DARK SIDE OF MOON ENIGMA SOLVED
Penn State
The markings on the visible face of the Moon that are sometimes
supposedly regarded as portraying the 'Man in the Moon' were created
in the early history of the Solar System when large meteoroids struck
the Earth-facing side of the Moon and created large 'seas' (long since
solidified, of course) of basalt that we see as dark areas called
maria. But no such 'face' exists on the far side of the Moon -- a
problem that has been called the Lunar Farside Highlands Problem and
dates back to 1959, when the Soviet spacecraft Luna 3 transmitted the
first images of the `back' side of the Moon back to Earth. It was
immediately obvious that far fewer 'seas' or maria existed on that
side of the Moon that always faces away from the Earth. The general
consensus on the Moon's origin is that it probably formed shortly
after the Earth and was the result of a Mars-sized object hitting the
Earth with a glancing but devastating impact. That 'Giant Impact
Hypothesis' suggests that the outer layers of the Earth and the object
were flung into space and eventually formed the Moon. After the great
impact, the Earth and Moon were very hot. The objects did not just
melt -- parts of them were vaporized, creating a disc of rock, magma
and vapour around the Earth.
The Earth and Moon loomed large in each other's skies when they
formed. The Moon was 10 to 20 times closer to Earth than it is now,
and the researchers found that it quickly entered a tidally locked
situation, with the rotation period of the Moon equal to the orbital
period of the Moon around the Earth. The same side of the Moon has
probably always faced the Earth ever since. Tidal locking is a
product of the gravity of both objects. The Moon, being much smaller
than the Earth, cooled more quickly. Because the Earth and Moon were
tidally locked from the beginning, the still-hot Earth -- more than
2500 C -- radiated towards the near side of the Moon. The far side,
away from the boiling Earth, slowly cooled, while the Earth-facing
side was kept molten -- there was an important temperature difference
between the two sides. The Moon's crust has high concentrations of
aluminium and calcium, elements that are hard to vaporize. When rock
vapour starts to cool, the very first elements that snow out are
aluminium and calcium. They would preferentially have condensed on
the far side of the Moon because the near side was still too hot.
Thousands to millions of years later, those elements combined with
silicates in the Moon's mantle to form plagioclase feldspars, which
eventually moved to the surface and formed the Moon's crust. The
far-side crust had more of those minerals and is thicker. The Moon
has now completely cooled and is not molten below the surface.
Earlier in its history, large meteoroids that struck the near side of
the Moon punched through the crust, releasing the vast lakes of
basaltic lava that formed the maria that make up the 'Man in the
Moon'. When meteoroids struck the far side, in most cases the crust
was too thick and no magmatic basalt welled up, so they created
craters, valleys and highlands, but almost no maria.
NEW ISOTOPIC EVIDENCE OF MOON FORMATION
European Association of Geochemistry
A new series of measurements of oxygen isotopes supports the
suggestion that the Moon formed from the collision of the Earth with
another large, planet-sized astronomical body, for which the name
Theia has been proposed, around 4.5 billion years ago. Efforts to
confirm that such an impact had taken place had centred on measuring
the ratios between the isotopes of oxygen, titanium, silicon and other
elements. The ratios are known to vary throughout the Solar System,
but their close similarity between the Earth and the Moon conflicted
with theoretical models of the collision that indicated that the Moon
would form mostly from Theia, and thus could be expected to be
compositionally different from the Earth.
Now a group of German researchers has used more refined techniques to
compare the ratios of the oxygen isotopes of masses 17 and 16 in lunar
samples with those found on the Earth. The team initially used lunar
samples which had arrived on the Earth as meteorites, but as such
samples could have exchanged their isotopes with terrestrial water,
fresher samples were sought. They were provided by NASA from the
Apollo 11, 12 and 16 missions, and were found to contain significantly
higher proportions of oxygen 17 than their Earthly counterparts. The
differences are small but definite. That helps towards two
conclusions. First, we can now be reasonably sure that the giant
collision took place. Secondly, it gives us an idea of the chemistry
of Theia. Theia seems to have been similar to what we call E-type
chondrites. If so, we can now try to make sense of the chemical and
isotopic composition of the Moon, because the present Moon is a
mixture of Theia and the early Earth. The next goal is to find out
how much material of Theia is in the Moon. Most models estimate that
the Moon it is composed of around 70% to 90% material from Theia, with
the remaining 10% to 30% coming from the early Earth. However, some
models argue for as little as 8% Theia in the Moon.
MILKY WAY MAY HAVE 100 MILLION HABITABLE PLANETS
Cornell University
There are some 100 million other places in the Milky Way galaxy that
could support complex life, according to the report of a group of
university astronomers in the previously unheard-of journal
'Challenges' (apparently based in Switzerland and started in 2010, in
which year two out of the total of three articles were Editorials).
They have examined data on planets orbiting other stars. Their study
provides the first quantitative estimate of the number of planets in
our Galaxy that could harbour life above the microbial level. The
study does not indicate that complex life exists on any planets, but
simply that there are many places with conditions that could support
it. Origin-of-life questions are not addressed -- only the conditions
to support life. Complex life does not mean intelligent life --
though it does not rule out that or even animal life -- but simply
that organisms larger and more complex than microbes could exist in a
number of different forms, for example, organisms that form stable
food webs like those found in ecosystems on the Earth.
The scientists surveyed more than 1,000 planets and used a formula
that considers planet density, temperature, substrate (liquid, solid
or gas), chemistry, distance from its central star and age. From that
information, they developed and computed the Biological Complexity
Index (BCI). The BCI calculation revealed that 1 to 2 per cent of the
planets showed a BCI rating higher than Europa, a moon of Jupiter
thought to have a subsurface global ocean that could harbour forms of
life. With about 10 billion* stars in the Milky Way galaxy, a great
extrapolation of the BCI yields 100 million plausible planets.
Despite the large number of planets that could harbour complex life,
the Milky Way is so vast that planets with high BCI values are very
far apart, according to the scientists. One of the closest and most
promising extra-solar systems, called Gliese 581, has two planets with
the apparent possible capacity to host complex biospheres. The
distance from here to Gliese 581 is about 20 light-years. It seems to
some people unlikely that we are alone, but we are in any case likely
to be so far away from alien life at our level of complexity that any
meeting with such life forms seems improbable for the foreseeable
future.
*The figure often suggested is 100 (US) billion, or 10 to the power 11
[ED.]
DWARF GALAXIES DO NOT FIT STANDARD MODEL
Rochester Institute of Technology
Dwarf galaxies that orbit the Milky Way and the Andromeda galaxies
defy the accepted model of galaxy formation, and recent attempts to
wedge them into the model are flawed, reports an international team of
astrophysicists. The study pokes holes in the current understanding
of galaxy formation and questions the accepted model of the origin and
evolution of the Universe. According to the standard paradigm, 23% of
the mass of the Universe is shaped by invisible particles known as
dark matter. The model predicts that dwarf galaxies should form
inside small clumps of dark matter and that the clumps should be
distributed randomly about their parent galaxy -- but what is observed
is very different. The dwarf galaxies belonging to the Milky Way and
Andromeda are seen to be orbiting in huge, thin disc-like structures.
The study criticizes three recent papers by different international
teams, all of which concluded that the satellite galaxies support the
standard model. The critique found "serious issues" with all three
studies.
The team of 14 scientists from six different countries replicated the
earlier analyses using the same data and cosmological simulations and
came up with much lower probabilities -- roughly one tenth of a
percent -- that such structures would be seen in the Milky Way and the
Andromeda galaxy. The earlier papers found structures in the
simulations that no one would say really looked very much like the
observed planar structures. The team writes that, "Either the
selection of model satellites is different from that of the observed
ones, or an incomplete set of observational constraints has been
considered, or the observed satellite distribution is inconsistent
with basic assumptions. Once these issues have been addressed, the
conclusions are different: features like the observed planar
structures are very rare." The standard cosmological model is the
frame of reference for generations of scientists, some of whom are
beginning to question its ability to reproduce what is actually
observed in the 'nearby' Universe. The team's conclusion tends to
favour an alternative, much older, model: that the satellites were
pulled out from another galaxy when it interacted with the Local-Group
galaxies in the distant past. That 'tidal' model can explain
naturally why the observed satellites are orbiting in thin discs.
'RED-NUGGET' GALAXIES
Harvard-Smithsonian Center for Astrophysics
An astronomer used sometimes to have to travel to a remote location
and work through long, cold nights, patiently guiding a telescope to
collect precious photons of light. Now, a proliferation of online
archives allows people to make discoveries from the comfort of their
own offices. By mining such archives, a team of astronomers has found
a lot of 'red-nugget' galaxies -- galaxies that are compact and
densely packed with old, red stars. Their abundance provides new
constraints on theoretical models of galaxy formation and evolution.
When the Universe was young, dense, massive galaxies nicknamed 'red
nuggets' were common. They are ten times as massive as the Milky Way,
but their stars are packed into a volume a hundred times smaller than
that of our Galaxy. However, astronomers searching the older, nearer
Universe could not find any such objects. Their apparent
disappearance, if real, signalled a surprising turn in galaxy
evolution.
To find nearby examples, the team combed through the data base of the
largest survey of the Universe, the Sloan Digital Sky Survey. The
red-nugget galaxies are so small that they appear like stars in Sloan
pictures. However, their spectra give away their true nature. The
team identified several hundred red-nugget candidates in the Sloan
data. Then they searched a variety of online telescope archives in
order to confirm their findings. In particular, high-quality images
from the Canada-France-Hawaii Telescope and the Hubble Space Telescope
showed that about 200 of the candidates were galaxies very similar to
their red-nugget cousins in the distant, young Universe. The large
number of red nuggets discovered in Sloan told the team how abundant
those galaxies were in the middle-aged Universe. That number then can
be compared to computer models of galaxy formation. Different models
for the way galaxies grow predict very different abundances. The
picture that matches the observations is one where red nuggets begin
their existence as very small objects in the young Universe. During
the next ten billion years some of them collide and merge with other,
smaller and less massive galaxies. Some red nuggets manage to avoid
collisions and remain compact as they age. The result is a variety of
elliptical galaxies with different sizes and masses, some very compact
and some more extended.
LIGHT ARRIVES FROM EXPLOSION 12 BILLION YEARS AGO
Southern Methodist University, Dallas
Intense light from a gamma-ray burst, the enormous explosion of a star
more than 12 billion years ago -- 'shortly' after the Big Bang --
recently reached here and was visible in the sky. The light travelled
for 12.1 billion years before it was observed by a telescope called
ROTSE-IIIb. Gamma-ray bursts are believed to be the catastrophic
collapse of a star at the end of its 'life'. Designated GRB 140419A,
the burst was observed on April 19. Gamma-ray bursts are the most
powerful explosions in the Universe since the Big Bang. They release
more energy in 10 seconds than our Sun will do during its entire
expected lifespan of 10 billion years. Some bursts appear to be
related to supernovae, and correspond to the demise of a massive star.
Gamma-ray bursts may be particularly massive cousins to supernovae, or
may correspond to cases in which the explosion ejecta are more beamed
in our direction. By studying them, we may learn about supernovae.
Scientists did not detect optical light from gamma-ray bursts until
the late 1990s. Among all radiation in the electromagnetic spectrum,
gamma rays have the shortest wavelengths and are visible only to
special detectors. As the gamma radiation declines, the explosion
produces an afterglow of visible optical light which, in turn, fades
very quickly, after a few seconds to a few hours. Sometimes optical
telescopes can capture the spectrum of an afterglow, which gives the
red-shift of the light, an indirect indication of the distance from us.
The red-shift of the recent burst was 3.96.
SPHERE EXO-PLANET IMAGER
ESO
SPHERE -- the Spectro-Polarimetric High-contrast Exo-planet REsearch
instrument -- has been installed on ESO's Very Large Telescope (VLT)
at the Paranal Observatory in Chile and has achieved first light.
The new facility for finding and studying exo-planets uses several
advanced techniques in combination, giving better performance than
existing instruments, and has produced impressive views of dust discs
around nearby stars and other objects during the very first days of
observations. It is hoped to revolutionize the detailed study of
exo-planets and circumstellar discs. The first of three novel
techniques exploited by SPHERE is extreme adaptive optics to correct
for the effects of the Earth's atmosphere so that images are sharper
and the contrast of the exo-planet increased. Secondly, a coronagraph
is used to block out the light from the star and increase the contrast
still further. Finally, a technique called differential imaging is
applied that exploits differences between planetary and stellar light
in terms of its colour or polarisation, and those subtle differences
can also be exploited to reveal an otherwise invisible exo-planet.
Topic: Late June Astronomy Bulletin (Read 1930 times)