TUNGUSKA EVENT CAUSED BY COMET
Geophysical Research Letters
In recent years there has increasingly been a consensus that the
object that caused the 1908 Tunguska explosion that levelled a large
area of Siberian forest was of the nature of a comet. That conclusion
is supported by Cornell University research from an unlikely direction
--- the exhaust plume from the space shuttle launched a century later.
The research connects the two events by what followed each about a day
later: bright night-visible clouds, or noctilucent clouds, that are
made up of ice particles and form only at very high altitudes and in
extremely cold temperatures. The researchers contend that the massive
amount of water vapour thrown into the atmosphere by the comet's icy
nucleus was caught up in eddies by a process called two-dimensional
turbulence, which explains why the noctilucent clouds formed a day
later many thousands of miles away. Noctilucent clouds are the
highest type of clouds, forming naturally in the mesosphere at about
55 miles over the polar regions during the summer months when the
mesosphere is around -117 C.
A single space-shuttle flight injects 300 tons of water vapour into
the Earth's thermosphere, and the water particles have been found to
travel to the Arctic and Antarctic regions, where they form the clouds
after settling into the mesosphere. Researchers saw the noctilucent-
cloud phenomenon days after the space shuttle Endeavour was launched
on 2007 August 8. Similar cloud formations had been observed after
launches in 1997 and 2003. Following the 1908 explosion, known as the
Tunguska Event, the night skies shone brightly in the north for several
days across Europe, particularly Great Britain.
LANDFORMS INDICATE 'RECENT' WARM CLIMATE ON MARS
Science and Technology Facilities Council
New research indicates that Mars had a significantly warmer climate in
its recent past than previously thought. Scientists came to that
conclusion by studying high-resolution images, obtained by the Mars
Reconnaissance Orbiter, of equatorial landforms that formed by melting
of ice-rich soils. The work indicates that the Martian surface
experienced freeze-thaw cycles as recently as 2 million years ago, and
that Mars has not been locked in permafrost conditions for billions of
years as had been previously thought.
The features were previously interpreted to be the result of volcanic
processes but are now attributed to the expansion and contraction of
ice, and by thawing of ice-rich ground, and suggest a climate very
different from what we find today. All of the landforms observed are
in an outflow channel, thought to have been active as recently as 2 to
8 million years ago. Since the landforms exist within, and cut
across, the pre-existing features of the channel, they too must be
supposed to have been created within that time. The pictures show
polygonally patterned surfaces, branched channels, blocky debris and
mound/cone structures, all of which are similar to landforms on Earth
typical of areas where permafrost terrain is melting. The observations
demonstrate not only that there was ice near the Martian equator in the
last few million years, but also that the ice melted to form liquid
water and then re-froze, probably for many cycles.
LARGEST MAP OF COLD DUST REVEALED
ESO
A new atlas has been produced, showing the inner regions of the Milky
Way at sub-millimetre wavelengths. It covers a strip about two
degrees wide and over 40 degrees long along the galactic plane, and
was created from observations made with the APEX telescope in Chile.
APEX is located at an altitude of 5100 m on the arid plateau of
Chajnantor in the Chilean Andes -- a site that permits observations in
the sub-millimetre wavelength range. The Universe is relatively
unexplored at sub-millimetre wavelengths, as extremely dry atmospheric
conditions and advanced detectors are required for such observations.
Images at those wavelengths are useful for studies of the birthplaces
of stars and the structure of the crowded Galactic core. The new
atlas, known as the APEX Telescope Large Area Survey of the Galaxy
(ATLASGAL), offers a fresh view of the Milky Way. It should give us
an overview of the large-scale structure of our Galaxy, and provide a
guide for observations to be made with the forthcoming ALMA telescope
and with the recently launched Herschel space telescope.
The interstellar medium is composed of gas and grains of cosmic dust,
rather like fine sand or soot. The gas is mostly hydrogen, which is
difficult to detect, so the dense regions are often identified instead
from the faint heat glow of the cosmic dust grains. Sub-millimetre
radiation shows the dust clouds as shining, even though they obscure
the view of the Universe at visible-light wavelengths. Accordingly,
the ATLASGAL map includes the denser central regions of our Galaxy, in
the direction of the constellation Sagittarius, that are otherwise
hidden behind a dark shroud of dust clouds. The map also shows
thousands of dense dust clumps, many never seen before, which mark the
future birthplaces of massive stars. The clumps are typically a
couple of light-years in size and have masses of between ten and a few
thousand times the mass of our Sun.
THE NATURE OF 'BLOBS' AND THE LIMIT TO THE SIZES OF GALAXIES
Chandra X-Ray Center
About ten years ago, astronomers who were making surveys of young
distant galaxies found great reservoirs of hydrogen gas, which they
called 'blobs', near them. The blobs glowed brightly in optical
light, but their nature and the source of the energy that powered the
glow and were unknown. It is thought that recent observations by the
Chandra X-ray satellite, the Spitzer infrared satellite, and other
instruments have answered the main questions. They have observed 29
blobs in one particular field. The blobs appear to be galaxies in
course of formation, several hundred thousand light-years across, and
are seen as they were when the Universe was only about two billion
years old. In five of the blobs, Chandra saw a point-like source of
X-ray emission, which has been taken to be the signature of a growing
super-massive black hole. Other observations indicate that there is a
lot of star-formation going on in the blobs. The radiation and
outflows from the black holes and the star formation is calculated to
be sufficient to light up the surrounding hydrogen in the blobs. It
may also be enough to arrest the infall of further gas and prevent the
blobs growing indefinitely into galaxies much larger than are actually
observed; some such process must be at work to limit the sizes of
individual galaxies.
ULYSSES SPACECRAFT ENDS MISSION
JPL
Ulysses, a joint NASA/ESA mission, ceased operations on June 30. The
spacecraft charted the unexplored regions of space above the poles of
the Sun for more than 18 years. When the space shuttle Discovery
launched Ulysses in 1990, the spacecraft had an expected lifetime of 5
years. It gathered information about the heliosphere -- the bubble
carved in space by the solar wind -- and made nearly three complete
orbits of the Sun in a plane almost perpendicular to the ecliptic..
The probe revealed for the first time the three-dimensional character
of Galactic cosmic radiation, energetic particles produced in solar
storms and the solar wind. In addition to measuring the solar wind
and charged particles, Ulysses measured small dust particles and
neutral gases that penetrate into the heliosphere from local
interstellar space. It also had three chance encounters with comet
tails, registered more than 1,800 cosmic gamma-ray bursts, and
provided findings for more than 1,000 scientific articles and two
books. On June 10, Ulysses became the longest-running ESA-operated
spacecraft, overtaking the International Ultraviolet Explorer, which
logged 18 years and 246 days of operations. Ulysses' orbital path is
carrying the spacecraft away from the Earth. The ever-widening gap
has progressively limited the amount of data transmitted, and the
project managers, with the concurrence of ESA and NASA, decided that
it was an appropriate time to end the mission..
Topic: Mid July Astronomy Bulletin (Read 985 times)