TELESCOPE FINDS FEWER ASTEROIDS NEAR EARTH
NASA
New observations by the Wide-field Infrared Survey Explorer (WISE)
indicate that there are significantly fewer near-Earth asteroids in
the mid-size range than previously thought. WISE scanned the entire
celestial sky twice in infrared light between 2010 January and 2011
February. It observed more than 100,000 asteroids in the main belt
between Mars and Jupiter, in addition to at least 585 'near-Earth'
ones. It observed in the infrared, detecting objects by their heat
rather than by reflected light, and is supposed to have taken
a more accurate census of the asteroid population than previous
visible-light surveys which were affected by the differing albedos of
asteroids. The WISE data suggest that more than 90% of the largest
near-Earth asteroids (1 km or larger), which would have global
consequences if they were to strike the Earth, have been found.
It is believed that all near-Earth asteroids as much as 10 kilometres
across, as big as the one that is thought to have wiped out the
dinosaurs, are now known. The new estimate for the number of mid-
sized near-Earth asteroids, about 20,000, is lower than the 35,000
previously suggested. However, the majority of mid-size asteroids
remains to be discovered.
FEATURES ON MERCURY ARE UNIQUE TO SOLAR SYSTEM
NASA
New observations from the Messenger spacecraft show that Mercury is
covered with pits that are unlike anything else in the Solar System.
They may have been formed by processes still active today, and change
our view of the small rocky planet's history. Messenger flew past
Mercury three times before settling into orbit on March 18. Its
fly-bys in 2008 and 2009 showed some craters that were filled with
bright material. Now, after more than a Mercury year (88 Earth days)
in orbit, Messenger's high-resolution camera has resolved the bright
patches into clusters of shallow, rimless, irregularly-shaped pits.
Each such hollow is from dekametres to a few kilometres across and
looks fresh and young by planetary standards. Other instruments on
Messenger have found that Mercury's rocks contain a lot more volatile
elements than expected. It is being suggested that the hollows formed
when such elements were liberated from the surface, leaving behind a
spongy, fragile matrix of soil that then collapsed. Micrometeoroids
and charged particles from the solar wind might have vaporised
volatiles in the rocks, triggering the collapses. Volatiles might
also have been concentrated in pockets by volcanic eruptions that
occurred during Mercury's long, frigid nights. In that case, volatile
volcanic gases from the eruptions could freeze solid and get buried
under lava flows. Later, a meteoroid impact could expose such
material to sunlight, causing it to evaporate and the surrounding rock
to crumble.
In any case, a lot of volatile stuff seems to be hiding beneath the
surface of Mercury. That presents problems for theories that attempt
to explain the planet's high density, which implies that two-thirds of
its mass must be made up of its metal core. One theory argued that,
early on, the Sun might have vaporised part of its rocky exterior.
Another suggested that a large proto-planet collided with it during
its formation phase, blasting away its outer layers. In both cases,
the heating involved would have evaporated volatile elements, allowing
them to escape into space.
ATMOSPHERE OF VENUS IS VARIABLE
NASA
The climate on Venus is widely known to be unpleasant -- at the
surface, the planet roasts at more than 400°C under a blanket of
sulphuric-acid clouds and an atmospheric pressure more than 90 times
the Earth's. The Earth has seasons because its rotation axis is
tilted by about 23 degrees, which changes the intensity of sunlight
and the length of the day in each hemisphere in the course of the
year. However, Venus has been tilted almost completely upside down,
leaving it with a nett tilt of less than three degrees from the Sun,
so the seasonal effect is negligible. Also, its orbit is even more
circular than Earth's, which prevents it from getting significantly
hotter or cooler by moving closer to or further away from the Sun.
And while you might expect things to cool down at night -- especially
since Venus rotates so slowly that its night lasts almost two Earth
months -- the thick atmosphere and sulphuric-acid clouds act like a
blanket while winds move heat around, keeping temperatures pretty
even. Finally, almost all the planet's water has escaped to space, so
there are no storms or precipitation.
However, higher up, the weather gets more interesting, according to a
new study of old data by NASA and international scientists. The team
detected strange things going on in data from telescopic observations
of Venus in infrared light at about 110 km above the planet's surface,
in cold, clear air above the acid clouds, in two layers called the
mesosphere and the thermosphere. Although the air over the polar
regions in the upper atmospheric layers on Venus was colder than the
air over the equator in most measurements, occasionally it appeared to
be warmer. In the Earth's atmosphere, a circulation pattern called a
'Hadley cell' occurs when warm air rises over the equator and flows
toward the poles, where it cools and sinks. Since the atmosphere is
denser closer to the surface, the descending air gets compressed and
warms the upper atmosphere over Earth's poles. The opposite was seen
on Venus. In addition, although the surface temperature is fairly
even, substantial changes have been observed -- up to 30 K change
within a few Earth days in the mesosphere--thermosphere layers over
low latitudes on Venus. The poles appeared to be more stable, but
still had changes up to 15 K .
The mesosphere and thermosphere of Venus are dynamically active. Wind
patterns resulting from solar heating and east-to-west zonal winds
compete, possibly resulting in altered local temperatures and their
variability over time. Such upper-atmospheric variability could have
many possible causes, according to the team. Turbulence from global
air currents at different altitudes flowing at more than 200 miles per
hour in opposite directions could exchange hot air from below with
cold air from above to force changes in the upper atmosphere. Also,
giant vortices swirl round each pole; they, too, could generate
turbulence and change the pressure, causing the temperature to vary.
Since the atmospheric layers that the team observed are above the
cloud blanket, they may be affected by changes in sunlight intensity
as day gives way to night, or as latitude increases toward the poles.
The layers are high enough that they could even be affected by solar
activity (the solar cycle), such as flares and coronal mass ejections.
Changes were seen over periods spanning days, to weeks, to a decade.
The team concludes that a lot more observations are needed to
determine which of so many phenomena mainly affect Venus' upper
atmosphere.
SPINNING HOURGLASS OBJECT
RAS
Kuiper Belt Objects (KBOs) orbit the Sun beyond Neptune and are the
best-preserved left-overs of the formation of the planets. 2001QG298
is a remarkable KBO made up from two components that orbit each other
very closely, possibly touching. In 2004, researchers noticed that
2001QG298's apparent brightness periodically tripled every 7 hours or
so. The object is so distant that its shape cannot be resolved, but
its light-curve reveals the strange shape of 2001QG298 as it spins
round. The object appears faint at times because one lobe is hidden
behind the other, so less area is reflecting sunlight. As the hidden
component rotates back into view, we can see the full hour-glass
shape: the reflecting area increases and the whole thing looks
brighter.
However, the new study shows that 2001QG298's rotational axis lies
almost in the plane of its orbit around the Sun. The object's
light-curve was observed again in late 2010 and had changed from the
2004 observation. The variation has become visibly shallower. It was
impossible to tell from the original observations if the rotational
axis and the orbital plane of 2001QG298 were perpendicular or aligned,
i.e. whether the object spun round about a vertical axis like the
blades on a helicopter or a horizontal one like the propeller of an
old-fashioned aeroplane. The changes to the light-curve variation
show that it must be approximately horizontal. As 2001QG298 moves
round in its orbit of the Sun, our viewing geometry of the 'propeller'
is gradually opening out from edge-on to seeing the rotating 'blades'
full-face. This means that more of the reflecting surface is becoming
visible at all times so the variation in the object's brightness
gradually disappears.
GIANT STAR EXPELS MULTIPLE DUST SHELLS
ScienceDaily
Astronomers using the Herschel space observatory have discovered at
least a dozen cold dust arcs around the giant star CW Leo. The
different shells were ejected by the star at intervals of 500 to 1,700
years. The faintest shell we can see now was, according to the team,
expelled about 16,000 years ago. In the meantime it has drifted away
from the star the best part of a light-year. Shells so far away from
the star are very cold, about -248°C, but Herschel has an instrument
that was purposely designed to make images of the far-infrared light
emitted by dust that cold. The astronomers believe that even fainter
shells exist further out, up to the violent bow shock where the
material expelled by the star collides with the interstellar medium.
The oldest shells have probably disappeared in the bow shock already.
QUASARS MEASURE COSMIC DISTANCES
Astropublishing
A method to enable quasars to be used to measure cosmic distances
seems to have been found, by astronomers at the Niels Bohr Institute
in Copenhagen. Quasars are sources of intense radiation that is
thought to be emitted by clouds of material that are falling towards
the super-massive black holes at the centres of a galaxies. They are
transient phenomena that may occur several times during the 'lifetime'
of a galaxy, and are most commonly observed in the distant Universe,
when galaxies were younger and their more dynamic evolution caused gas
clouds to pass more frequently near the central black hole. The
conviction that there is a relationship between the radius of the
region from which clouds fall (the so-called 'broad-line region', from
the widening caused to the spectral lines by the high velocities
involved) and the luminosity of the quasar, has grown with time, and
now astronomers have managed to measure that radius and use it to
estimate the luminosity of the quasars. The combination of the
absolute luminosity and the observed apparent luminosity leads
naturally to an estimate of the distance to the quasar.
Because of their huge luminosity, the method can be used to measure
distances well beyond those probed by supernovae, even out to a
redshift of 4. The size of the broad-line region is determined by a
technique called 'reverberation mapping', that makes use of an
observed time delay between brightness variations of the central
quasar source and the spectral lines that come from the surrounding
clouds. As the clouds are illuminated from the central source by
radiation which we know travels at the speed of light, so the radius
of the broad-line region may be found.
Topic: Early October Astronomy Bulletin (Read 866 times)