see through them. Although radar systems have
provided high-resolution maps of the surface, Venus Express is the
first orbiting spacecraft to produce a map that hints at the chemical
composition of the rocks. The new data are consistent with suspicions
that the highland plateaus of Venus are ancient continents once
surrounded by oceans and produced by past volcanic activity.
The eight Russian landers of the 1970s and 1980s touched down away
from the highlands and found only basalt-like rock beneath their
landing pads. The new map shows that the rocks on the Phoebe and
Alpha Regio plateaus are lighter in colour and look old in comparison
with the majority of the planet. On Earth, such light-coloured rocks
are usually granite and form continents. Granite is formed on the
Earth when basalt rocks are driven deep below the surface by shifting
tectonic plates. Water combines with the basalt to form granite,
which is returned to the surface through volcanic eruptions. If there
is granite on Venus, there must have been an ocean and plate tectonics
in the past.
Over time, Venus' water has been lost to space, but there might still
be volcanic activity. Venus Express did not see any evidence of
ongoing volcanic activity, but that does not rule it out. Venus is a
big planet being heated by radioactive elements in its interior. It
should have as much volcanic activity as the Earth. Indeed, some
areas do appear to be composed of darker rock, which might be
relatively recent volcanic flows.
[Caveat by editor: not only does this item seem to build a very large
castle on very thin air, since the only actual fact seems to be that
when observed in the infrared some areas appear lighter than others,
but it also seems doubtful whether the writer of the item understood
what he was writing about. Granite is not made in the way that the
article says; it exists as a magma, i.e. rock liquefied by heat at
depth (nothing to do with water), and it is an igneous rock, that is
to say it is emplaced, and solidifies, at considerable depth (NOT
extruded at the surface volcanically) and is seen on the surface only
when and if the overburden has been eroded away.]
COMET-LIKE OBJECTS IN THE ASTEROID BELT MAY HAVE FORMED BEYOND NEPTUNE
Southwest Research Institute
Many of the objects found today in the asteroid belt between the
orbits of Mars and Jupiter *may* have formed in the outer part of
the Solar System, according to an international team of astronomers.
The team made numerical simulations that seemed to show that some
comet-like objects that might have existed in a disc outside the
original orbits of the planets could have been scattered across
the Solar System and into the asteroid belt during a violent phase of
planetary evolution. Usually, the Solar System is considered a place
of relative permanence, with any changes occurring gradually over
hundreds of millions to billions of years. New models of planet
formation indicate, however, that at specific times, the architecture
of the Solar System underwent dramatic upheaval.
In particular, it now seems possible that approximately 3.9 billion
years ago, the giant planets -- Jupiter, Saturn, Uranus and Neptune --
rearranged themselves. Evidence for such an event was first
identified in the samples returned from the Moon by the Apollo
astronauts. They tell us about an ancient cataclysmic bombardment
where large asteroids and comets rained down on the Moon. [That's not
the same thing as the giant planets rearranging themselves! And the
evidence for there having been a rain of large objects onto the Moon
was there for all to see just by looking through a telescope -- nodody
needed to go there in person for such evidence. Remarkably, however,
until the Apollo era many soi-disant astronomers clung contumaciously
to the idea that the impact craters were volcanic. -- ED.] Scientists
[and others] now recognize that that event was not limited to the
Moon, but also affected the Earth and many other Solar-System bodies.
The same dynamical conditions that devastated the planets also led to
the capture of some would-be impactors in the asteroid belt. Once in
the asteroid belt, the embedded comet-like objects collided with both
themselves and the asteroids. The model shows that comets are
relatively easy to break up when hit by something, at least when
compared to typical asteroids. It is unavoidable that some of the
debris went on to land on asteroids, the Moon and the Earth. In fact,
some of the leftovers may still be arriving today.
The team believes that the similarities between some micrometeorites
landing on Earth and comet samples returned by the Stardust mission
are no accident. Some of the meteorites that once resided in the
asteroid belt show signs that they were hit 3.5 to 3.9 billion years
ago. The scientists try to make the case for the hits to have been
made by captured comets or perhaps their fragments. If that is so,
they are telling us the same sort of story as the lunar samples,
namely that there was a lot of dynamic activity in the Solar System
about 4 billion years ago.
Overall, the main asteroid belt contains a surprising diversity of
objects ranging from primitive ice/rock mixtures to igneous rocks.
The standard model used to explain it assumes that most asteroids
formed in place from a primordial disc that had radical chemical
variations. The model now being advanced, however, prefers the
observed diversity of the asteroid belt not to be a direct result of
an intrinsic compositional variation of the proto-planetary disc.
IMPACT ON JUPITER
Spaceweather.com
Evidence is mounting that something hit Jupiter just a few days ago.
The impact site (a dark 'scar' in Jupiter's clouds) was discovered on
July 19 by Australian amateur astronomer Anthony Wesley, and other
astronomers quickly confirmed the find. Infrared images posted on
Spaceweather.com are consistent with an asteroid or comet strike. The
debris zone in Jupiter's clouds is itself as wide as a small planet,
making it an easy object for amateur telescopes.
NEW KIND OF ASTRONOMICAL OBJECT PROPOSED
Science Daily
An international team of astronomers suggests that lots of
'hypercompact stellar systems', appearing as faint star clusters,
might be detected at optical wavelengths in our immediate cosmic
environment. Some such objects may already have been observed in
astronomical surveys. Hypercompact stellar systems could supposedly
result if a supermassive black hole were violently ejected from a
galaxy, following a merger with another galaxy that also contained
such a hole. The evicted hole would rip stars from the galaxy as it
was thrown out. The stars closest to the hole would move in tandem
with the massive object and become a permanent record of the velocity
at which the kick occurred. It would be possible to quantify the kick
by measuring how fast the stars move round the hole. Only stars
orbiting faster than the kick velocity could remain attached to the
hole after the kick. They carry with them a kind of fossil record of
the kick, even after the hole has slowed down. Finding such objects
would be like discovering DNA from a long-extinct species.
The best place to find hypercompact stellar systems, the authors
argue, is in clusters of galaxies like the Coma and Virgo clusters,
dense regions containing thousands of galaxies that have been merging
for a long time. Merging galaxies result in interactions between
their black holes (if they have any), a pre-requisite for the kicks.
Even if a hole got kicked out of a galaxy, it would still be
gravitationally bound to the whole cluster of galaxies and would still
be there somewhere in the cluster.
The scientists would like to think that hypercompact stellar systems
may already have been seen and nobody has realized it. The objects
would be easy to mistake for common star systems like globular
clusters. The key signature of hypercompact stellar systems would be
a high internal velocityy dispersion. It would be detectable only by
measuring the velocities of stars, which would be obliged to circulate
far more quickly around a massive hole than in the self-gravity of a
normal cluster. Determination of the velocity dispersion in a star
cluster is a difficult measurement that would in most cases require a
lot of observing time on a large telescope. At rare intervals, a
hypercompact stellar system would make its presence known in a much
more dramatic way, when one of the stars was tidally disrupted by the
hole, causing a beacon-like flare. The only evidence of such floating
black holes would be their armada of stars, with a display of stellar
fireworks to signal their existence more obviously perhaps once in a
million human lifetimes.
FERMI FINDS GAMMA-RAY GALAXY SURPRISES
NASA/Goddard Space Flight Center
In 1991, just before the launch of the Compton gamma-ray observatory,
astronomers knew of gamma-rays from exactly one galaxy beyond our own.
To their surprise and delight, the satellite observed similar
emissions from dozens of other galaxies. Now its successor, the Fermi
space telescope, is filling in the picture with new finds of its own.
Compton showed us that two classes of active galaxies emitted gamma
rays -- blazars and radio galaxies. Fermi has found a third, and
opened a new window in the field.
Active galaxies are those with unusually bright centres that show
evidence of particle acceleration to speeds approaching that of light
itself. In 1943, astronomer Carl Seyfert described the first two
types of active galaxy on the basis of the widths of their spectral
lines, a measure of the rapidity of gas motion in their cores. Today,
astronomers recognize many additional classes, but they now believe
that the types represent the same essential phenomenon seen at
different viewing angles. They think that, at the centre of each
active galaxy, there is a black hole of upwards of a million times the
Sun's mass. Through processes not understood, some of the matter
headed for the black hole blasts outward in fast, oppositely-directed
particle jets. For the most luminous active-galaxy classes -- blazars
-- astronomers are looking right down the particle beam.
Fermi has detected gamma-rays from a 'Seyfert 1' galaxy catalogued as
PMN J0948+0022, which lies 5.5 billion light-years away in the
constellation Sextans. Its spectrum shows narrow lines, which
indicates slower gas motions which had been taken to argue against the
presence of a particle jet. But, unlike most narrow-line Seyfert 1
galaxies, PMN J0948 also produces strong and variable radio emission
which suggests that the galaxy is indeed producing such a jet. The
gamma-rays seen by Fermi confirm the existence of particle
acceleration to near the speed of light in that type of galaxy.
Another case where Fermi sees something new involves NGC 1275, a
massive Seyfert galaxy much closer to us. Also known as Perseus A,
one of the sky's loudest radio sources, NGC 1275 lies at the centre of
the Perseus cluster of galaxies, about 225 million light-years away.
Compton's EGRET instrument did not detect gamma-rays from NGC 1275,
although some were detected by another instrument sensitive to lower
energies. But Fermi clearly shows the galaxy to be a gamma-ray source
at the higher energies for which EGRET was designed, with a flux about
seven times higher than the sensitivity threshold of EGRET. If NGC
1275 had been that bright when EGRET was operating, it would have been
seen. The implied change in the galaxy's output suggests that its
particle beam was either inactive or much weaker a decade ago. The
time-scales of such changes indicate the maximum sizes of the emitting
regions. If gamma-rays can switch on in NGC 1275 in ten years they
must arise from a source no more than ten light-years across. That
means that the radiation must come from a small region in the galaxy
-- presumed to be near its supposed black hole -- rather than being
emitted by hot gas throughout the galaxy.
Topic: Late July Astronomy Bulletin (Read 1233 times)