POSSIBLE FRAGMENTS FROM 1908 TUNGUSKA METEOR EXPLOSION
Universe Today
The 1908 explosion over the Tunguska region in Siberia has always been
an enigma. The leading theories of what caused the high-altitude
explosion are that an asteroid or comet shattered in an airburst
event, but no reliable trace of such a body has ever been found. A
newly published paper, however, refers to three different potential
meteorite fragments found in the sand bars in a body of water in the
area, the Khushmo River. The fragments have all the hallmarks of
being meteorites from the event, but there is an oddity inasmuch as
the researcher actually found the fragments 15 years ago but has only
recently published his findings. Like the recent Chelyabinsk
air-burst event, the Tunguska event probably produced a shower of
fragments from the exploding parent body. The explosion flattened
trees over a radius of 25 km. Luckily that region was largely
uninhabited; only one person was said to have been killed, and there
were very few people who reported the explosion. Forensic-like
research has suggested that the blast was 1,000 times more powerful
than the Hiroshima bomb explosion, and it created an earthquake that
registered 5 on the Richter scale.
Previous expeditions to the region did not find any meteorites, though
one expedition in 1939 by Russian mineralogist Leonid Kulik found a
piece of melted glassy rock containing bubbles, which was considered
evidence of an impact event. But the sample was somehow lost and is
not available for a modern analysis. In 1998 Andrei Zlobin from the
Russian Academy of Sciences made several drill holes in the peat bogs
in the area, and while he found evidence of the explosion, he did not
at first find any meteorites. He gathered from a river shoal about
100 samples of rocks that had features of potential meteorites, but
further examination reduced the number to just three rocks with
tell-tale features like melting and regmaglypts (impressions on the
surfaces of meteorites, caused by ablation as the hot rock tears
through the atmosphere at high speed). Zlobin claims that after the
expedition he investigated thermal processes and mathematical
modelling of the Tunguska impact; he used tree-ring evidence to
estimate the temperatures from the event, and concluded that rocks
already on the ground would not have been changed or melted from the
blast, and therefore any rocks showing evidence of melting should be
from the impactor itself. Zlobin says that he has not yet carried out
a detailed chemical analysis of the rocks, to determine their chemical
and isotopic composition. But he does say that the stony fragments do
not rule out a comet, since the nucleus could easily contain rock
fragments. There is nothing actually definitive from Zlobin's new
paper, and there is the question of why he waited so long to conduct
his study, but his work does offer some hope for a fuller explanation
of the Tunguska event than has yet been obtained. Clearly there is
more work to be done, particularly the chemical analysis, perhaps with
international cooperation and corroboration.
EARTH AND MOON HAVE COMMON WATER SOURCE
Brown University
New research finds that water inside the Moon's mantle came from
primitive meteorites, the same source as is thought to have supplied
most of the water on the Earth. The findings raise new questions
about the process that formed the Moon. The Moon is thought to have
formed from a disc of debris left when another object hit the Earth
4.5 billion years ago, very early in the Earth's history. Scientists
have long assumed that the heat from such an impact would cause
hydrogen and other volatile elements to boil off into space, meaning
that the Moon must have started off completely dry. But recent
spacecraft and new research on samples from the Apollo missions have
shown that the Moon actually has water, both on its surface and
beneath. By showing that water on the Moon and on Earth came from the
same source, the new study offers evidence that the Moon's water has
been there all along.
The simplest explanation for what the researchers found is that there
was water on the proto-Earth at the time of the impact. Some of that
water survived the impact, and is what we see in the Moon. To find
the origin of the Moon's water, the team looked at melt inclusions
found in samples brought back from the Apollo missions. Melt
inclusions are tiny dots of volcanic glass trapped within crystals
called olivine. The crystals prevent water escaping during an eruption
and enable researchers to get an idea of what the inside of the Moon
is like. Research in 2011 found that the melt inclusions have plenty
of water -- as much water in fact as lavas forming on the Earth's
ocean floor. To find the origin of that water, the team looked at the
isotopic composition of the hydrogen trapped in the inclusions. They
measured the amount of deuterium in the samples in comparison with the
amount of ordinary hydrogen. Water molecules originating from
different places in the Solar System have different amounts of
deuterium. In general, bodies that formed closer to the Sun have less
deuterium than ones formed farther out. They found that the
deuterium/hydrogen ratio in the melt inclusions was relatively low and
matched the ratio found in carbonaceous chondrites, meteorites
originating in the asteroid belt near Jupiter and thought to be among
the oldest objects in the Solar System. That means that the source of
the water on the Moon is primitive meteorites, not comets as some
scientists thought. Comets, like meteorites, are known to carry water
and other volatiles, but most comets formed in the far reaches of the
Solar System and tend to have high deuterium/hydrogen ratios -- much
higher than in the Moon's interior, where the samples in this study
are supposed to have come from. The new data provide the best
evidence yet that the carbon-bearing chondrites were a common source
for the volatiles in the Earth and Moon, and perhaps the entire inner
Solar System. Recent research has found that as much as 98% of the
water on Earth also comes from primitive meteorites, suggesting a
common source for water on Earth and water on Moon. The easiest way to
explain that is that the water was already present on the early Earth
and was transferred to the Moon. If the Moon was formed by a massive
impact with the early Earth, it makes sense that the water in both
would share a common source, but there is still a question as to how
that water was able to survive such a violent collision.
CASSINI OBSERVES METEOROIDS HITTING SATURN'S RINGS
NASA
The Cassini spacecraft has provided the first direct evidence of small
meteoroids crashing into Saturn's rings and breaking into streams of
rubble. The meteoroids Cassini detected range in size from about a
centimetre to several metres. The new results imply that present-day
impact rates for small particles at Saturn are about the same as those
at the Earth. The Saturnian equinox in 2009 was an especially good
time to see the debris left by meteoroid impacts. The edge-on
illumination of the rings by the Sun made clouds of debris look bright
against the darkened rings. Researchers now think that meteoroids
probably break up on a first encounter with the rings, creating
smaller, slower pieces that then enter into orbit around Saturn. The
impact into the rings of those secondary meteoroids creates clouds of
debris. The tiny particles forming the clouds have a range of orbital
speeds around Saturn. As a result they are soon are pulled into
diagonal, extended bright streaks such as Cassini observed.
HUBBLE FINDS DEAD STARS POLLUTED WITH PLANETARY DEBRIS
RAS
The Hubble telescope has found signs of Earth-like planets in an
unlikely place -- the atmospheres of two white-dwarf stars. The
white dwarfs are small, dim remnants of stars and are about 150
light-years away in the Hyades star cluster, in Taurus. Hubble's
spectroscopic observations identified silicon in their atmospheres;
silicon is a major ingredient of the rocky material of the Earth and
other terrestrial planets in the Solar System. The silicon may have
come from asteroids that were shredded by the white dwarfs' very
strong gravity when they approached too close to the stars.
FIRST BIOLOGICAL EVIDENCE OF A SUPERNOVA
Technische Universitaet Muenchen
In fossil remnants of iron-loving bacteria, researchers have found a
radioactive iron isotope that they trace back to a supernova in our
cosmic neighbourhood. This is the first possible biological signature
of a supernova. The age determination of a deep-drill core from the
Pacific Ocean showed that the supernova must have occurred about 2.2
million years ago, roughly around the time when the modern human
developed.
Most of the chemical elements have their origins in core-collapse
supernovae. When a star ends its life in a gigantic explosion, it
throws most of its mass into space. The radioactive iron isotope
Fe-60 is produced almost exclusively in such supernovae. Because its
half-life of 2.62 million years is short compared to the age of the
Solar System, no supernova iron should be present on Earth.
Therefore, any discovery of Fe-60 on Earth would indicate a relatively
recent supernova in our cosmic neighbourhood. In 2004, Fe-60 was
discovered in a ferro-manganese crust obtained from the floor of the
equatorial Pacific Ocean. Its geological dating puts the event around
2.2 million years ago. Some support for an event around that time has
come from Pacific Ocean sediment brought up as a drill core.
So-called magnetotactic bacteria live on the ocean floor. They make
within their cells hundreds of tiny crystals of magnetite (Fe3O4),
each approximately 80 nanometres in diameter. The bacteria obtain the
iron from atmospheric dust that enters the ocean. Astrophysicists
conjectured, therefore, that Fe-60 should also occur within those
magnetite crystals produced by magnetotactic bacteria extant at the
time of the supernova interaction with our planet. The bacterially
produced crystals, when found in sediments long after their host
bacteria have died, are called 'magnetofossils'. The drill core
referred to above showed indications of Fe-60 at about the right date.
A second and much larger core has been obtained but not yet analysed.
CLUSTER OF HYDROGEN CLOUDS BETWEEN NEARBY GALAXIES
National Radio Astronomy Observatory.
In a dark, starless patch of intergalactic space, astronomers have
discovered a cluster of hydrogen clouds strewn between two nearby
galaxies, Andromeda (M31) and Triangulum (M33). The researchers
speculate that the rarefied clouds -- each about as massive as a dwarf
galaxy -- condensed out of a vast and as-yet undetected reservoir of
hot, ionized gas, which could have accompanied an otherwise invisible
band of dark matter. Astronomers have known for some time that many
seemingly empty stretches of the Universe contain vast but diffuse
patches of hot, ionized hydrogen. Earlier observations of the area
between M31 and M33 suggested the presence of colder, neutral
hydrogen, but they could not see any details to determine if it had a
definitive structure or represented a new type of cosmic feature.
Now, with high-resolution images, they were able to detect discrete
concentrations of neutral hydrogen emerging out of what was thought to
be a mainly featureless field of gas.
Astronomers are able to observe neutral atomic hydrogen by the
characteristic signal it emits at radio wavelengths. Though hydrogen
is abundant throughout the cosmos, between galaxies it can be very
tenuous, and the faint signal it emits is difficult to detect. But
last year the Green Bank Telescope found that there was a lot of
hydrogen between M31 and M33. More thorough studies of the region
indicated that much of the gas is clumped together into discrete and
apparently self-gravitating clouds that apart from their lack of stars
would be called dwarf galaxies. It was possible to track the motion
of the clouds, which proved to be travelling through space at
velocities similar to M31 and M33, suggesting that they are
independent entities and not constituent parts of either galaxy.
EINSTEIN WAS RIGHT -- AGAIN
ESO
An international team has discovered a double object that consists of
a tiny, but unusually massive, neutron star that spins 25 times each
second and is observed as a pulsar, orbited every two and a half hours
by a white-dwarf star. The unusual pair constitutes a unique
laboratory for testing the limits of physical theories. The pulsar,
called PSR J0348+0432, is the remains of a supernova explosion. It is
twice as massive as the Sun, but just 20 kilometres across. The
gravity at its surface is more than 300 billion times stronger than
that on the Earth and at its centre any volume the size of a sugar
cube has more than a billion tons of matter in it. Einstein's general
theory of relativity, which explains gravity as a consequence of the
curvature of space-time, has passed every test since it was first
published almost a century ago, but the extreme gravity of PSR
J0348+0432 offers an opportunity to push tests into new territory.
The team combined Very Large Telescope observations of the white dwarf
with very precise timing of the pulsar from radio telescopes. Such a
close binary is expected to radiate gravitational waves and lose
energy. That causes the orbital period to lengthen very gradually,
and the predictions of the change from general relativity and other
competing theories are different. The radio observations are so
precise that astronomers have already been able to measure a change in
the orbital period of 8 microseconds per year, exactly what Einstein's
theory predicts.
PLANETS
By Alan Clitherow, SPA Planetary Section Director
Saturn remains the dominant planet in the night sky through late May
and into June. As the sky darkens on the 27th of May Saturn becomes
visible close to due south and at a magnitude of around 0.47 it should
be the first “star” visible in this location; it will be at around 27
degrees above the horizon when first visible from the south of the UK
and 22 degrees from central Scotland from around 2230UT. At these
elevations Saturn is rather low in the sky which does present some
problems with observation. Since we are viewing at a slant through our
thick atmosphere, it can be badly affected by any turbulence along the
line of sight. With that in mind the air can often be very steady
shortly after sunset so it is worth making the effort as the rings are
well presented at the moment and with a good view of Saturn’s
northern hemisphere all the way up to and slightly beyond the pole.
When the Voyager 1 probe passed Saturn it imaged a strange hexagonal
pattern of clouds surrounding the North Pole. Much clearer images have
been made by the Cassini probe, currently orbiting Saturn, and it
seems this feature is a stable hollow hexagon of cloud, slowly
rotating around the pole and containing a major rotating storm
hovering over the pole itself. Each side of this hexagon is some
14,000 Km long and one or two of its ‘points’ have recently been
imaged from Earth by amateurs from areas with good or excellent
‘seeing’ conditions. As we move into our summer Saturn will sink lower
into the south-western sky and if, like me, you are viewing from
Scotland, the planet will be affected by the very short summer nights
so I recommend you take the chance to view it while you still can.
Another planetary event of interest is the conjunction of Venus,
Mercury and Jupiter low in the north-western sky shortly after sunset.
Sadly this will not be visible from anywhere other than the extreme
south of the UK and even then only fleetingly. If you do live on the
south coast and can find a nice high observation site with a clear
horizon it would be worth attempting to see this event on the 27th of
May. Venus will become visible as a brilliant point of light, at a
magnitude of -3.8, at around 2100UT or shortly thereafter sitting just
above the horizon. With the Sun some 8 degrees below the horizon, use
binoculars to view Venus and, shortly thereafter, you should see
Jupiter and then tiny Mercury in the same field of view. For the rest
of the UK this event is likely to be lost in the bright summer evening
sky being below the horizon by the time semi-darkness arrives.
Venus and Mercury will improve in visibility as we move into June but
this is not a good evening apparition for Mercury from the UK. It will
be at its greatest separation from the Sun on the 12th of June (28
degrees behind the Sun) but it will not rise far above the horizon and
will be hard to observe in the bright skies. Venus too suffers from
its low elevation but will be much easier to find as a brilliant point
of light low on the north-western horizon; its elevation rising slowly
through the month.
UK FIREBALL OF 2013 MAY 8
By Tony Markham, SPA Meteor Section Director
Many people across the Midlands and south of England and south Wales
saw a bright fireball heading in a westerly direction at about
20:47 UT on the evening of May 8. Many of the witnesses described it
as having a short 'tail' and being green in colour.
Some of the news reports the following morning claimed that the
fireball was debris from Comet Halley. The claim was presumably based
on someone looking up a list of meteor showers and seeing that the
Eta Aquarid meteor shower (which is linked to Comet Halley) is active
in early May. However, the radiant of that meteor shower does not rise
until the early hours of the morning, so Eta Aquarid meteors cannot
be seen during the evening hours.
There was no indication that the fireball was related to the re-entry
of man-made space debris. It was most likely a (large) member of the
background sporadic meteor activity that is active throughout the
year. Indeed, most meteors that appear during the year are part of
the sporadic background. Major meteor showers, though briefly
impressive, provide only a small fraction of the overall total.
Since the fireball appeared at a time of night when the sky was not
yet fully dark, not all automated meteor-camera systems were active.
However, the cameras of the UK meteor-observation network had some
success in capturing images of the fireball. Their initial analysis
of its atmospheric trajectory, which suggests that the fireball passed
over Dorset and Devon, can be found at
http://snipurl.com/271sjpq .
Bear in mind, however, their caution that none of the cameras captured
the whole path, so there is some uncertainty in the calculated path.
If you saw that fireball, but have not reported your observation,
please do so via the SPA's fireball report form :
http://snipurl.com/271skj3ETA AQUARID OUTBURST
By Tony Markham, SPA Meteor Section Director
The Eta Aquarids are one of the year's stronger meteor showers but are
only well seen from tropical and southern latitudes. Few are usually
seen from the UK because the Eta Aquarid radiant only rises late in
the night when morning twilight is already encroaching.
Although the above-noted fireball was not related to the Eta Aquarids,
it appears that Eta Aquarid activity was unusually high this year.
There was some advanced warning that that might happen. A few days
before the shower peak, Mikiya Sato published predictions that, during
the course of May 6, the Earth would encounter several dust trails
ejected by Comet Halley 8-11 centuries ago. Given the age of those
filaments it was quite possible that, rather than seeing sharp peaks
in activity, observers would see a broad peak of enhanced activity
(owing to the dust having spread out). Sato's best estimate was that
the peak would be around twice its normal level -- somewhat similar to
that seen for enhancements of the Orionid meteor shower (the Earth's
autumn encounter with Comet Halley's dust stream) during the years
2006-2010.
The predictions proved accurate. Alex Pratt (in Leeds), who had never
previously imaged any Eta Aquarids, has reported that this year he
managed to image several of them late in the nights of May 4-5, 5-6
and 6-7. High rates were reported from Canada and the USA. Radio and
radar observers also reported higher than usual activity. For the
latest visual results, see the IMO activity curve at
http://snipurl.com/271slyp .
Topic: Mid May Astronomy Bulletin (Read 1672 times)