TITAN'S METHANE SEAS
BBC News
Cassini measurements of the Ligeia Mare sea, which is the second-
largest sea on Saturn's moon Titan, suggest that it contains about
9,000 cubic kilometres of liquid, mostly methane. That is a huge
volume, equivalent to about 40 times the proven reserves of oil and
gas on the Earth. The body of liquid methane known as Kraken is
perhaps five times as big. The Cassini spacecraft has been slowly
mapping Titan's surface, using radar to see through its thick, hazy
atmosphere. The survey work for the northern hemisphere, the region
where the seas and lakes are found, has now achieved nearly full
coverage. That has enabled scientists to assess the liquid bodies'
distribution, and to say something more detailed about their size and
evolution. For example, the radar instrument has been able to look
through Ligeia Mare to the sea-floor to determine its depth. It comes
out at 160-170m. The signal also carries information about
composition. Researchers say it is clear that the liquid is dominated
by methane, but that the modelling suggests there ought to be
contributions from a chemical cocktail that includes ethane, propane,
butane, ethylene and even a trace of cyanogens. What is intriguing
about the distribution of the large seas is that they appear to be
clustered in a block, measuring roughly 900 by 1,800 km. That may
suggest that the liquid is pooling in fissures in the 'bed rock'
created by tectonic forces on the moon. The smaller lakes, on the
other hand, may cluster in areas where the ground has been dissolved
through some as-yet-unexplained chemical process - but one that
produces deep depressions in a way not dissimilar to the sink holes
that are weathered out of limestone on the Earth by the action of
water. The holes may then act as repositories for the liquid methane.
The Cassini mission is getting low on fuel and is due to be
terminated in 2017 by putting the probe on a destructive dive into
Saturn's atmosphere. In the meantime, its radar passes will continue
to fill in gaps in the Titan surface map. One key flyover next
year will allow the probe to take bathymetric measurements for
Kraken, similar to those recently made for Ligeia.
ARGON MOLECULE DISCOVERED IN SPACE
Cardiff University
A molecule containing an atom of a noble gas has been discovered in
space by a team using an instrument aboard the Herschel space
observatory. The molecule, argon hydride, was seen in the Crab
Nebula, the remains of the supernova of 1054, about 2 kiloparsecs
away. Previously, molecules of that kind have been only laboratory
curiosities. The noble gases, which include helium, argon, krypton
and xenon, do not react easily with other chemical elements, and are
usually found on their own. In the right circumstances, however, they
can form molecules with other elements, but it has seemed up till now
that the right conditions do not occur naturally. Last year, Herschel
was used to study the intricate network of filaments that surround the
remnant of the Crab supernova. They are mainly composed of cold
molecuar hydrogen. Herschel also obtained spectra, which showed
emission lines that were identified as arising from argon hydride.
At first sight, the discovery seemed bizarre -- with hot gas still
expanding at high speeds, a supernova remnant is a harsh, hot and
hostile environment, not a promising place to find such a molecule.
It seems that the argon was produced in the initial stellar explosion,
and is ionised, making it much more reactive than it is in the neutral
state. Where the ionised argon was mixed with the cold molecular
hydrogen the conditions existed for argon hydride to form. The
Herschel spectra show the argon formed in the supernova explosion to
be of a different isotopic composition from the argon present in the
Earth's atmosphere, where it is the fourth-largest constituent after
nitrogen, oxygen, and (usually) water vapour.
FOMALHAUT'S FAINT COMPANION HAS COMETS AS WELL AS A PLANET
RAS
The 'nearby' star Fomalhaut has a planetary system containing both an
exo-planet and a ring of comets. Astronomers who have been using
Herschel have now discovered that the least massive of the three stars
in the Fomalhaut system, Fomalhaut C, has a comet belt. Fomalhaut A
is one of the brightest stars in the sky. Located 25 light years away
in the constellation Piscis Austrinus, it shines blue-white and is
prominent from the southern hemisphere. From northern latitudes it
appears low down in the south during autumn evenings. In contrast,
Fomalhaut C, also named LP 876-10, is a dim red-dwarf star invisible
without a telescope, and was found to be part of the Fomalhaut system
only in October this year. Fomalhaut A's prominence prompted
observations with the Hubble telescope, which showed the ring of
comets and a direct image of the planet, Fomalhaut b, in 2008.
(Astronomers use upper-case letters for stars and lower-case letters
for planets.)
MASSIVE STARS MARK OUT MILKY WAY'S 'MISSING ARMS'
RAS
A study of massive stars has reaffirmed that our Galaxy has four
spiral arms, following years of debate sparked by images taken by the
Spitzer space telescope that showed only two arms. Astronomers cannot
see what our Galaxy (the Milky Way) looks like, because we are on the
inside looking out. But they can try to deduce its shape by
observation of its stars and their distances from us. In the 1950s
astronomers used radio telescopes to map our Galaxy. Their
observations focussed on clouds of gas in which new stars are born,
revealing four major arms. Spitzer, on the other hand, looked at
infrared light emitted by stars. It was announced in 2008 that
Spitzer had detected about 110 million stars, but had found evidence
of only two spiral arms. The astronomers behind the new study used
several radio telescopes to observe individually about 1650 massive
stars. The distances and luminosities of the stars were calculated,
and showed a distribution across four spiral arms.
It isn't simply a case of the new results being right and Spitzer's
being wrong. The two surveys were looking at different things.
Spitzer sees only much cooler, lower-mass stars -- stars like the Sun,
which are much more numerous than the massive stars that the radio
astronomers were observing. Massive stars are much less common than
lower-mass ones because they only last a 'short' time (about 10
million years). The shorter lifetimes of massive stars means that
they are found only in the arms in which they formed, which could
explain the discrepancy in the number of galactic arms that different
research teams have claimed. Lower-mass stars last much longer and
move round the Galaxy many times, spreading out in the disc. The
gravitational effects of the two stellar arms that Spitzer revealed is
enough to pile up the majority of stars in those arms, but not in the
other two. However, the gas is sufficiently compressed in all four
arms to lead to the formation of massive stars.
NEW CLASS OF SUPERNOVA
University of California - Santa Barbara
Astronomers affiliated with the Supernova Legacy Survey (SNLS) have
discovered two of the brightest and most distant supernovae ever
recorded, 10 billion light-years away and a hundred times more
luminous than normal supernovae. They are puzzling because the
mechanism that powers most supernovae -- the collapse of a giant star
to a black hole or normal neutron star -- cannot explain their extreme
luminosity. Discovered in 2006 and 2007, the supernovae were so
unusual that astronomers initially did not know what they were or how
far away they are. One of them, named SNLS-06D4eu, is the most
distant and possibly the most luminous member of an emerging class of
explosions called superluminous supernovae. Its spectrum shows no
hydrogen. The new study finds that the objects may be powered by the
creation of a magnetar, an extraordinarily highly magnetized neutron
star spinning hundreds of times per second. Magnetars have the mass
of the Sun packed into a star the size of a city and have magnetic
fields 10*14 (10 to the power 14) times the Earth's.. While several
such superluminous supernovae have been seen since they were first
announced in 2009, and the creation of a magnetar had been postulated
as a possible energy source, the recent work is the first to try to
match detailed observations to models of what such an explosion might
look like.
Astronomers created models that explained the data as the explosion of
a star only a few times the mass of the Sun and rich in carbon and
oxygen. The star was probably much bigger to begin with, but shed its
outer layers long before exploding, leaving only a 'small', naked
core. What may have made the star special was an extremely rapid
rotation. When it ultimately died, the collapsing core could have
spun up the resulting neutron star to an enormous degree. Discovered
as part of the SNLS -- a five-year programme with several large
telescopes to study thousands of supernovae -- the two supernovae
could not initially be properly identified nor could their exact
locations be determined. It took subsequent observations of the faint
host galaxy with the VLT in Chile for astronomers to determine the
distance and energy of the explosions. Years of subsequent
theoretical work were required to try to understand how such an
astounding amount of energy could be produced.
The supernovae are so far away that the ultraviolet (UV) light emitted
in the explosion was stretched out by the expansion of the Universe
until it was redshifted into the part of the spectrum our eyes and
telescopes on Earth can see. That explains why the astronomers were
initially baffled by the observations: they had never seen a supernova
so far into the UV before. It gave them a glimpse into the inner
workings of the supernovae. Superluminous supernovae are so hot that
the peak of their light output is in the UV part of the spectrum, but
because UV light is blocked by the Earth's atmosphere, it had never
been fully observed before. The supernovae exploded when the Universe
was 'only' 4 billion years old -- before the Sun even existed. Such
superluminous supernovae are rare, occurring perhaps once for every
10,000 normal supernovae. They seem to explode preferentially in more
primitive galaxies -- those with low abundances of heavy elements --
which were more common in the early Universe.
CHINA LANDS ROVER ON MOON
BBC News
China has successfully landed a craft carrying a robotic rover on the
surface of the Moon, the first soft landing on the Moon for 37 years.
The touchdown took place on a plain called Sinus Iridum. According to
Chinese space scientists, the mission is designed to test new
technologies, gather scientific data and build intellectual expertise,
as well as scouting for mineral resources that could eventually be
mined. The 120-kg 'Jade Rabbit' rover can reportedly climb slopes of
up to 30 degrees and travel at 200 metres per hour. Its name --
chosen in an online poll of 3.4 million voters -- is derived from an
ancient Chinese myth about a rabbit living on the Moon as the pet of
the lunar goddess Chang'e. The rover and lander are powered by solar
panels but some sources suggest that they also carry radioisotope
heating units containing plutonium-238 to keep them warm during the
cold lunar night.
The landing site is a flat volcanic plain, an apparent embayment of
the Mare Imbrium; it forms the right eye of the fanciful portrait of
the 'Man in the Moon'. The lander will operate there for a year, but
the rover is expected to work only for some three months. Next, a
mission to bring samples of lunar soil back to the Earth is planned
for 2017.
UK FIREBALL: DECEMBER 19, 07:58 GMT
By Tony Markham, SPA Meteor Section Director
Many people witnessed a bright fireball shortly before 8am on the
morning of Thursday December 19. The timing of the appearance of the
fireball meant that it was seen by people travelling to work or
walking their dogs. One downside of its timing, though, was that many
of the camera systems that monitor overnight for fireballs had been
switched off by that time. However, William Stewart and Alex Pratt of
the NEMETODE network have reported that they did obtain images of
parts of the fireball's path. Twitter proved valuable in determining
the extent of the fireball's visibility. It was seen from near
Aberdeen in the north down to Kent in the south and from Dublin and
Belfast in the west across to Norfolk in the east. The indications are
that the fireball crossed northern England in a west to east or south-
west to north-east direction and most likely ended over the North Sea.
A meteorite fall is considered unlikely and, in any case, had a fall
occurred it would almost certainly have been into the sea. Several
eyewitness reports of the fireball have been received by the SPA
Meteor Section. If you saw the fireball, but have not reported your
observation, please do so via the Fireball report form at
http://snipurl.com/28cve3i (don't worry if you can't answer all of the questions - just answer those that you can).
QUADRANTIDS 2014
The Quadrantid meteor shower is one of the most active meteor showers
of the year, with a peak Zenithal Hourly Rate (ZHR) often in excess
of 80. Prospects for 2014 are quite good, with the shower peaking
during the evening of January 3. With New Moon having occurred on
Jan. 1, moonlight will not be a problem. The only downside is that
the radiant of the Quadrantid meteor shower is at its lowest point in
the northern sky at around 8pm, so observed rates will be reduced by
obstructions and haze low in the sky. Such effects will be reduced
as the radiant rises later in the night. However, the Quadrantid peak
is rather narrow, so the falling ZHR will offset the improving
geometry. The Quadrantid radiant, at RA 15h20m, Dec +49, lies in a
bland area of sky between Draco, Bootes and Ursa Major. A finding
chart can be found at
http://snipurl.com/28cvehn
Topic: Late December Astronomy Bulletin (Read 1213 times)