SIERRA FIREBALL DECODED
Spaceweather.com
On Sunday morning, April 22, just as the Lyrid meteor shower was dying
down, a spectacular fireball exploded over California's Sierra Nevada
mountain range. The loud explosion rattled buildings from central
California to Reno, Nevada, and beyond. Scientists searched for very-
low-frequency infrasound signals which can travel great distances from
an explosion. There were strong signals at two stations, enabling a
triangulation of the energy source at 37°.6N, 120°.5W. The energy
dissipated before the meteor broke apart in the atmosphere is
estimated to be equivalent to the explosion of about 4000 tons of TNT.
The fact that sonic booms were heard indicates that the meteor
penetrated low into the atmosphere, which implies a speed less than 15
km/s. On the assumption of that value for its speed, the mass of the
meteor was around 70 tons; if it had the density of solid rock, its
size would have been about 3-4 metres. There seem to be no reports so
far of any meteorite recoveries.
DAWN GETS EXTRA TIME TO EXPLORE VESTA
NASA/Jet Propulsion Laboratory
NASA is allowing 40 extra days for its spacecraft 'Dawn' to explore
Vesta, the second-most-massive object in the main asteroid belt. Dawn
will continue in orbit around Vesta until August 26. The extra time
will allow more observations to be made from Dawn's current low-
altitude mapping orbit, will enable the gamma-ray and neutron detector
to build better maps of the elemental composition of Vesta's surface,
and will improve data for the gravity experiment. The spacecraft's
camera and spectrometer will continue to obtain high-resolution
images. Additional time will also be spent in the planned second
high-altitude mapping orbit later this year. When Dawn arrived at
Vesta last July, much of the northern hemisphere was in shadow, but
with the passage of time more of that area will be sunlit. The
spacecraft is due to leave Vesta in August to go to Ceres, and despite
the delay in starting it is still intended to arrive there at the
originally scheduled date in 2015 February.
CASSINI FINDS TITAN LAKE TO BE LIKE A NAMIBIA MUD FLAT
JPL/NASA
Data from Cassini suggest that a lake on Titan, called Ontario Lacus,
has similarities to what we call a salt pan on Earth. Scientists have
found evidence for long-standing channels etched into the lake bed.
That suggests that Ontario Lacus, previously thought to be completely
filled with liquid hydrocarbons, could actually be a depression that
drains and refills from below, exposing liquid areas ringed by
materials like saturated sand or mudflats. They conclude that the
solid floor of Ontario Lacus is probably exposed in those areas.
Those characteristics make Ontario Lacus very similar to the Etosha
salt pan on Earth, which is a lake bed that fills with a shallow layer
of water from groundwater levels that rise during the rainy season.
That layer then evaporates and leaves sediments like tide marks
showing the previous extent of the water. On Earth, salt pans tend to
form in deserts where liquids can suddenly accumulate, and it appears
that the same thing is happening on Titan. While the liquid on Titan
is methane, ethane and propane rather than water, the cycle appears to
work in a very similar fashion to the water cycle on Earth. Apart
from the Earth, Titan is the only other body known to bear stable
liquids on its surface. There, the full hydrocarbon cycle is based on
hydrogen, carbon and nitrogen, and takes place between the atmosphere,
the surface and the sub-surface. Titan's lakes are an integral part
of that process. It will be interesting to see whether Titan, like
the Earth, changes with the seasons.
URANUS AURORAE GLIMPSED FROM EARTH
Science Daily
Last year, for the first time, scientists using the Hubble telescope
observed aurorae above Uranus. They saw them twice, as luminous spots
on the day side of Uranus -- the only side visible from here.
Previously, the distant aurorae had been observed only by instruments
on a passing spacecraft. Unlike aurorae on Earth, which can last for
hours, the newly detected ones on Uranus appeared to last only for a
couple of minutes. In general, aurorae are a feature of the
magnetosphere, the area surrounding a planet that is controlled by its
magnetic field and shaped by the solar wind (a steady flow of charged
particles emanating from the Sun). Aurorae are produced in the
atmosphere when charged solar-wind particles are accelerated in the
magnetosphere and are guided by the magnetic field close to the
magnetic poles -- that is why terrestrial aurorae are seen at high
latitudes - but the magnetosphere of Uranus is very poorly known.
Aurorae were last seen on Uranus 25 years ago when Voyager 2 passed
the planet and recorded spectra of them. Researchers suspect that the
unfamiliar appearance of the newly observed aurorae is due to the
unusual orientations of the planet's rotational and magnetic axes.
The magnetic axis is both offset from the centre of the planet and at
60 degrees to the rotational axis -- an extreme tilt compared to the
11-degree difference on Earth. The 2011 aurorae differed not only
from the Earth's but also from the Uranian ones detected by Voyager 2,
which made its fly-by when Uranus was near a solstice -- its
rotational axis was pointed toward the Sun. In that configuration,
the magnetic axis was always at a large angle to the solar-wind flow,
producing a magnetosphere similar to Earth's magnetosphere, although
more dynamic. In those conditions, the aurorae lasted longer than the
recently witnessed ones and were mainly seen on the night side of the
planet. Now Uranus is near its equinox, when its rotational axis is
almost crosswise to the flow of the solar wind. Because the magnetic
axis is so tilted, the rotation of the planet now causes each of its
magnetic poles to point once per Uranian day toward the Sun, which
may cause a type of aurora very different from the one that was seen
near the solstice.
SOME STARS MAY CAPTURE ROGUE PLANETS
The Astrophysical Journal
There has been speculation that ever so many stars in our Galaxy might
have captured rogue planets which were ejected from the star systems
in which they formed, but it is not yet time to get too excited about
it because astronomers have not detected even a single clear-cut case
of a captured planet yet. Astronomers who tried to model young star
clusters containing free-floating planets found that, _if_ there were
as many rogue planets as stars, then 3-6% of the stars would capture a
planet over time. The more massive a star, the more likely it would
be to capture planets. Rogue planets should be a natural consequence
of star formation, because newly formed star systems might contain
multiple planets, and if two planets interact gravitationally in a
certain way, one can be ejected, and _if_ it later encounters a
different star moving in the same direction at the same speed (a
seemingly unlikely coincidence) it can be captured by that star.
It would tend to end up in an orbit hundreds or thousands of times
farther from its star than the Earth is from the Sun, and its orbit
would be likely to be tilted relatively to those of any native
planets.
SERIOUS BLOW TO DARK-MATTER THEORIES
ESO
A new study of the motions of some of the stars in the Milky Way has
found no evidence for dark matter in a large volume around the Sun.
According to some theories, the solar neighbourhood was expected to be
filled with dark matter, a mysterious invisible substance that can be
detected only indirectly by its gravity. But a new study has found
that such theories do not fit the observational facts. A team using
the 2.2-m telescope at La Silla in Chile has mapped the motions of
more than 400 stars up to 4 kiloparsecs away, and has calculated the
mass of material in the vicinity of the Sun, in a volume four times
larger than has been considered in such a calculation before. That
mass matches very well an estimate from what is actually seen in
stars, dust and gas in the region around the Sun, leaving no scope for
extra 'dark matter'.
'Dark matter' is just a name for a fudge-factor in calculations; it is
supposed to be a substance that cannot be seen, but shows itself by
its gravitational attraction for the material around it. It was
originally postulated to explain why the outer parts of galaxies,
including our own Milky Way, rotate as quickly as they do, but it also
got used in efforts to explain how galaxies formed and evolved. Today
it is even suggested that a dark component constitutes about 80% of
the mass in the Universe, despite the facts that it has resisted all
attempts to clarify its nature and that all attempts to detect it in
laboratories on Earth have failed. Models of how galaxies form and
rotate suggest that the Milky Way is surrounded by a halo of dark
matter, of which significant amounts would be expected in the region
around the Sun, where actually there isn't any according to the new
result. All the same, the Milky Way does rotate faster than can be
accounted for by the visible matter alone, so some solution to the
'missing-mass problem' needs to be found.
DID EXPLODING STARS HELP LIFE ON EARTH?
RAS
A Danish physicist named Svensmark has suggested that supernovae near
the Solar System might have had a strong influence on the development
of life. Supernovae produce a lot of high-energy cosmic rays, and
those from a supernova close enough to the Solar System could have a
direct impact on the atmosphere of the Earth. The researcher looked
back through 500 million years of geological and astronomical data and
tried to deduce how the rate at which supernovae exploded near the
Solar System might have varied over time, by considering the possible
proximity of the Sun to supernovae as it moved round the Galaxy and
passed through the spiral arms of the Milky Way,
Comparing that with the geological record, he found that whenever the
Sun visited regions of enhanced star-formation in the Milky Way, where
supernovae are most common, life had prospered; the evolution of life
seemed to mirror the evolution of the Galaxy. The diversity of life
over the last 500 million years seemed to be related to tectonics
affecting the sea level together with variations in the supernova
rate, and to virtually nothing else. The variety of life seemed to be
greatest when supernovae were plentiful. A suggested, if rather
vague, reason was that the cold climate associated with high supernova
rates brought a greater variety of habitats between polar and
equatorial regions.
Although the new analysis suggests, perhaps surprisingly, that
supernovae are on the whole good for life, high supernova rates could
bring the cold and changeable climate of prolonged glacial episodes,
and could produce nasty shocks. Scientists have been puzzled by
many relatively brief falls in sea-level by 25 metres or more that
show up in seismic soundings as eroded beaches. They might occur if
chilling caused by 'nearby' supernovae causes short-lived glacial
episodes; with water temporarily bottled up on land as ice, the
sea-level drops. The data also support the idea of a long-term link
between cosmic rays and climate, with climatic changes underlying the
biological effects.
Topic: Late April Astronomy Bulletin (Read 1502 times)