NEW EVIDENCE FOR ICE ON MERCURY
NASA
Owing to its proximity to the Sun, Mercury would seem an unlikely
place to find ice. But the tilt of Mercury's rotational axis is
almost zero -- less than one degree -- so there are pockets at the
planet's poles that never see sunlight. Scientists suggested decades
ago that water ice might be trapped in those shadowed areas at
Mercury's poles. The idea received a boost in 1991, when the Arecibo
radio telescope in Puerto Rico detected unusually radar-bright patches
at Mercury's poles, spots that reflected radio waves in the way one
would expect if there were water ice. Many of those patches
corresponded to the locations of large impact craters mapped by the
Mariner 10 spacecraft in the 1970s. But researchers weren't sure if
the radar-bright patches detected by Arecibo corresponded to shadowy
places in the craters. Messenger's arrival at Mercury last year
changed that. Images taken by the spacecraft in 2011 and this year
show that radar-bright features at Mercury's north and south poles are
within shadowed regions of the surface. Now, the most recent data
confirm that water ice is the major constituent of Mercury's north-
polar deposits. In the coldest places, the ice is exposed on the
surface. In slightly warmer spots, some kind of dark insulating
material appears to cover the ice.
Messenger uses neutron spectroscopy to measure average hydrogen
concentrations within the radar-bright regions. Ice concentrations
are derived, in turn, from the hydrogen measurements. The neutron
data indicate that the radar-bright polar deposits contain, on
average, a hydrogen-rich layer more than tens of centimetres thick
beneath a surface layer 10 to 20 centimetres thick that is less rich
in hydrogen. The buried layer has a hydrogen content consistent with
nearly pure water ice. Data from Messenger's laser altimeter, which
has made detailed maps of the planet's topography, corroborate the ice
hypothesis.
DWARF PLANET MAKEMAKE LACKS ATMOSPHERE
ESO
Astronomers across South America have observed the dwarf planet
Makemake as it occulted a star. Makemake is beyond Pluto but closer
to the Sun than Eris, the most massive known dwarf planet. Previous
observations of Makemake have shown it to be similar to other dwarf
planets, leading some astronomers to expect its atmosphere, if
present, to be similar to that of Pluto. However, at the occultation
the star disappeared and reappeared very abruptly, rather than fading
and brightening gradually; that shows that Makemake has no significant
atmosphere. Its lack of moons and its great distance from us make it
difficult to study, and what little we do know about it is only
approximate. The new observations determine its size more accurately,
put constraints on a possible atmosphere, give an estimate of its
density, and show its albedo to be about 0.77. That is comparable to
that of dirty snow, higher than that of Pluto, but lower than that of
Eris. Occultations are particularly uncommon in the case of Makemake,
because it is in a rather barren area of the sky. It was quite a
triumph to predict the rare occultation event accurately and have it
observed by a coordinated team scattered at many sites across South
America.
DO MISSING JUPITERS MEAN MASSIVE COMET BELTS?
RAS
Last year, the infrared Herschel spacecraft found that the dusty belt
surrounding the nearby star Fomalhaut must be maintained by collisions
between comets. In a new Herschel study, two more nearby planetary
systems, Gliese 581 and 61 Vir, have been found to have a lot of
cometary debris. Herschel detected the signatures of cold dust at
-200C, in quantities that imply that those systems have at least 10
times more comets than in our own Solar System's Kuiper Belt, a
reservoir of cometary nuclei beyond the orbit of Neptune.
Gliese 581 is a low-mass red dwarf star, the most common type of star
in the Galaxy, in the constellation of Libra. Earlier studies have
shown that it hosts at least four planets. Two planets are now
confirmed around 61 Vir, which is just a little less massive than our
Sun. The planets in both systems are known as super-Earths, covering
a range of masses between 2 and 18 times that of the Earth.
Interestingly, however, there is no evidence for giant Jupiter- or
Saturn-mass planets in either system. The gravitational interplay
between Jupiter and Saturn in our own Solar System is thought to have
been responsible for disrupting a formerly populous Kuiper Belt and
sending a deluge of comets towards the inner planets in a cataclysmic
event that lasted several million years.
EVEN BROWN DWARFS MAY GROW ROCKY PLANETS
ESO
Astronomers using the Atacama Large Millimetre Array (ALMA) have found
that the outer region of a dusty disc encircling the young brown dwarf
Rho-Oph 102, in the Rho Ophiuchi star-forming region, contains
millimetre-sized solid grains like those found in denser discs around
newborn stars. The finding flies in the face of theories of how
rocky, Earth-scale planets form. Rocky planets are thought to form
through the random collision and sticking together of what are
initially microscopic particles in the disc of material around a star.
The tiny grains, known as cosmic dust, are similar to very fine soot
or sand. However, astronomers expected that in the outer regions
around a brown dwarf grains could not grow because the discs were too
sparse, and particles would be moving too fast to stick together after
colliding. Also, prevailing theories say that any grains that manage
to form should move quickly towards the central brown dwarf, disappear-
ing from the outer parts of the disc where they could be detected.
Astronomers were therefore surprised to find millimetre-sized grains
in a thin little disc. It is not clear whether a complete rocky
planet could develop there, or already has done, but we are seeing the
first steps. ALMA also identified carbon monoxide around the brown
dwarf -- the first time that cold molecular gas has been detected in
such a disc. That discovery, and that of the millimetre-size grains,
suggest that the disc is more similar to the ones around young stars
than might have been expected.
GIANT PLANET ORBITING KAPPA ANDROMEDAE
The Register
Canadian astronomers, using the Japanese Subaru telescope in Hawaii,
have discovered a planet with a mass at least 13 times that of Jupiter
orbiting the star Kappa Andromedae, which is 2.5 times the mass of our
Sun and is the most massive star known to have a planet. The planet,
labelled by the researchers as Kappa And b, is one of the few directly
observed exo-planets. It shares the considerable proper motion of the
star, showing that the two objects are related. Although astronomers
have found over 750 planets around other stars, they have detected
light directly from only six of them. The formation of the system is
also unusual, as Kappa And b is located farther from the star than any
of the Solar-System planets are from the Sun.
MILKY WAY BLACK HOLE IS ACTIVE
NASA
At the centre of the Milky Way galaxy, a hot vortex of matter swirls
around a black hole four million times as massive as the Sun. Many
galaxies contain such objects, which sustain themselves by
swallowing stars, planets, asteroids, comets and clouds of gas that
pass by the crowded galactic core. NuSTAR is an orbiting observatory
designed to take pictures of violent, high-energy phenomena. Launched
in June, it is the only telescope capable of producing focused images
of the highest-energy X-rays produced by dying stars and black holes.
The observatory has pinpointed a burst of hard X-rays coming from the
Galactic Centre during an observing campaign in July. Lower-energy
X-ray observations made by the Chandra X-ray observatory and infrared
data from the Keck telescope in Hawaii confirmed the outburst. The
destruction of objects falling into a black hole is a violent process
in which the objects are ripped apart by powerful tides and heated to
millions of degrees as they disappear into the gravitational
singularity. In this case, NuSTAR picked up X-rays emitted by matter
being heated to about 100 million degrees Celsius. More active black
holes at the centres of other galaxies accrete matter in prodigious
quantities, but the Milky Way's is relatively quiet. Asteroids could
be a major element in its diet. Chandra indeed detected flares
consistent with asteroids 10 km or so in diameter falling into the
black hole.
BLACK HOLE COULD UPSET GALAXY EVOLUTION MODELS
Max Planck Institute
It is suggested that almost every galaxy contains in its central
region a black hole with a mass between hundreds of thousands and
billions of Suns. The best-studied super-massive black hole is in
the centre of our home galaxy and has a mass of about four million
Suns. But the newly discovered black hole in the centre of the disc
galaxy NGC 1277 appears to have a mass 17 billion times that of the
Sun, about 14% of the total galaxy mass, whereas usual values are
around 0.1%. If there prove to be many black holes that are
comparably disproportionate, astronomers will need to rethink their
models of galaxy evolution.
MOST POWERFUL QUASAR YET DISCOVERED
ESO
Quasars are the intensely luminous centres of distant galaxies that
are powered by massive black holes. A new study has looked at one
such object, known as SDSS J1106+1939, with the VLT at Paranal
Observatory in Chile. Although black holes pull material in, most
quasars also accelerate some of the material around them and eject it
at high speed. The study has found the most energetic quasar outflow
known to date. The rate that energy is carried away by the huge mass
of material ejected at high speed from the SDSS object is equivalent
to at least two million million (what in England we used to call two
billion) times the power output of the Sun. That is much higher than
the total power output of the Milky Way galaxy. This is the first
time that a quasar outflow has been measured to have the very high
energies that are hoped for by theory. Many theoretical simulations
suggest that the impact of such outflows on the galaxies around them
*may* resolve some enigmas in modern cosmology. The newly discovered
outflow lies about a thousand light-years away from the super-massive
black hole at the heart of the quasar concerned. The outflow is at
least five times more powerful than any known before. It appears that
about one solar mass per day (an Earth mass every quarter of a second)
is streaming away from the quasar at a speed of 8000 km/s.
QUADRANTID METEORS 2013
By Tony Markham, SPA Meteor Section Director
The year starts with this short lived but intense meteor shower.
Activity is evident from Jan 1-6, with a sharp peak occurring on Jan 3.
The peak itself occurs during the daytime from the UK and so the best
observed rates are likely to occur late in the night of Jan 2-3.
The Quadrantid radiant, lies at Declination +50, in a rather bland
area of the sky between Draco, Bootes and Ursa Major, and is
circumpolar for observers in the UK. A chart showing the location of
the radiant can be found at
http://snipurl.com/25t884q .
The radiant is at its lowest altitude at around 20h local time and is
highest in the sky at the end of the night. Peak rates vary from year
to year. The ZHR for the 2012 peak only seemed to reach around 80
whereas the 2009 peak was broader than usual with the ZHR being above
100 for nearly 12 hours.
The 2013 peak occurs with the Moon, in the Leo/Vir area, approaching
Last Quarter, and so moonlight will be a problem in the second half of
the night. However, you can minimise the effect of the moonlight by
observing with your back to the Moon.
Clear skies in January do of course mean freezing temperatures, so
make sure you wrap up very well.
PLANETS
By Alan Clitherow, SPA Planet Section Director
December has a number of interesting sights for observation. Jupiter
reaches opposition, i.e. is directly opposite the Sun in the sky and
is due south at midnight, on the 3rd of the month. In fact, Jupiter
can be observed all night, reaching an altitude of nearly 60.
Telescopically, there is much on display, with the North Equatorial
Belt (NEB) showing a renewed level of activity; a number of grey-white
oval anti-cyclonic atmospheric disturbances has appeared and at least
two are in the process of merging. The Great Red Spot (GRS), which is
currently a slightly muted pinkish colour, will usually be visible at
some point in the evening owing to the shortness of the rotation
period, a little under 10 hours. Best times to see the GRS, when it
is on the central meridian, are listed below. All times are UT and
accurate to the nearest minute; any errors are entirely my own! Try
using different coloured filters.
Dec 08 00:44 and 20:35
Dec 09 06:31 and 16:25
Dec 10 02:22 and 22:13
Dec 11 18:04
Dec 12 04:00 and 23:51
Dec 13 19:42
Dec 14 05:38
Dec 15 01:29 and 21:20
Dec 16 17:12
Dec17 03:07 and 22:59
Dec 18 18:50
Dec 19 04:46
Dec20 00:36 and 20:28
Dec21 06:24 and 16:19
Dec22 02:15 and 22:06
Dec23 17:58
Dec24 03:53 and 23:45
Dec 25 19:36
Dec26 05:31
Dec27 01:23 and 21:15
Dec28 17:05
Dec29 03:01 and 22:52
Dec30 18:44
Dec 31 04:39
Venus is a brilliant morning object but is getting lower in the sky
each day. Between the 10th and the 12th of the month there is
something of a planetary dance as Mercury, Venus and Saturn all lie
low down in the south-eastern sky near the waning crescent Moon.
Observe from around 05:30 local time until just before dawn to see
this lovely collection of objects rising. For British observers
Mercury has not been very well placed during this morning apparition,
rising little more than about 10 above the horizon. From the start
of the month the planet sinks rapidly, when viewed each day from
before sunrise, and is perhaps too low to observe effectively by
Christmas. Venus is also sinking but is still a spectacular 'morning
star' from early in the month. Saturn, on the other hand, is rising
higher each morning and is well worth viewing if one is up in the
early morning. If you are setting up to watch Jupiter in the evening,
you might also seek out the aqua-marine-coloured Uranus, which is
reasonably well placed in the evening sky among the stars well below
the Square of Pegasus. Its great distance means that it always
appears tiny when compared to the more obvious planets.
Topic: December Astronomy Bulletin (Read 1508 times)