SPACE HAS NEVER BEEN CLOSER
NASA-GSFC
Observations made by instruments on a US air-force satellite have
shown that the boundary between the Earth's upper atmosphere and space
has moved to extraordinarily low altitudes. The satellite was
designed to study disturbances in the Earth's ionosphere, which is a
gaseous envelope of electrically charged particles that surrounds the
Earth and is important because radar, radio waves, and signals for
global positioning systems can be disrupted by ionospheric
disturbances. The satellite's first discovery was, however, that
the ionosphere was not where it had been expected to be. The
transition between the ionosphere and space was found to be at about
260 miles altitude during the night-time, rising to about 500 miles
during the day. Those altitudes were unusually low compared with the
more typical values of 400 miles at night and 400 miles by day.
COMET IMPACTS MAY HAVE TRIGGERED ANCIENT FAMINE
New Scientist
It has been proposed that multiple comet impacts around 1500 years ago
may have triggered a 'dry fog' that plunged half the world into famine.
Historical records tell us that from the beginning of 536 March a fog
of dust blanketed the atmosphere for 18 months. During that time, the
Sun (it was said) gave no more light than the Moon, global
temperatures plummeted and crops failed. The cause has been unknown,
but theories have included a great volcanic eruption or an impact from
space. Now scientists have found the first, indirect, evidence that
multiple impacts caused the haze. They found tiny balls of condensed
rock vapour or 'spherules' in debris inside Greenland ice cores dating
back to early 536 AD. Though the spherules' chemistry suggests that
they did not belong to an impactor, they do point to terrestrial
debris ejected into the atmosphere by an impact event. That
represents the first actual geological evidence for an impact in 536.
The fallout material was laid down over several years, and some layers
were particularly densely deposited. That suggests that more than one
impactor was involved -- probably comets, which do tend to fragment.
Two possible submarine craters whose age ranges fit the global dimming
event have been identified -- one is in the Gulf of Carpentaria in
Australia, and the other in the North Sea near Norway. Marine
micro-fossils found with the impact spherules are consistent with an
ocean impact.
ASTEROIDS WITH EARTH-LIKE CRUST
Carnegie Institution
It has sometimes been assumed that the small size of asteroids limited
the types of rock that could form in their crusts, but researchers
report that two newly discovered meteorites are ancient asteroid
fragments consisting of feldspar-rich rock called andesite, previously
known only from the Earth. The two light-coloured meteorites were
discovered in a region of the Antarctic ice known as the Graves
Nunatak ice-field. The unusual thing about them is that they have
compositions similar to the Earth's continental crust. Andesite is an
igneous rock common on Earth in areas where colliding tectonic plates
generate volcanoes, such as those of the Andes. The meteorites
contain minerals thought to require large-scale processes such as
plate tectonics to concentrate the right chemical ingredients. In
view of that, some researchers suggested that the meteorites are
fragments of a planet or the Moon, not an asteroid, but analysis of
the meteorites' oxygen isotopes ruled out that possibility, and it
now seems likely that the formation of andesite crust can occur by
processes other than plate tectonics.
Several Solar-System objects, including parent bodies of meteorites,
planets, moons, and asteroids, have characteristic oxygen-isotope
signatures, and just by analyzing the isotopic ratios we can tell
if a meteorite came from Mars, from the Moon, or from a particular
asteroid. One extensively studied parent is the asteroid 4 Vesta.
In the majority of cases the actual location of the parent body is
unknown but, even so, a particular group of meteorites may still be
assigned to the same parent body on the basis of the isotopic ratios.
The ratios in the newly found meteorites are nearly coincident with
those of meteorites from 4 Vesta.
The meteorites' age, more than 4.5 billion years, suggests that they
formed very soon after the birth of the Solar System, making it
unlikely that they came from the crust of a differentiated planet.
The chemical signature of some rare metals, notably osmium, in the
meteorites also points to their origin on an asteroid that was not
fully differentiated. The researchers hypothesize that that the
asteroid had a diameter somewhat larger than 100 kilometres, which
would be sufficient to hold enough heat for the asteroid's rocks to be
partially, but not completely, melted. The asteroid would remain
undifferentiated, but the melted portions could erupt onto the
asteroid's surface to form the andesitic crust.
JUPITER-LIKE PLANETS MUST FORM QUICKLY
Harvard-Smithsonian Center for Astrophysics
Jupiter must have gained mass very quickly when it was forming, since
the rest of the material from which it formed probably dispersed in
just a few million years, according to a new study of planet formation
around young stars. Astronomers examined the 5-million-year-old star
cluster NGC 2362 with the Spitzer space telescope, which can detect
the signatures of actively forming planets in infrared light. They
found that all stars with the mass of the Sun or greater have lost
their protoplanetary (planet-forming) discs. Only a few stars less
massive than the Sun still retain their discs. Such discs provide the
raw material for forming gas giants like Jupiter. Therefore, if gas
giants do not form in less than 5 million years they probably won't
form at all. Even though nearly all the discs that could form
gas-giant planets in NGC 2362 have disappeared, several stars in the
cluster have 'debris discs', which indicates that smaller rocky or icy
bodies may still be forming.
FEW BROWN DWARFS ACCOMPANY STARS
Space Telescope Science Institute
The Hubble telescope has been surveying the complete sample of stars
known to be nearer than 10 parsecs (32.6 light-years). Part of the
intention is to discover and characterize 'missing' members of the
sample. The sample includes 239 known red dwarf stars (normal stars
that are about half the diameter and a fifth of the mass of the Sun and
with much lower surface temperatures), and there are 12 known brown
dwarfs. Only two brown dwarfs were found as companions to normal
stars. Since brown-dwarf binaries do exist, the fact that there are
very few binaries whose components lie on opposite sides of the
hydrogen-burning limit -- the division between normal stars and brown
dwarfs -- must be seen as significant, although what the significance
actually is remains unknown.
DISCS ROUND DEAD STARS MAY BE FROM DEAD ASTEROIDS
NASA/JPL
Observations made with the Spitzer telescope are interpreted as
indicating the existence of the remains of shredded asteroids around
six dead white-dwarf stars. Spitzer's infrared spectrograph observed
dusty discs around eight white dwarfs and found that the dust contains
a glassy silicate mineral similar to olivine and commonly found on
Earth. The Spitzer data also suggest that there is no carbon in the
rocky debris. The gravitational tides that could be raised by a white
dwarf in a rocky body that got too close would be enough to tear that
body apart. It is being proposed the asteroid-like bodies have met
such a fate within the last million years in the cases of the white
dwarfs recently investigated by Spitzer. The amount of material is
such that in one case it might have constituted an asteroid 200 km in
diameter. So far, the results suggest that the same materials that
make up the Earth and our Solar System's other rocky bodies could be
common in the Universe.
ASTROPHYSICISTS MAP MILKY WAY'S 4 SPIRAL ARMS
Iowa State University
As the Sun and other stars revolve around the centre of the Milky Way,
we cannot see the spiral arms directly, but have to rely on indirect
evidence to find them. In visible light, the Milky Way appears as an
irregular, densely populated strip of stars. Dark clouds of dust
obscure the Galaxy's central region, so it cannot be observed in
visible light; if it were not for the obscuring clouds its brightness
would be comparable with that of the Moon and be a great hindrance to
astronomers. Infrared light, however, can penetrate the dust clouds,
so astrophysicists have used satellite-infrared data to model the
distribution of molecular gas in the Galaxy and thereby to map the
Galaxy's spiral arms.
The stars in the Milky Way are thought to be distributed as a disc
with a central bulge dominated by a bar-shaped arrangement of stars.
Outside the central area, stars are located along spiral arms. In
addition to the two main spiral arms in the inner Galaxy, there are
thought to be two weaker arms which end about 10,000 light-years from
the Galaxy's centre. (The Sun is about 25,000 light-years from the
Galactic centre.) One of the weaker arms has been known for a long
time, but there has been concern over its large deviation from
circular motion, which is now attributed to gravitational effects of
the newly identified central bar. The other weak arm, on the far side
of the Galaxy, was recently found in gas data. The discovery of that
second arm was something of a relief, affirming that the inner Galaxy
is indeed quite symmetrical in structure as had been supposed.
MILKY WAY SPINS FASTER AND IS MORE MASSIVE THAN WAS THOUGHT
NRAO
In some regions of prolific star-formation in the Galaxy there are
cosmic masers, areas where gas molecules are strengthening naturally-
occurring radio emission in the same way as lasers strengthen light
beams. By repeatedly measuring maser positions very accurately by
radio interferometery using intercontinental baselines, at times when
the Earth is at opposite sides of its orbit around the Sun, radio
astronomers can measure parallaxes (and thereby distances) for objects
at great distances in the Galaxy. Some of such direct distance
measurements differ from earlier, indirect measurements, sometimes by
as much as a factor of two. The star-forming regions harbouring the
cosmic masers define the spiral arms of the Galaxy, so the
measurements are helping to improve our understanding of the structure
and motions of the Galaxy. They also indicate that the Milky Way is
rotating somewhat faster than had been thought previously, such that
the circular velocity in the vicinity of the Sun is more like 260 km/s
rather than the 200 km/s that has long been accepted. The increase
implies that our Galaxy is correspondingly more massive, so it is now
seen as being on a par with the Andromeda Nebula.
MARS ROVERS HAVE LASTED FIVE YEARS
JPL
Spirit and Opportunity approach the fifth anniversaries of their
landings on Mars. No one expected, when they landed early in 2004,
with a three-month nominal service life, that they would still both be
operating in 2009, The rovers have made important discoveries about wet
and violent environments on ancient Mars and have returned a quarter of
a million images, driven more than 21 kilometres, climbed a mountain,
descended into craters, struggled with sand traps and ageing hardware,
survived dust storms, and relayed a great deal of data via the Mars
Odyssey orbiter.
JUNE RESTART FOR LHC
Web User
The Large Hadron Collider, the world's biggest particle accelerator,
may be started again by the summer. Soon after it was switched on in
September last year, it suffered a catastrophic malfunction whereby
super-cold helium leaked into the system. Many of the magnets used to
accelerate the particles needed to be repaired as a result, at a cost
of around £20m.
Topic: Mid January Astronomy News (Read 1142 times)