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Author Topic: Mid April Astronomy Bulletin  (Read 963 times)

Offline Clive

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Mid April Astronomy Bulletin
« on: April 17, 2017, 13:34 »

Scientists have long thought that Ceres, the largest object in the
asteroid belt and now classified as a 'dwarf planet', may have a very
weak, transient atmosphere, but uncertainties have lingered about its
origin and why it is not always present.  Now, researchers suggest
that the temporary atmosphere is related to the behaviour of the Sun,
rather than Ceres' proximity to the Sun.  The study was conducted by
scientists from the Dawn mission and others who previously identified
water vapour on Ceres.  When energetic particles from the Sun hit ice
on or near the surface of Ceres, they transfer energy to the water
molecules as they collide.  That frees the water molecules from the
ground, allowing them to escape and create a tenuous atmosphere that
may last for a week or so.  The results also have implications for
other airless, water-rich bodies of the Solar System, including the
polar regions of the Moon and some asteroids.  Atmospheric releases
might be expected from their surfaces, too, when solar activity
occurs.  Before Dawn arrived in orbit round Ceres in 2015, evidence
for an atmosphere had been detected by some observatories at certain
times, but not at others, suggesting that it is a transient
phenomenon.  The IUE satellite detected hydroxyl emission from Ceres
in 1991, but not in 1990.  Then, in 2007, ESO's VLT searched for a
hydroxide emission, but did not find any.  ESA's Herschel observatory
detected water in the possible weak atmosphere, or 'exosphere', of
Ceres on three occasions, but did not on a fourth attempt.

As Dawn began its study of Ceres in 2015 March, scientists found ample
evidence for water in the form of ice.  The spacecraft's gamma-ray and
neutron detector (GRaND) found that the surface is rich in hydrogen,
which is consistent with broad expanses of water ice.  The ice is
nearer to the surface at higher latitudes, where temperatures are
lower.  Ice has been detected directly in the small bright crater
called Oxo and in at least one of the craters that are persistently in
shadow in the northern hemisphere.  Other research has suggested that
persistently shadowed craters are likely to harbour ice.  Addition-
ally, the shapes of craters and other features are consistent with
significant water-ice content in the crust.  Because of that evidence
for abundant ice, many scientists think that Ceres' exosphere is
created in a process similar to the one that occurs on comets, even
though they are much smaller.  In that model, the closer Ceres gets to
the Sun, the more water vapour is released because of ice sublimating
near or at the surface.  But the new study suggests comet-like
behaviour may not explain the mix of detections and non-detections of
a weak atmosphere.  Scientists showed that past detections of the
transient atmosphere coincided with higher concentrations of energetic
protons from the Sun.  Non-detections coincided with lower concen-
trations of such particles.  Also, the best detections of Ceres'
atmosphere did not occur at its closest approach to the Sun.  That
suggests that solar activity, rather than Ceres' proximity to the Sun,
is a more important factor in generating an exosphere.  GRaND data
suggested that, during a six-day period in 2015, Ceres accelerated
electrons from the solar wind to very high energies.  In its orbital
path, Ceres is currently getting closer to the Sun.  But the Sun is
now in a particularly quiet period, expected to last for several more
years.  Since their results indicate Ceres' exosphere is related to
solar activity, study authors are predicting that the dwarf planet
will have little to no atmosphere for some time.  However, they
recommend that other observatories monitor Ceres for future emissions.

Keele University

Astronomers have detected an atmosphere around the super-Earth planet
GJ 1132b.  That marks the first detection of an atmosphere around an
Earth-like planet other than the Earth itself, and thus is a
significant step on the path towards the detection of life outside
the Solar System.  Astronomers' current strategy for finding life on
another planet is to detect the chemical composition of that planet's
atmosphere, on the look-out for chemical imbalances which could be
caused by living organisms.  In the case of our own Earth, the
presence of large amounts of oxygen is a tell-tale sign of life.
Until these findings, the only previous detections of exo-planet
atmospheres all involved gas giants reminiscent of a high-temperature
Jupiter.  GJ 1132b is significantly hotter and a bit larger than the
Earth, so one possibility is that it is a 'water world' with an
atmosphere of hot steam. 

The planet GJ 1132b orbits the low-mass star GJ 1132 in the southern
constellation Vela, 39 light-years away.  The team used the GROND
imager at the 2.2-m ESO telescope in Chile to observe the planet
simultaneously at seven different wavelength bands spanning the
optical and near-infrared.  As GJ 1132b is a transiting planet, it
passes directly between the Earth and its host star every 1.6 days,
blocking a small fraction of the star's light.  From the amount of
light lost, astronomers can deduce the planet's size -- in this case
1.4 times that of the Earth.  The new observations showed the planet
to be larger in one of the seven wavelength bands.  That suggests the
presence of an atmosphere that is opaque at that specific wavelength
(making the planet appear larger), but transparent at all the others.
The discovery of that atmosphere is encouraging.  Very-low-mass stars
are extremely common (much more so than Sun-like stars), and are known
to host lots of small planets.  But they also show a lot of magnetic
activity, causing high levels of X-rays and ultraviolet light to be
produced, which might completely evaporate the planets' atmospheres.
However, the properties of GJ 1132b show that an atmosphere can endure
that for millions of years without being destroyed.  In view of the
huge number of very-low-mass stars and planets, that might imply that
the conditions suitable for life are common in the Universe.


1350 light-years away, in the constellation Orion, lies a dense and
active star-formation factory called Orion Molecular Cloud 1 (OMC-1),
part of the same complex as the famous Orion Nebula.  Stars are born
when a cloud of gas hundreds of times the mass of the Sun begins to
collapse under its own gravity.  In the densest regions, proto-stars
ignite and begin to drift about randomly.  Over time, some stars begin
to fall toward a common centre of gravity, which is usually dominated
by a particularly large proto-star -- and if the stars have a close
encounter before they can escape their stellar nursery, violent
interactions can occur.  About 100,000 years ago, several proto-stars
started to form deep within OMC-1.  Gravity began to pull them
together with ever-increasing speed, until 500 years ago two of them
finally clashed.  Astronomers are not sure whether they merely grazed
each other or collided head-on, but either way it triggered a powerful
eruption that launched other nearby proto-stars and hundreds of
colossal streamers of gas and dust out into inter-stellar space at
over 150 km/s.  That cataclysmic interaction released as much energy
as the Sun emits in 10 million years.  Fast forward 500 years, and a
team of astronomers has used the Atacama array of radio telescopes
(ALMA) to observe the heart of the cloud.  There they found the
flung-out debris from the explosive birth of that clump of massive
stars, looking like a cosmic version of fireworks with giant streamers
rocketing off in all directions.

Such explosions are expected to be relatively short-lived, the
remnants like those seen by ALMA lasting only centuries.  But although
they are fleeting, such proto-stellar explosions may be relatively
common.  By destroying their parent cloud, such events might also help
to regulate the pace of star formation in giant molecular clouds.
Hints of the explosive nature of the debris in OMC-1 were first
revealed by the 'Submillimeter Array' in Hawaii in 2009.  The team
also observed the object in the near-infrared with the Gemini South
telescope in Chile, revealing the remarkable structure of the
streamers, which extend nearly a light-year from end to end.  The new
ALMA images, however, show the explosive nature in high resolution,
unveiling important details about the distribution and high-velocity
motion of the carbon monoxide (CO) gas inside the streamers.  That may
help astronomers understand what impact such events could have on star
formation across the galaxy.

Space Telescope Science Institute (STScI)

Astronomers have uncovered a super-massive black hole that may have
been propelled out of the centre of a distant galaxy by gravitational
waves.  Though there have been several other suspected, similarly
booted, black holes elsewhere, none has been confirmed so far.
Astronomers think that the object, detected by the HST, is a very
strong case.  With a mass of more than 1 billion suns, it is the
most massive black hole ever detected to have been kicked out of its
central home.  Researchers estimate that it took energy equivalent to
100 million supernovae exploding simultaneously to jettison the black
hole.  The most plausible explanation for such propulsive energy is
that the object was given a kick by gravitational waves unleashed by
the merger of two hefty black holes at the centre of the host galaxy.
First predicted by Einstein, gravitational waves are ripples in space
that are created when two massive objects collide.  The ripples are
similar to the concentric circles produced when a rock is thrown into
a pond.  Last year, the Laser Interferometer Gravitational-Wave
Observatory (LIGO) proved that gravitational waves exist by detecting
them emanating from the union of two black holes which were several
times more massive than the Sun.

Hubble images taken in visible and near-infrared light provided the
first clue that the galaxy was unusual. The images revealed a bright
quasar, the energetic signature of a black hole, far from the galactic
core.  Black holes cannot be observed directly, but they are the
energy source at the heart of quasars -- intense, compact sources of
radiation that can outshine an entire galaxy. The quasar, named
3C 186, and its host galaxy are 8 billion light-years away in a
galaxy cluster.  The team discovered the galaxy's peculiar features
while conducting a Hubble survey of distant galaxies unleashing
powerful blasts of radiation in the throes of galaxy mergers.  The
team calculated the black hole's distance from the core by comparing
the distribution of starlight in the host galaxy with that of a normal
elliptical galaxy from a computer model.  The black hole had travelled
more than 35,000 light-years from the centre, which is more than the
distance between the Sun and the centre of the Milky Way.  On the
basis of spectroscopic observations taken by Hubble and the Sloan
survey, the researchers estimated the black hole's mass and measured
the speed of gas trapped near it.  They found that the gas around the
black hole was flying away from the galaxy's centre at 4.7 million
miles an hour.  That measurement is also a gauge of the black hole's
velocity, because the gas is gravitationally locked to the hole.  At
that speed an object would travel from the Earth to the Moon in three
minutes!  That is fast enough for the black hole to escape from the
galaxy in 20 million years and roam through the Universe forever. 
The researchers are lucky to have caught the event, because not every
black-hole merger produces imbalanced gravitational waves that propel
a black hole in the opposite direction.


Enigmatic dark energy, despite being thought by some people to make up
68% of the Universe, may not exist at all, according to a Hungarian--
American team.  The researchers believe that standard models of the
Universe fail to take account of its changing structure, but that once
that is done the need for dark energy disappears.  Our Universe was
formed in the Big Bang, 13.8 billion years ago, and has been
expanding ever since.  The key piece of evidence for the expansion is
Hubble's 'law', that the speed with which a galaxy moves away from us
is proportional to its distance.  From the 1920s, mapping the
velocities of galaxies led scientists to conclude that the whole
Universe is expanding, and that it began as a vanishingly small point.
In the second half of the twentieth century, astronomers found
evidence for unseen ('dark') matter by observing that something extra
was needed to explain the motion of stars within galaxies.  Dark
matter is now thought by some to make up 27% of the content of
Universe (in contrast, 'ordinary' matter amounts to only 5%).
Observations in the 1990s of the explosions of white dwarf stars in
binary systems, so-called Type Ia supernovae, then led scientists to
the conclusion that a third component, dark energy, made up 68% of the
cosmos, and is responsible for driving an acceleration in the
expansion of the Universe.
In the new work, the researchers question the existence of dark energy
and suggest an alternative explanation.  They argue that conventional
cosmology relies on approximations that ignore the Universe's
structure and in which matter is assumed to have a uniform density.
Einstein's equations of general relativity that describe the expansion
of the Universe are so complex mathematically that for a hundred years
no solutions accounting for the effect of cosmic structures have been
found.  We know from very precise supernova observations that the
Universe is accelerating, but at the same time we rely on coarse
approximations to Einstein's equations which may introduce serious
side-effects, such as the need for dark energy, in the models designed
to fit the observational data.  In practice, normal and dark matter
appear to fill the Universe with a foam-like structure, where galaxies
are located on the thin walls between bubbles, and are grouped into
super-clusters.  The insides of the bubbles are, in contrast, almost
empty of both kinds of matter.  Using a computer simulation to model
the effect of gravity on the distribution of millions of particles of
dark matter, the scientists reconstructed the evolution of the
Universe, including the early clumping of matter and the formation of
large-scale structure.  Unlike conventional simulations with a
smoothly expanding Universe, taking the structure into account led to
a model where different regions of the cosmos expand at different
rates.  The average expansion rate, though, is consistent with present
observations, which suggest an overall acceleration.  The theory of
general relativity is fundamental to understanding the way the
Universe evolves.  We do not question its validity; we question the
validity of the approximate solutions.  The findings rely on a
mathematical conjecture which permits the differential expansion of
space, consistent with general relativity, and they show how the
formation of complex structures of matter affects the expansion.
Those issues were previously swept under the rug but taking them into
account can explain the acceleration without the need for dark energy.
If that finding is upheld, it could have a significant impact on
models of the Universe and the direction of research in physics.
For the past 20 years, astronomers and theoretical physicists have
speculated on the nature of dark energy, but it remains an unsolved
problem.  With the new model, the team expects at the very least to
start a debate.
A group of solar and climate scientists argues that the whole concept
of the 'Little Ice Age' is misleading, as the changes were small-
scale, seasonal and insignificant compared with present-day global
warming.  Explanations for the cooling to the Earth's climate,
thought to have occurred between the 16th and 19th centuries, include
low solar activity, volcanic eruptions, human changes to land use and
natural climatological change.  But researchers note that the
temperature shift was smaller than that seen in recent decades
resulting from the emission of greenhouse gases, and that although low
solar activity may have been one driving factor, it certainly was not
the only one.  Researchers scrutinised historical records, such as the
accounts of 'frost fairs' when the River Thames froze solid, and
looked at the paintings from the era, such as the landscapes of Pieter
Bruegel the Elder, with 'Hunters in the Snow' depicting a cold winter
scene.  Both of those are cited in support of the Little Ice Age
concept.  From around 1650 to 1710, and to a lesser extent from 1790
to 1825, periods respectively known as the Maunder and Dalton Minima,
sunspot numbers were unusually low, an indication that the surface of
the Sun was slightly cooler.  That external influence is often
suggested as an explanation for the colder conditions.  Scientists
looked at the various pieces of evidence in more detail.  They
compared direct temperature records and proxy data such as ice
records, with the years when the Thames was frozen over (whether or
not a frost fair took place), and with the indications of solar
activity.  Historical climate change is assessed through a variety of
means.  The Central England Temperature (CET) data set tracks
temperature from 1659, making it the oldest and longest-running
meteorological instrumental data sequence in the world.  That direct
record is supplemented by studies of biological proxies such as tree
rings, corals, insect numbers and molluscs, all sensitive to climate
The authors draw comparisons with the ice ages proper.  Cores taken
from Antarctic ice allow global temperatures to be inferred, by
measuring the proportions of deuterium and of the oxygen isotope 18O,
compared with their lighter (normal) counterparts.  It takes more
energy to evaporate water with a higher proportion of the heavier
atoms, and they are more easily lost from rainfall, before they are
deposited in ice found nearer the poles.  The changing proportions of
those atoms allows researchers to assess how the temperature has
changed over millions of years.  From those comparisons, the
scientists argue that the description of the period as an Ice Age is
misleading, as temperatures in that period fell far less than in a
glaciation.  During the Little Ice Age (LIA), the average temperature
in the northern hemisphere fell by around 0.5 degrees. In contrast, in
the most recent major glaciation that came to an end around 12,000
years ago, global temperatures were typically 8C colder than today.
Frost fairs also seem to be a poor indication of overall climate, as
they often did not take place despite the Thames freezing, partly for
many reasons, including puritanical authorities, or safety, as lives
were lost when the ice melted.  The ending of the frost fairs had
nothing to do with climate change or solar activity, instead being
due to the increased river flow when the original London Bridge was
demolished in 1825, and the first Victoria embankment opened in 1870.
Both of those prevented the river from freezing completely, despite
many subsequent cold winters.  Selective use of art-historical
evidence appears to reinforce the illusion of a prolonged cold spell.
Yet 'Hunters in the Snow', depicting a January scene, is part of a
series by Bruegel known as 'The Twelve Months'.  Seven of the
paintings may have been lost, but those of February, July and November
all give no indication of unusually cold conditions.  Consistently
with that, the team notes that even at the height of the LIA period,
colder European winters were still accompanied by many warm summers.
For example, 1701 is close to the lowest point of the Little Ice Age,
yet in both Paris and London the summer was reported as being
unbearably hot and the CET for July that year is the tenth-hottest
on record, with average temperatures for the month reaching 18.3C.
The year 1676 is the second-hottest June on record at 18.0C, yet it
too was in the middle of a run of cold winters. Such high summer
temperatures do not fit at all with the name 'Little Ice Age'.  Much
more dramatic variations can result from large volcanic eruptions.
Samalas, a volcano which erupted in 1257 in what is now Indonesia,
ejected large amounts of dust into the atmosphere, causing a temporary
cooling effect.  The years between 1570 and 1730, corresponding to the
coldest part of the LIA, also saw continuous lower-level volcanic
activity that may have suppressed temperatures.  Volcanic eruptions
undoubtedly cause both cold winters and cold summers.  One of the
clearest examples was the Tambora eruption of 1815 July, which caused
the next year to be called the year without a summer.


The Cassini spacecraft, in orbit around Saturn since 2004, is about
to begin the final chapter of its remarkable story.  On April 26
the spacecraft will make the first of a series of dives through the
2,400-km gap between Saturn and its rings as part of the mission's
grand finale.  During its time at Saturn, Cassini has made numerous
dramatic discoveries, including a global ocean that shows indications
of hydrothermal activity within the icy moon Enceladus, and liquid
methane seas on its moon Titan.  Now, 20 years since launch and after
13 years orbiting Saturn, Cassini is running low on fuel.  In 2010,
NASA decided to end the mission with a purposeful plunge into Saturn
this year in order to protect and preserve the planet's moons for
future exploration -- especially the potentially habitable Enceladus.
But the beginning of the end for Cassini is, in many ways, like a
whole new mission.  Using expertise gained during the mission, Cassini
engineers designed a flight plan that will maximize the scientific
value of sending the spacecraft on its plunge into the planet on
September 15.  As it ticks off its terminal orbits during the next
five months, the mission is expected to rack up an impressive list of
scientific achievements.

The mission team hopes to gain insights into the planet's internal
structure and the origins of the rings, obtain the first-ever sampling
of Saturn's atmosphere and particles coming from the main rings, and
capture the closest-ever views of Saturn's clouds and inner rings.
Cassini will begin its grand-finale orbits, with a last close fly-by
of the giant moon Titan, on April 22.  As it has done many times over
the course of the mission, Titan's gravity will affect Cassini's
flight path.  Cassini's orbit then will shrink so that instead of
making its closest approach to Saturn just outside the rings, it will
begin passing between the planet and the inner edge of its rings.
Astronomers expect the gap to be clear of particles large enough to
damage the spacecraft.  In mid-September, following a distant
encounter with Titan, the spacecraft's path will be altered so that
it dives into the planet.  When Cassini makes its final plunge into
Saturn's atmosphere, it will send data from several instruments --
most notably, data on the atmosphere's composition -- until its signal
is lost.
« Last Edit: April 17, 2017, 13:37 by Clive »
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