Topic: Mid July Astronomy Bulletin

[Clive]

ICE AT MOON'S SOUTH POLE
NASA

Measurements by the Lunar Reconnaissance Orbiter (LRO) spacecraft
indicate that ice may make up as much as 22% of the surface material
in the crater Shackleton, near the Moon's south pole.  The crater is
named after the Antarctic explorer Ernest Shackleton; it is two miles
deep and more than 12 miles in diameter.  The small tilt of the Moon's
spin axis makes Shackleton crater's interior permanently shadowed from
the Sun and therefore extremely cold, like several other craters at
the Moon's south pole, Light from LRO's laser altimeter showed the
floor of Shackleton to be brighter than the floors of other nearby
craters, which is consistent with the presence of small amounts of
ice.  The spacecraft mapped the crater by using the laser to
illuminate the crater's interior and measure its albedo or natural
reflectance.  The altimeter mapped the topography of the crater by
timing the reflection back from the ground.  In addition to suggesting
the existence of ice, the observations of Shackleton showed a well
preserved crater that has remained relatively unscathed since its
formation more than three billion years ago, though the crater's floor
is itself pocked with several small craters.

While the crater's floor was relatively bright, its walls were even
brighter.  Scientists had thought that if ice were anywhere in a
crater, it would be on the floor, where no direct sunlight penetrates.
The upper walls of Shackleton crater receive occasional illumination,
which could evaporate any ice that was there.  A theory offered to
explain the puzzle is that 'moonquakes' -- seismic shaking brought on
by meteorite impacts or gravitational tides raised by the Earth -- may
have caused Shackleton's walls to slough off older, darker soil,
revealing newer, brighter soil underneath.  There may be multiple
explanations for the observed brightness throughout the crater, for
example, newer material may be exposed along its walls, while ice may
be mixed in with its floor.


NO EARTH IMPACT BY ASTEROID
NASA

It has now been decided that the 'potentially hazardous asteroid'
(PHA) 2011 AG5 will fly safely past and not hit the Earth in 2040.
PHAs are a subset of the larger group of near-Earth asteroids, coming
within 8 million kilometres.  They are large enough to traverse the
atmosphere intact and cause damage on at least a local scale.  The
impact of an asteroid the size of 2011 AG5 could damage a region at
least a hundred miles across.  Several years ago another asteroid,
named Apophis, was thought to pose an impact threat in 2036, but
additional observations taken in 2005-08 enabled refinement of
its path, and showed that there is negligible likelihood of impact.

Measuring approximately 140 metres in size, 2011 AG5 was discovered in
2011 January by scientists at the University of Arizona.  Several
observatories monitored its path for nine months before it got too far
away and too faint to see.  Observations of 2011 AG5 have been limited
so far, because it is presently beyond the orbit of Mars and in the
daytime sky on the other side of the Sun.  In autumn 2013, conditions
will improve to allow space- and ground-based telescopes to track it.
It will then be at about the same distance as the Sun but favourably
located for observations in the late-evening sky.  The orbit will be
still further refined in 2023, when the asteroid comes within 1.8
million kilometres, but already according to the present orbit there
is only a very remote chance of impact.


WATER IN MARS' INTERIOR
Carnegie Institution.

Scientists have analyzed the water content of two meteorites that
originating from inside Mars. They found that the amount of water in
parts of the Martian mantle is vastly larger than previous
estimates. The results not only affect what we know about the history
of Mars, but also have implications for how water got to the Martian
surface.  The scientists analyzed what are called shergottite
meteorites, fairly young meteorites that originated by partial melting
of the Martian mantle -- the layer under the crust -- and crystallized
in the shallow subsurface and on the surface.  They came to the Earth
after ejection from Mars by an impact approximately 2.5 million years
ago.  Meteorite chemistry tells scientists a lot about the processes
the planet underwent.  Analyses of two meteorites that had very
different processing histories were carried out.  One had undergone
considerable mixing with other elements during its formation, while
the other had not.  The water content of the mineral apatite was much
the same in both even though the chemistry of trace elements was
markedly different.  The results suggest that water was incorporated
during the formation of Mars and that the planet was able to store
water in its interior during its differentiation.

The scientists estimated from the minerals' water content that the
Martian mantle source from which the rocks were derived contained
between 70 and 300 parts per million (ppm) water. For comparison, the
upper mantle of the Earth contains approximately 50-300 ppm water.
There has been substantial evidence for the presence of liquid water
on the Martian surface for some time, so it has been puzzling why
previous estimates for the planet's interior have been so dry.  The
new research makes sense and suggests that volcanoes may have been the
primary vehicle for getting water to the surface.


TWO PLANETS IN CLOSE ORBIT AROUND STAR
University of Washington

A research team has discovered a bigger version of the Earth orbiting
very close to a much larger, Neptune-sized, planet around a star about
1,200 light-years away.  The planets have nearly the same orbital
plane and at their closest approach come within about 1.2 million
miles of each other -- just five times the Earth-Moon distance and
about 20 times closer to one another than any two planets in our Solar
System -- but the timing of their orbits ensures that they will never
collide.  Orbiting a star in Cygnus referred to as Kepler-36a, the
planets are designated Kepler-36b and Kepler-36c.  Planet b is a rocky
planet like the Earth, though 4.5 times more massive and with a radius
1.5 times greater.  Kepler-36c, which could be either gaseous like
Jupiter or watery, is 8.1 times more massive than the Earth and has a
radius 3.7 times greater.  The larger planet was originally observed
in data from the Kepler satellite, whose photometer can detect a
planet from its transits in front of the parent star.  Scientists
checking planetary systems already in the Kepler data saw a clear
signal in the Kepler-36a system, a slight dimming every 16 days, the
length of time it takes the larger Kepler-36c to circle its star.
Kepler-36b circles the star seven times for each six orbits of 36c,
but it was not discovered initially because of its small size and
gravitational perturbations by its orbital companion.  The fact that
the two planets are so close to each other and exhibit specific
orbital patterns allowed the scientists to make fairly precise
estimates of each planet's characteristics, on the basis of their
gravitational effects on each other and the resulting variations in
the orbits.  To date, this is the best-characterized system with small
planets.

The experts believe that the smaller planet is 30% iron, less than 1%
atmospheric hydrogen and helium and probably no more than 15% water.
The larger planet, on the other hand, probably has a rocky core
surrounded by a substantial amount of atmospheric hydrogen and helium.
The planets' densities differ by a factor of eight but their orbits
differ by only 10%, which makes the differences in composition
difficult for the scientists to explain by current models of planet
formation.  The team also calculated specific information for the star
itself, determining that Kepler-36a is about the same mass as the Sun
but is only a quarter as dense.  It is also slightly hotter and has
slightly less metal content.  The researchers concluded that the star
is a few billion years older than the Sun and no longer burns hydrogen
at its core, so has entered a sub-giant phase in which its radius is
60% greater than the Sun's.


DIRECT OBSERVATION OF EXO-PLANET
ESO

A new technique has allowed astronomers to study the atmosphere of an
exo-planet in detail -- even though it does not pass in front of its
parent star.  An international team has used the Very Large Telescope
(VLT) in Chile to observe the planet Tau Boötis b directly.  That was
one of the first exo-planets to be discovered, in 1996, and it is
still one of the closest known.  Its parent star is easily visible
with the naked eye, but up to now the planet was detected only by its
gravitational effects on the star.  Tau Boötis b is a large 'hot
Jupiter' planet orbiting very close to its star.  Like most
exo-planets, this one does not transit the disc of its star.  Up to
now, transits were essential to allow the study of 'hot-Jupiter'
atmospheres: when a planet passes in front of its star it imprints the
properties of the atmosphere onto the starlight, but no starlight
shines through Tau Boötis b's atmosphere towards us.  The team used an
instrument on the VLT to make infrared observations (at wavelengths
around 2.3 microns), and used a new method to distinguish the weak
signal of the planet from the much stronger one from the parent star.
Seeing the planet's light directly has allowed the astronomers to
measure the inclination of the planet's orbit to the line of sight and
hence to determine its mass precisely.  The inclination is found to be
44° and the mass six times that of Jupiter.  The team has also been
able to measure the amount of carbon monoxide present, as well as the
temperature at different altitudes by an oblique method involving
theoretical models.  It appears that the temperature falls off with
height, as in the Earth's atmosphere, but that is the opposite of the
temperature inversion found for other 'hot-Jupiter' exo-planets by
observations of transits.


COSMIC WEB OF THE FIRST STARS
RAS

Scientists have discovered a new way to detect the first stars when
the Universe was in its infancy at a mere 1% of its present age.
Until recently, astronomers believed that it was impossible to observe
stars when the Universe was so young and just coming out of its
so-called dark age -- a time when it was permeated by hydrogen gas and
before any light sources such as stars had switched on.  Now, however,
computer models show that an expected difference in the speed of gas
and dark matter caused the first stars to clump together into a
prominent cosmic web.  The discovery of such web-like structures makes
it feasible for radio astronomers to detect the 21-cm-wavelength light
from the first stars when the Universe was only 200 million years old
and still emerging from its dark age.