NEW REVELATIONS ABOUT MERCURY
Carnegie Institute
Analysis of data from Messenger's third and final fly-by of Mercury
last September has found evidence of later vulcanism than had
previously been recognized, new magnetic information, and the first
observations of emission from an ionized species in Mercury's very
thin atmosphere or exosphere. During its first two fly-bys of
Mercury, Messenger captured images showing that pervasive vulcanism
occurred early in the planet's history. The third fly-by revealed a
290-km-diameter impact basin, among the youngest basins yet seen and
recently named Rachmaninoff, having an inner floor filled with
spectrally distinct smooth plains.
The sparsely cratered Rachmaninoff plains post-date the formation
of the basin and apparently formed from material that once flowed
across the surface. Astronomers interpret these plains to be the
youngest volcanic deposits yet found on Mercury. Moreover, an
irregular depression surrounded by a diffuse halo of bright material
northeast of the basin marks a candidate explosive volcanic vent
larger than any previously identified on Mercury. These observations
suggest that vulcanism on the planet lasted a lot longer than had been
thought.
During the fly-by, the magnetometer documented for the first time a
build-up of magnetic energy in Mercury's magnetic tail. The increases
in energy in Mercury's the tail were large (factors of two to three),
and they occurred very quickly, lasting only two or three minutes from
beginning to end. The increases in tail magnetic energy at Mercury
are about 10 times greater than at the Earth, and the events run their
course about 50 times more rapidly.
Observations were also made of emission from ionized calcium in
Mercury's exosphere. The emission was concentrated over a relatively
small area one to two Mercury radii anti-sunward of the planet, with
most of the emission occurring close to the equatorial plane. The
concentrated distribution cannot be explained by in-situ conversion of
local calcium atoms to calcium ions and instead points to
magnetospheric transport of the ions as a mechanism for concentrating
them. Although such transport is common in planetary magnetospheres,
the degree to which it can affect the distribution of species in
Mercury's exosphere was not fully appreciated.
POTENTIALLY HAZARDOUS ASTEROID
Science Daily
A lot of interest has been shown in recent years in 'potentially
hazardous asteroids', which might at some future time hit the Earth,
although so far none of any significant size has been found to have
any but an infinitesimal chance (which reduces more and more as
observations accumulate) of an actual impact.
Asteroid (101955) 1999 RQ36 is such an object. It was discovered in
1999 and is around 560 metres across. Its orbit is actually well
determined by 290 optical observations and 13 radar measurements, but
there is a significant uncertainty in extrapolating it into the far
future because the paths of small asteroids are slightly influenced by
the Yarkovsky effect. That effect arises on a rotating body because
the 'afternoon' side of the body is warmer than the morning side,
having been in sunshine longer, so it radiates more, and the infrared
photons that constitute its radiation carry away a certain amount of
momentum. By the law of conservation of momentum, that has to be
balanced by momentum imparted to the asteroid in the opposite
direction, that is, 'downwards' from the point of view of the evening
hemisphere.
Spanish scientists have published a paper in the journal 'Icarus',
showing that there is a tiny possibility that the relevant asteroid
could collide with the Earth in 2182. They give reasons for saying
that, if it were wished to try to modify the path of the asteroid by
artificial means to make it even more certain that it will miss the
Earth, it would be much less difficult and expensive to do so before
2060. Although of course even a small asteroid has what is by
ordinary standards an enormous mass, so it would not be accelerated
much by forces of ordinary magnitude, a very slight change in velocity
accumulates to a large distance in the course of a hundred years.
BROWN DWARF ORBITING YOUNG SUN-LIKE STAR
University of Arizona
An international team of astronomers using the 8-m Gemini-South
telescope in Chile has discovered a 36-Jupiter-mass brown-dwarf
companion to the star PZ Tel, which is an object quite like the Sun
but very young -- about 12 million years old. The team used an
adaptive-optics system coupled to a coronagraph in their efforts to
detect faint objects close to bright ones. The companion is 18
astronomical units (Earth--Sun distances) from the primary star,
subtending a third of a second of arc at the distance of PZ Tel.
Because PZ Tel is a rare type of object, being both close and very
young, it has been imaged several times in the past, so astronomers
were surprised to see a companion around what was thought to be a
single star. The orbit appears to be eccentric, such that in the past
the angular separation of the system has been smaller and the
companion's image has been lost in the glare of the primary's.
ASTRONOMERS FIND VERY MASSIVE STAR
RAS
A UK-led international team of astronomers has discovered the most
massive stars to date, including one which they think at birth had
more than 300 times the mass of the Sun, well above the currently
accepted limit. The astronomers studied two young clusters of stars,
NGC 3603 and RMC 136a. NGC 3603 is 22 000 light-years away;
RMC 136a is located inside the Tarantula Nebula, in one of our
neighbouring galaxies, the Large Magellanic Cloud, 165 000 light-
years away.
The team found several stars with surface temperatures over 40 000
degrees and luminosities several million times as great as the Sun's.
Comparisons with models seem to imply that the stars were born with
masses in excess of 150 solar masses. One of them, in the R136
cluster, is proposed to be the most massive star ever found -- to have
a current mass of about 265 solar masses and a birth mass of as much
as 320 times that of the Sun. But that is only estimated from
computer models. In NGC 3603, the astronomers could determine
directly from the orbital parameters the masses of two stars that
belong to a double-star system. Three objects in the cluster have
estimated masses at birth above or close to 150 solar masses, but one
of them is double, with an orbital period of 3.77 days. The two stars
in that system are 120 and 92 times the mass of the Sun.
HOT EXOPLANET WITH COMET-LIKE TAIL
NASA
Astronomers using the Cosmic Origins Spectrograph (COS) on the Hubble
telescope have confirmed the existence of an object that could be
called a "cometary planet". The gas-giant planet, named HD 209458b,
is orbiting close to its star and its heated atmosphere is
escaping into space. The planet is slightly less massive than Jupiter
but orbits 100 times closer to its star in a period of just over three
days. It has been intensely scrutinized because it is one of the few
extra-solar planets that transit their respective stars. During a
transit, astronomers may be able to study the structure and chemical
makeup of the planet's atmosphere by observing the starlight that
passes through it. Hubble detected carbon and silicon in the planet's
1100°C atmosphere. The detection showed that the parent star is
heating the entire atmosphere, dredging up the heavier elements and
allowing them to escape from the planet. The data also showed that
the material leaving the planet was not all travelling at the same
speed. Some gas was found to be escaping at high velocities, with a
large amount flowing toward us at 10 km/s. The most likely
interpretation is that the gas is being swept up by the stellar wind
to form a comet-like tail trailing the planet.
UNUSUAL COSMIC LENS
ScienceDaily
Astronomers have discovered the first known case of a distant
galaxy being magnified by a quasar acting as a gravitational lens.
The first gravitational lens was discovered in 1979, and produced an
image of a distant quasar that was magnified and split by a
foreground galaxy. Hundreds of cases of gravitationally lensed
quasars are now known. But, until the current work, the reverse
process -- a background galaxy being lensed by the massive host
galaxy of a foreground quasar -- had never been detected.
Quasars, which are extraordinarily luminous objects in the distant
Universe, are thought to be powered by super-massive black holes
in the cores of galaxies. A single quasar can be a thousand times
brighter than an entire galaxy of a hundred billion stars, which
makes studies of their host galaxies very difficult. The
researchers say that the significance of the discovery is that it
may provide a way to understand such host galaxies.
RECORD-BREAKING X-RAY BLAST
Penn State University
A blast of the brightest X-rays ever detected from beyond our Milky
Way Galaxy's neighbourhood temporarily blinded the X-ray eye on the
Swift space observatory earlier this summer. The X-rays travelled
through space for 5 billion years before overwhelming Swift's X-ray
telescope on June 21. The blindingly bright blast, named GRB 100621A,
came from a gamma-ray burst, a violent eruption of energy thought to
come from the explosion of a massive star turning into a new black
hole. It was by far the brightest source ever seen in X-ray
wavelengths at cosmological distances.
Gamma-ray bursts typically begin with a bright flash of high-energy
gamma-rays and X-rays, and then fade away, sometimes leaving
behind a rapidly declining afterglow in less-energetic wavelengths,
including optical and ultraviolet. Surprisingly, although the energy
from the June burst was the brightest ever in X-rays, it was merely
ordinary in optical and ultraviolet wavelengths. The Swift scientists
were able to estimate the overall brightness of GRB 100621A by
sampling the photons at some distance from its overexposed centre
and using a standard correction technique.
JAPAN TO LAUNCH MERCURY MISSION IN 2014
Indiavision News
Japan is preparing to launch a space mission to Mercury in 2014, using
a craft covered in mirrors to reflect the 450°C heat from the planet.
According to the Japan Aerospace Exploration Agency, the mirrors will
help the probe to survive temperatures on the surface of the planet.
Scientists have calculated that, by reflecting the intense heat of the
Sun, the temperature of the mirrors can be kept at about 160°C.
Inside the body of the spacecraft, where the observation equipment
will be housed, temperatures should be below 60°C. The craft stands
around six feet high and is powered partly by solar energy collected
by panels that are wrapped around its body. It is designed constantly
to rotate to prevent one side becoming too hot. JAXA scientists hope
to be ready to launch the probe -- which is at present unnamed -- in
2014. JAXA has been boosted by the impressive performance of
Hayabusa, which returned to Earth in June after a seven-year journey
to recover particles of an asteroid, and Ikaros, which was launched in
May and is the first spacecraft to draw its energy from a solar-
powered sail.
Topic: Mid August Astronomy Bulletin (Read 629 times)