ASTEROIDS ARE MOON'S MAIN WATER SUPPLY
Moscow Institute of Physics and Technology
Water found on the Moon is now thought to have been brought there by
impacting asteroids and not by comets as was previously thought.
Using computer simulation, scientists have discovered that one large
asteroid can deliver more water to the lunar surface than the
cumulative fall of comets over a thousand-million-year period. At the
beginning of the space age, during the days of the Apollo programme,
scientists believed the Moon to be completely dry. At the earliest
stages in satellite evolution, the absence of an atmosphere and the
influence of solar radiation were thought to be enough to evaporate
all volatile substances into space. However, in the 1990s, scientists
obtained data from the Lunar Prospector probe that were indicative of
a larger fraction of hydrogen at the near-surface soil of some regions
of the Moon, which one could interpret as a sign of the presence of
water. In order to explain how water could be kept on the Moon's
surface, scientists formulated a theory known as 'cold traps'. The
axis of the Moon's rotation is nearly vertical, which is why in the
polar regions there are craters whose floors are never exposed to
sunlight. When comets consisting mostly of water ice fall, evaporated
water can gravitate into those 'traps' and remain there indefinitely,
as solar rays do not evaporate it.
In recent years, lunar missions (the Indian Chandrayan probe, the
American LRO, data from the Cassini probe and Deep Impact) have
brought scientists two new pieces of information. The first is that
there are indeed considerable quantities of water and hydroxyl groups
in the near-surface soil on the Moon. The LCROSS experiment, in which
a probe was purposely crashed onto the Moon resulting in the release
of a cloud of gas and dust that was later studied with a spectrometer,
directly confirmed the existence of water and other volatile
substances. The second piece of new information came when the Russian
LEND apparatus on board LRO generated a map of water distribution on
the Moon's surface. The scientists first considered whether comets
could be the main water suppliers. The typical velocity of an ice
comet ranges from 20 to 50 km/s; estimates suggested that such a high
impact velocity causes from 95 to 99.9% of the water to evaporate into
space beyond retrieve. There is a family of short-period comets whose
velocity is much lower -- 8-10 km/s. Such comets could account for
about 1.5% of lunar craters. Nevertheless, the simulation showed that
when such short-period comets do impact, almost all the water
evaporates and less than 1% of it remains at the impact point.
Scientists concluded that only a very small amount of water that
arrives with a comet stays on the Moon, so they decided to explore the
possibility of an asteroid origin of lunar water. They noticed that
asteroids consist of initially non-differentiated materials typical of
the Solar System and contain a rather considerable proportion of
water. In particular, carbonaceous chondrite, the most common type of
asteroids and meteorites, can contain up to 10% water. However, water
in chondrites is effectively protected: it is in a chemically bonded
condition, and it is 'blocked' in a crystal lattice of minerals.
Water starts to be lost only when it is heated to 300-1200 C depending
on the type of mineral, so it could potentially remain in the crater
together with the rest of the asteroid. Calculations indicate that
between 2 and 4.5% of lunar craters could contain considerable
supplies of water in the form of hydrated minerals. They are stable
enough to contain water even in areas exposed to the Sun. That is
important in relation to any project to establish a 'lunar base',
because the polar cold traps are not convenient areas for such
constructions: there is little solar energy, it is difficult to
organize radio communication, and lastly, there are dramatically low
temperatures. The possibility of obtaining lunar water in regions
exposed to the Sun could make the issue of lunar exploration less
difficult,
AIDA DOUBLE MISSION TO DIVERT DIDYMOS ASTEROID'S DIDYMOON
RAS
An ambitious joint US-European mission, called AIDA, is being planned
to divert the orbit of a binary asteroid's small moon, as well as to
give us new insights into the structure of asteroids. A pair of
spacecraft, the ESA-led Asteroid Impact Mission (AIM) and NASA-led
Double Asteroid Redirection Test (DART), will rendezvous with the
asteroid Didymos nd its small natural satellite, known informally as
Didymoon. Following a period of study of both asteroids and detailed
mapping of Didymoon by AIM, DART will impact onto Didymoon and AIM
will assess the mission's effectiveness in diverting the moon's orbit
around Didymos. To protect the Earth from potentially hazardous
impacts, we need to understand asteroids much better -- what they are
made of, their structure, origins and how they respond to collisions.
AIDA will be the first mission to study a binary asteroid system, as
well as the first to test whether we can deflect an asteroid by an
impact with a spacecraft. The European part of the mission, AIM, will
study the structure of Didymoon and the orbit and rotation of the
binary system, providing clues to its origin and evolution. Asteroids
represent different stages in the road to planet formation, so they
may be offering snapshots of the Solar System's history.
AIM is due for launch in 2020 October and to reach the binary system
(65803) Didymos in 2022 May. Binary systems make up around 15% of the
asteroid population. Egg-shaped Didymoon (about 160 metres in
diameter) orbits the diamond-shaped Didymos asteroid (about 750 metres
in diameter) every 12 hours at an altitude of 1.1 kilometres.
Ground-based observations show that Didymos is probably a common
chondrite, or stony asteroid, formed of dust from the primitive Solar
System. At present, Didymoon's mass and density are unknown.
BLUE SKIES ON PLUTO
Spaceweather.com
The first colour images of Pluto's atmosphere have been beamed back to
Earth by the New Horizons spacecraft just after it sped by Pluto on
July 14. The spacecraft's cameras were looking back at Pluto's night
side as sunlight illuminated the fringe of blue around Pluto's
circumference. A blue sky often results from scattering of sunlight
by very small particles. On the Earth, those particles are nitrogen
molecules. On Pluto they appear to be soot-like particles we call
tholins. The term 'tholin' was coined to describe organic substances
obtained in experiments on gas mixtures akin to the atmosphere of
Saturn's moon Titan. On Pluto, tholins form high in the atmosphere
where UV sunlight breaks apart nitrogen and methane molecules. The
fragments re-combine to form complex macromolecules, which continue to
combine and grow until they become tholins. Ironically, tholins
themselves are not blue -- they merely scatter blue light. When
tholins fall to the ground they show their true colours, grey or red.
At least some of Pluto's patchy red colouring is thought to result
from a gentle rain of such particles from the planet's atmosphere.
MASSIVE MAGNETIC STARS
RAS
A consortium known as 'Binarity and Magnetic Interactions in various
classes of Stars' (BinaMIcS) has observed Epsilon Lupi, a third-
magnitude B-type system whose duplicity was discovered more than 100
years ago. It is a pair of massive stars which have magnetic fields.
Around 1/3 of stars in our Galaxy are thought to be in binary systems.
They are interesting, because astronomers can in some cases determine
their masses and relate that to their brightness -- which helps us
understand how stars evolve.
Epsilon Lupi is the fourth-brightest star system in the southern
constellation Lupus. The pair of stars is about 500 light-years away;
both are B-type stars having between 7 and 8 times the mass of the
Sun, and together the pair is around 6000 times as luminous as the
Sun. Only comparatively recently it was discovered that the two giant
stars have substantial magnetic fields. In cool stars, such as the
Sun, magnetic fields are generated by 'dynamos' powered by strong
convection in the outer layers of the star, where hot material rises,
cools and falls back. But there is no convection in the envelopes of
massive stars, so there is no support for a magnetic dynamo.
Nevertheless, approximately 10% of massive stars have strong magnetic
fields.
Two explanations have been proposed for their origin, both variants on
the idea of a 'fossil' magnetic field, a field generated at some point
in the star's past and then locked into the star's surface. The first
hypothesis is that the magnetic field is generated while the star is
being formed; a second is that the magnetic field originates in
dynamos driven by the violent mixing of material when two already-
formed stars in a close binary merge. The Epsilon Lupi discovery
allows us to rule out the binary-merger scenario, but it does not
change the basic finding of the BinaMIcS collaboration: fewer than 2%
of massive stars in close binaries have magnetic fields, and we still
do not know why that is. The research shows that the strengths of the
magnetic fields are similar in the two stars, but their magnetic axes
are anti-aligned, with the south magnetic pole of one star pointing in
approximately the same direction as the north pole of the other. It
may even be that the two stars share a single magnetic field, in any
case it probably points to something significant about how the stars
are interacting with one another. The stars are close enough that
their magnetospheres are likely to be interacting during the whole of
their orbit around each other. That means that their magnetic fields
may act as a brake, slowing down the stars. As a result, in the long
term, the two stars may be gradually spiralling in towards one
another.
RIPPLES RACING THROUGH PLANET-FORMING DISC
ESO
Using images from the Very Large Telescope and the Hubble telescope,
astronomers have discovered never-before-seen structures within a
dusty disc surrounding a nearby star. The fast-moving wave-like
features in the disc of the star AU Microscopii are unlike anything
ever observed, or even predicted, before. The origin and nature of
the features is unknown. AU Mic is a young, nearby star surrounded by
a large disc of dust. Studies of such debris discs can provide
valuable clues about how planets, which form from such discs, are
created. Astronomers have been searching AU Mic's disc for any signs
of clumpy or warped features, as such signs might give away the
location of possible planets. In 2014 they used the more powerful
high-contrast imaging capabilities of ESO's newly installed SPHERE
instrument, mounted on the Very Large Telescope for their search --
and discovered something very unusual. Five wave-like arches at
different distances from the star show up in the new images,
reminiscent of ripples in water. After observing the features in the
SPHERE data the team turned to earlier images of the disc taken by
Hubble in 2010 and 2011 to see whether the features were also visible
in those. They were not only able to identify the features on the
earlier images, but they also discovered that they had changed in the
four-year interval -- in fact they are moving very fast! They found
that the arches are racing away from the star at speeds of up to about
10 km/s. The features further away from the star seem to be moving
faster than those closer to it. At least three of the features are
moving so fast that they could well be escaping from the gravitational
attraction of the star. Such high speeds rule out the possibility
that these are conventional disc features caused by objects such as
planets disturbing material in the disc while orbiting the star.
There must have been something else involved to speed up the ripples
and make them move so quickly, implying that they are a sign of
something truly unusual. The team does not know what caused the
ripples, but it has considered and ruled out a series of phenomena as
explanations, including the collision of two massive and rare
asteroid-like objects releasing large quantities of dust, and spiral
waves triggered by instabilities in the disc. One explanation for the
strange structure links them to the star's flares. AU Mic is a star
with high flaring activity -- it often lets off huge and sudden bursts
of energy from on or near its surface. Such a flare could perhaps
have triggered something on one of the planets -- if there are planets
-- like a violent stripping of material which could now be propagating
through the disc, propelled by the flare's force. The team plans to
continue to observe the AU Mic system with SPHERE and other
facilities, including ALMA, to try to understand what is happening.
RADIO TELESCOPES COULD OBSERVE STARS IN THE GALACTIC CENTRE
RAS
The centre of our Milky Way galaxy is a mysterious place. Not only is
it thousands of light-years away, it is also cloaked in so much dust
that most stars there are rendered invisible. Harvard researchers are
proposing a new way to penetrate the fog and observe stars hiding
there. They suggest looking for radio waves coming from supersonic
stars. The long path from the centre of our Galaxy to the Earth is so
choked with dust that in visible light there is about thirty
magnitudes of extinction. Radio waves, however, can pass through the
dust unimpeded. On their own, stars are not bright enough in the
radio for us to detect them at such distances. However, if a star is
travelling through gas faster than the speed of sound, material
blowing off the star as a stellar wind can plough into the interstellar
gases and create a shock wave. Through a process called
synchrotron radiation, electrons accelerated by that shock wave
produce radio emission that we can potentially detect. In a sense, it
is the cosmic equivalent of a sonic boom from an aeroplane.
To create a shock wave, the star would have to be moving at a speed of
thousands of km/s. That is possible in the Galactic Centre since the
stars there are influenced by the strong gravity of a supermassive
black hole. When an orbiting star reaches its closest approach to the
black hole, it may well acquire the required speed. The researchers
suggest looking for that effect from one already-known star called
S2. That star, which is hot and bright enough to be seen in the
infrared despite all the dust, and will make its closest approach to
the Galactic Centre in late 2017 or early 2018. When it does, radio
astronomers can look for emission from its shock wave. S2 will be a
test: if it is seen in the radio, then potentially we can use the same
method to find smaller and fainter stars that can not be seen in any
other way.
BLACK HOLE IS 30 TIMES EXPECTED SIZE
RAS
The central supermassive black hole of a recently discovered galaxy
has been found to be far larger than should be possible, according to
current theories of galactic evolution. The galaxy, SAGE0536AGN, was
initially discovered with the Spitzer space telescope in infrared
light. Thought to be at least 9 billion years old, it contains an
active galactic nucleus (AGN), an incredibly bright object resulting
from the accretion of gas by a central supermassive black hole. The
gas is accelerated to high velocities, and caused to emit light, by
the black hole's immense gravitational field. The team has confirmed
the presence of the black hole by measuring the speed of the gas
moving around it. Using the Southern African Large Telescope, the
scientists observed a hydrogen emission line whose broadening implies
that the gas is moving at a speed that implies that the mass of the
black hole in SAGE0536AGN is about 350 million times the mass of the
Sun. But the mass of the galaxy itself, obtained through measurements
of the movement of its stars, has been calculated to be 25 thousand
million solar masses. That is seventy times that of the black hole,
but the black hole is still thirty times more massive than expected
for that size of galaxy. In ordinary galaxies the black hole would
grow at the same rate as the galaxy, but in SAGE0536AGN the black hole
has grown much faster, or the galaxy stopped growing prematurely.
Because that galaxy was found by accident, there may be more such
objects waiting to be discovered. Time will tell whether SAGE0536AGN
really is an oddity, or simply the first in a previously unknown class
of galaxies.
BARREN DWARF GALAXY FORMS BRILLIANT STAR CLUSTERS
National Radio Astronomy Observatory.
An international team of astronomers using the Atacama Large
Millimetre/submillimetre Array (ALMA) has discovered an unexpected
population of compact interstellar clouds hidden within the nearby
dwarf irregular galaxy Wolf-Lundmark-Melotte, more commonly known as
WLM. The clouds, which are within a heavy blanket of interstellar
material, help to explain how dense star clusters are able to form in
the tenuous environs of a galaxy thousands of times smaller and far
more diffuse than our own Milky Way. For many reasons, dwarf
irregular galaxies like WLM are poorly equipped to form star clusters.
They are fluffy, with very low densities. They also lack the heavy
elements that contribute to star formation. Such galaxies should form
only dispersed stars rather than concentrated clusters, but that is
clearly not the case. By studying that galaxy with ALMA, the
astronomers were able to locate, for the first time, compact regions
that appear able to emulate the nurturing environments found in larger
galaxies. Those regions were discovered by pinpointing the almost
imperceptible and highly localized millimetre-wavelength light emitted
by carbon monoxide (CO) molecules, which are typically associated with
star-forming interstellar clouds.
Earlier, an affiliated team of astronomers first detected CO in the
WLM galaxy with the single-dish Atacama Pathfinder EXperiment (APEX)
telescope. Those initial, low-resolution observations could not
resolve where the molecules reside, but they did confirm that WLM
contains the lowest abundance of CO ever detected in any galaxy. The
lack of CO and other heavy elements should put a serious damper on
star formation, the astronomers note. Molecules, and carbon monoxide
in particular, play an important role in star formation. As gas
clouds begin to collapse, temperatures and densities rise, pushing
back against gravity. That is where molecules and dust particles come
to the rescue by absorbing some of the heat through collisions and
radiating it into space at infrared and sub-mm wavelengths. That
cooling effect enables gravity to continue the collapse until a star
forms. The problem previously was that in WLM and similar galaxies
with very low abundances of heavy elements, astronomers simply did not
see enough of that material to account for the new star clusters they
observed. The reason that the CO was initially so difficult to see,
the researchers discovered, is that unlike in normal galaxies, the WLM
clouds are tiny compared to their overlying envelopes of molecular and
atomic gas. To become effective star factories, the concentrated CO
clouds need the enormous envelopes of transitional gas to bear down on
them, giving the cores of CO a high enough density to allow them to
form a normal cluster of stars. Those bundles of star-forming gas are
under considerable pressure, even though the surrounding ocean of
interstellar gas is much more shallow. By discovering that the carbon
monoxide is confined to highly concentrated regions within a vast
expanse of transitional gas, we could finally understand the
mechanisms that led to the impressive stellar neighbourhoods we see in
the galaxy today. Further studies with ALMA will also help determine
the conditions that formed the globular clusters found in the halo of
the Milky Way. It has been suggested that those much larger clusters
may originally have formed in dwarf galaxies and later migrated to the
halo after their host dwarf galaxies dispersed. WLM is a relatively
isolated dwarf galaxy located approximately 3 million light-years away
on the outer edges of the 'Local Group' -- the collection of galaxies
that includes the Milky Way, the Magellanic Clouds, Andromeda, M33,
and dozens of smaller galaxies.
MOST EARTH-LIKE WORLDS YET TO BE BORN
RAS
According to a new theoretical study, when our Solar System was born
4.6 billion years ago only 8% of the potentially habitable planets
that will ever form in the Universe existed; the bulk of those
planets had yet to be born. That conclusion is based on data
collected by the Hubble and Kepler space telescopes. The main
motivation for the study was understanding the Earth's place in the
context of the rest of the Universe. Compared to all the planets that
will ever form in the Universe, the Earth was actually formed quite
early. Looking far away and far back in time, Hubble has given
astronomers a 'family album' of galaxy observations that chronicle the
Universe's star formation history as galaxies grew. The data show
that the Universe was making stars at a fast rate 10 billion years
ago, but the fraction of the Universe's hydrogen and helium gas that
was involved was very low. Today, star birth is happening at a much
slower rate than long ago, but there is so much leftover gas available
that the Universe will keep forming stars and planets for a very long
time to come. There is enough material remaining to produce even more
planets in the future, in the Milky Way and beyond.
Kepler's planet survey indicates that Earth-sized planets in a star's
'habitable zone', the distance at which water could pool on the
surface, are ubiquitous in our galaxy. On the basis of that survey,
scientists predict that there should be one billion Earth-sized
worlds in the Milky Way galaxy at present, a good portion of them
presumed to be rocky. That estimate skyrockets when extrapolated to
the other 100 billion galaxies in the observable Universe. That
leaves plenty of opportunity for untold more Earth-sized planets in
habitable zones to arise in the future. The last star is not expected
to burn out until 100 trillion years from now. That is plenty of time
for literally anything to happen on the planet landscape. The
researchers say that future Earths are more likely to appear inside
giant galaxy clusters and also in dwarf galaxies, which have yet to
use up all their gas for building stars and accompanying planetary
systems. By contrast, our Milky Way galaxy has used up much more of
the gas available for future star formation. A big advantage to our
civilization from arising early in the evolution of the Universe is
our being able to use powerful telescopes like Hubble to trace our
lineage from the Big Bang through the early evolution of galaxies.
The observational evidence for the Big Bang and cosmic evolution,
encoded in light and other electromagnetic radiation, will be all-but
erased away a trillion years from now by the expansion of space. Any
far-future civilizations that might arise will be largely clueless as
to how or if the Universe began and evolved.
LINK BETWEEN COMET AND ASTEROID SHOWERS & MASS EXTINCTIONS
RAS
For more than 30 years, scientists have argued about a controversial
hypothesis relating to periodic mass extinctions and impact craters --
caused by comet and asteroid showers -- on the Earth. Now scientists
offer new support linking the age of the craters with recurring mass
extinctions of life, including the demise of the dinosaurs. They show
a cyclical pattern over the studied period, with both impacts and
extinction events taking place every 26 million years. That cycle has
been linked to periodic passage of the Sun and planets through the
dense mid-plane of our Galaxy. Scientists have theorized that
gravitational perturbations of the distant Oort comet cloud that
surrounds the Sun lead to periodic comet showers in the inner Solar
System, where some comets strike the Earth. To test their hypothesis,
the researchers performed time-series analyses of impacts and
extinctions using newly available data offering more accurate age
estimates. The correlation between the impacts and extinction events
over the past 260 million years is striking and suggests a
cause-and-effect relationship. In particular, they found that six
mass extinctions of life during the studied period correlate with
times of enhanced impact cratering. One of the craters considered in
the study is the large (180 km diameter) Chicxulub impact structure in
the Yucatan, which dates from about 65 million years ago, the time of
a great mass extinction that included the dinosaurs. Moreover, they
add, five out of the six largest impact craters of the last 260
million years on Earth correlate with mass-extinction events.
Topic: Late October Astronomy Bulletin (Read 1532 times)