MERCURY'S HIDDEN WATER-ICE REVEALED
BBC NEWS
It might seem curious that the closest planet to the Sun -- where
temperatures soar above 400C -- could have ice on it. But some of the
craters on the planet are always shadowed from the Sun, turning them
into cold traps. Assuming very low levels of light-scattering off
crater walls, scientists were able to construct a picture of what any
frozen deposits might look like. Scientists suggested decades ago
that water-ice might be trapped in shadowed areas near the planet's
poles. Then, in the 1990s, data from the Arecibo radio telescope in
Puerto Rico revealed areas that strongly reflect radar -- a character-
istic of ice. Many of them corresponded to the locations of large
impact craters mapped by the spacecraft Mariner 10 in the 1970s.
After the Messenger probe entered orbit around the planet in 2011, it
deployed a range of techniques that seemed to show that there are
several billion (US, = 10 to the 9) tons of water ice locked up at the
north pole. In the latest study, researchers studied an impact crater
called Prokofiev, the largest such depression near the pole. The
uniform surface texture of presumed water-ice areas in Prokofiev
crater suggests that the deposits arrived relatively recently. In
other areas, water-ice is covered by a thin layer of dark material
rich in organic molecules. Those dark deposits display sharp
boundaries.
That result was a little surprising, because sharp boundaries indicate
that the volatile deposits at Mercury's poles are, on the cosmic
time-scale, young. A question not yet answerable with any certainty
is 'When were Mercury's ice deposits created?' or 'Are they billions
of years old, or were they emplaced only recently?' Understanding the
age of the deposits has implications for understanding the delivery of
water to all the terrestrial planets, including the Earth. Overall,
the images indicate that Mercury's polar deposits either were
delivered to the planet recently or are regularly restored at the
surface through an ongoing process. Distinctions between the features
of shadowed craters on the Moon and those on Mercury may also be
instructive about the history of water in the Solar System. The Moon,
too, is believed to have ice deposits in areas of permanent shadow.
They are colder than those on Mercury. But there are significant
differences in the properties of the surfaces as measured from orbit.
One explanation could be that the deposits on Mercury are more recent.
If Mercury's currently substantial polar volatile inventory is the
product of the most recent portion of a longer process, then a
considerable mass of volatiles may have been delivered to the inner
Solar System throughout its history. That is a key problem, because
if one could understand why one body looks one way and another looks
different, one might gain insight into the process behind it and thus
into the question of the age and distribution of ice in the Solar
System.
SMELLY COMET!
University of Bern
How does a comet smell? Since early August the Rosetta Orbiter Sensor
for Ion and Neutral Analysis (ROSINA) is analyzing the gases of the
comet Churyumov-Gerasimenko with its two mass spectrometers. The
chemistry of the coma of the comet is surprisingly rich already at
more than 400 million kilometres from the Sun. The gases detected
include hydrogen sulphide, ammonia, formaldehyde, hydrogen cyanide,
methyl alcohol, sulphur dioxide and carbon disulphide. Such a mixture
would make a most unattractive (not to say lethal) smell on Earth, but
the density of those molecules in the cometary coma is still very low,
and the main constituents of the coma are water and carbon dioxide
molecules mixed with carbon monoxide. That all makes a scientifically
interesting mixture that may be relevant to studies the origin of the
Solar System, the formation of the Earth and the origin of life. The
idea was that at distances beyond 3 astronomical units the comet would
mostly sublimate the very volatile molecules carbon dioxide and carbon
monoxide, so the fact that ROSINA is already detecting so many more
molecules at large distances from the Sun comes as quite a surprise.
A quantitative analysis will show how the comet compares with other
comets for which data are available (mostly from remote sensing). The
comparison will reveal whether Churyumov-Gerasimenko, being a Kuiper-
Belt comet, differs from the better-known Oort-Cloud comets, and that
may then shed light on some characters of the cloud from which the
Solar System emerged.
CASSINI HINTS AT WHAT LIES BENEATH THE SURFACE OF SATURN'S MOON MIMAS
Cornell University
Instruments on the Cassini spacecraft have measured the rotational
motion of Mimas, the closest of Saturn's moons. The team used a
technique called stereo-photogrammetry to interpret images taken by
the Cassini 'Imaging Science Subsystem' to measure the libration.
That technique employs images of Mimas taken at different times and
from various vantage points to build precise 3-D computer models of
the locations of hundreds of surface reference points. From those,
the researchers determined the moon's shape and were able to notice
that the satellite does not rotate smoothly but librates back and
forth a bit as well. In physical terms, the expected librational
motion would be about 3 kilometres of surface displacement, but the
observed displacement is about 6 kilometres. The measurement is
interesting because it may be telling us about the satellite's
insides; it suggests that Mimas' interior is not uniform. The extra
libration could be produced if the moon contains n non-spherical
rocky core or if a sub-surface ocean exists beneath its icy shell.
Mimas is about 400 km in diameter, and its possible internal global
ocean is located under an icy crust ranging in thickness between 25
and 30 km.
TWO FAMILIES OF COMETS ORBIT NEARBY STAR
ESO
A team of French astronomers has studied nearly 500 individual comets
orbiting the star Beta Pictoris and has discovered that they belong to
two distinct families: old ones that have made multiple passages near
the star, and younger ones that probably came from the recent break-up
of one or more larger objects. Different families of comets exist
also in the Solar System.
Beta Pictoris is a young star about 63 light-years away. It is
thought to be 'only' about 20 million years old and is surrounded by a
huge disc of material -- a very active young planetary system where
gas and dust are produced by the evaporation of comets and collisions
between asteroids. For almost 30 years astronomers have seen subtle
changes in the light from Beta Pictoris that were thought to be caused
by the passage of comets in front of the star itself. Comets are
small bodies of a few kilometres in size, but they are rich in ices,
which evaporate when they approach their star, producing great tails
of gas and dust that can absorb some of the light passing through them.
The dim light from the comets is swamped by the light of the brilliant
star, so they can not be imaged directly. To study the Beta Pictoris
comets, the team analyzed more than 1000 observations obtained between
2003 and 2011 with the HARPS instrument on the ESO 3.6-m telescope at
the La Silla Observatory in Chile.
The researchers selected a sample of 493 different comets. Some were
observed several times and for a few hours. Analysis provided
measurements of the speed and the size of the gas clouds. Some of the
orbital properties of each of the comets, such as the shape and the
orientation of the orbit and the distance to the star, could also be
deduced. The analysis of several hundred comets in a single planetary
system indicated the presence of two distinct families: one of old
bodies whose orbits are controlled by a massive planet, and another,
probably arising from the recent breakup of one or a few bigger
objects. The comets of the first family have a variety of orbits and
show a rather weak activity with low rates of production of gas and
dust. That suggests that they have exhausted their supplies of ices
during their repeated passages close to the star. Objects of the
second family are much more active and are also on nearly identical
orbits, which suggests that they all have the same origin, probably
from a larger object whose fragments are on an orbit grazing their
star.
A PARTICULARLY HUGE GALAXY CLUSTER IN THE EARLY UNIVERSE
ESO
Astronomers have used the APEX telescope to probe a huge galaxy
cluster that was forming in the early Universe and revealed that much
of the star formation taking place is not only hidden by dust, but is
also occurring in unexpected places. This is the first time that a
full census of the star formation in such an object has been possible.
Galaxy clusters are the largest objects in the Universe held together
by gravity but their formation is not well understood. The 'Spiderweb
Galaxy' (formally known as MRC 1138-262) and its surroundings have
been studied for twenty years and are thought to be one of the best
examples of a proto-cluster in the process of assembly, more than ten
billion (10 to the 10) years ago. But astronomers strongly suspected
that their understanding was far from complete. They wanted to find
out how much of the star formation taking place in the Spiderweb
Galaxy cluster is hidden from view behind dust. The team used the
LABOCA camera on the APEX telescope in Chile to make 40 hours of
observations of the Spiderweb Cluster at millimetre wavelengths --
wavelengths of 'light' that are long enough to 'see' right through
most of the thick dust clouds.
The APEX observations showed that there were about four times as many
sources detected in the area of the Spiderweb as in a similar area
of surrounding sky. By comparing the new data with complementary
observations made at different wavelengths the astronomers were able
to confirm that many of the sources were at the same distance as the
galaxy cluster itself and must be parts of the forming cluster. But
they were surprised to find that the star formation was concentrated
mostly in a single region, and that region is not even centred on the
central Spiderweb Galaxy in the proto-cluster. It seems that the
star-forming regions are developing asymmetrically.
NEW WINDOW ON EARLY UNIVERSE
University of Bonn
Scientists at the Universities of Bonn and Cardiff have been trying to
obtain information on galaxies seen at the 'edge of the Universe' or
(equivalently) near the beginning of cosmic time. That is a difficult
thing to do, because the large red-shifts of such objects 'dilute'
their signals on their way to us. Estimating how much molecular
hydrogen is present in the remotest galaxies is particularly
challenging, as the molecule emits almost no radiation. Nevertheless,
astrophysicists are keen to map the abundance of molecular hydrogen
because it is the fundamental building block for new stars; the more
of it is contained within a particular galaxy, the more stars that
galaxy can form.
Currently, astrophysicists are said to try to use a fudge to estimate
the abundance of molecular hydrogen in a galaxy: they use carbon
monoxide -- which emits far more light than molecular hydrogen -- as a
surrogate, and then, by a doubtful procedure, infer the molecular
hydrogen abundance from the carbon monoxide signal. It has been
proposed that the radiation of neutral carbon is better suited for the
observation of very distant galaxies. Again the measured values are
supposed to allow an estimation of how much molecular hydrogen is
present. Unfortunately, the radiation from neutral carbon is very
strongly absorbed by water vapour in the Earth's atmosphere. However,
at the site of the new radio telescope in the Chilean Atacama Desert,
the Atacama Large Millimetre/submillimetre Array (ALMA), the air is so
extremely dry that the telescope can easily observe the radiation from
extragalactic carbon atoms. So it is hoped that ALMA can detect the
carbon signal in distant galaxies, from which optimists hope to
estimate the amounts of molecular hydrogen in them, although that does
appear to be a roundabout and unsatisfactory procedure, with a weak
logical link to the quantity of actual interest.
UAE TO LAUNCH ROCKET TO MARS
GULF NEWS
The UAE has announced that it hopes to send a spacecraft to Mars by
2021, the year of the country's 50th anniversary. It also announced
that a UAE space agency will be created to supervise the mission and
coordinate the country's growing space technology. The UAE is one of
nine countries that now have space programmes relating to Mars.
Topic: Early November Astronomy Bulletin (Read 1736 times)