WAS VENUS ONCE A HABITABLE PLANET?
ESA
If Venus once had oceans, it might have been a habitable planet
similar to the Earth. The Earth and Venus seem completely different
today: here, we have a lush, clement world teeming with life, while
Venus is hellish, its surface roasting at high temperatures. But the
two planets are nearly identical in size, and now, thanks to the Venus
Express orbiter, planetary scientists are seeing other similarities.
One difference stands out: Venus has very little water. Were
the contents of the Earth's oceans to be spread evenly across the
world, they would create a layer 3 kilometres deep. If you could
condense the amount of water vapour in Venus' atmosphere onto its
surface, it would create a global puddle just 3 centimetres deep.
Yet, billions of years ago, Venus probably had more water. Venus
Express has found that the planet has lost a large quantity of water
into space. The loss occurs because ultraviolet radiation from the
Sun streams into Venus' atmosphere and breaks up the water molecules
into atoms, which then escape into space. Venus Express has measured
the rate of escape and found that roughly twice as much hydrogen as
oxygen is escaping. It is believed that water is the source of the
escaping ions. It has also shown that deuterium, a heavy isotope of
hydrogen, is progressively enriched in the upper layers of Venus'
atmosphere because the heavier isotope does not escape so easily.
Everything points to there having been large amounts of water on Venus
in the past, but that does not necessarily mean that there were oceans
on the planet's surface. A newly developed a computer model suggests
that the water was largely atmospheric and existed only during the
earliest times when the surface of the planet was completely molten.
Whether that is true or not is a key question. If Venus ever did
possess surface water, it may possibly have had an early habitable
phase. It is also possible that colliding comets brought additional
water to Venus after the surface crystallized, and that they created
bodies of standing water in which life might have been able to form.
ROSETTA TRIUMPHS AT ASTEROID LUTETIA
Science Daily
ESA's Rosetta mission has returned the first close-up images of the
asteroid Lutetia, showing that it is probably a primitive survivor
from the violent birth of the Solar System. The images show that
Lutetia is heavily cratered, having suffered many impacts during its
4.5 billion years of existence. As Rosetta drew close, a bowl-shaped
depression stretching across much of the asteroid rotated into view.
The images show that Lutetia is an elongated body, with its longest
dimension around 130 km. The pictures come from Rosetta's OSIRIS
instrument, which combines a wide-angle and a narrow-angle camera. At
closest approach, details down to a size of 60 m can be seen over the
entire surface of Lutetia. Rosetta raced past the asteroid at 15
km/s, completing the fly-by in just a minute, but the cameras and
other instruments had been working for hours and in some cases days
beforehand, and will continue afterwards. Ground telescopes have
shown Lutetia to present confusing characteristics. In some respects
it resembles a 'C-type' asteroid, a primitive body left over from the
formation of the Solar System. In others, it looks like an 'M-type',
which have been associated with iron meteorites, are usually reddish
and are thought to be fragments of the cores of much larger objects.
The fly-by marks the attainment of one of Rosetta's main scientific
objectives. The spacecraft will now continue to a 2014 rendezvous
with its primary target, Comet Churyumov-Gerasimenko. It will then
accompany the comet for months, from near the orbit of Jupiter
down to its closest approach to the Sun. In 2014 November
Rosetta will release its Philae lander to land on the comet nucleus.
PROCESSES OF MASSIVE-STAR FORMATION
Joint Astronomy Center, Hilo, Hawaii
Astronomers using the United Kingdom Infrared Telescope (UKIRT)
believe that they have identified the leading mechanism by which
massive stars form in our Galaxy: by collecting matter via discs
around their equatorial regions. That was revealed by the detection
of gas outflows and shocked regions associated with massive young
stars in formation, located in clouds of gas and dust. Scientists
know that lower-mass stars like our Sun form by gravitational collapse
of material inside clouds of gas and dust in space. The gas and dust
spiral down onto the equatorial regions of the young star by a
process known as accretion. At the same time, the accreting young
stars drive high-velocity jets of gas ('outflows') which radiate at
infrared wavelengths, so astronomers can use observations in the
infrared to search for not only the youngest stars, but also evidence
of the accretion process.
There have been misgivings over whether stars with masses larger than
10 times the mass of our Sun form in the same way, as it has been
supposed that the extreme energy output of such stars, which start
nuclear burning in their cores even before they complete their growth
through accretion, would prevent further growth by blowing away the
accretion discs. Hence, alternative mechanisms such as mergers of
lower-mass stars have been suggested as the main way in which massive
stars form. The presence or absence of outflows from massive young
stars should tell us whether accretion or some other method leads to
their formation. Most of the massive young stars being formed are
confined to the Galactic plane and are located in molecular clouds
extending over several or even tens of light-years. They are hidden
behind large amounts of gas and dust, which hamper their detection at
visible wavelengths but are penetrated by infrared light. The UKIRT
observers looked at 50 bright young stellar objects; 38 of them showed
molecular line emission, in most cases arising from outflows. Within
the sample, the outflows are seen to be well-defined irrespective of
the energy output of their central young stars and are nearly as
well-defined as those from low-mass stars. The outflows appear to be
driven by jets like those from low-mass stars. The astronomers have
concluded that massive stars up to at least 30 times the mass of the
Sun form through disc accretion.
ORIGIN OF MILKY WAY'S ANCIENT STARS
RAS
Scientists at Durham and collaborators at the Max Planck Institute and
Groningen University in Holland have tried to make computer
simulations of the beginnings of our Galaxy. The simulations have
suggested that ancient stars found in a stellar halo of debris
surrounding the Milky Way were ripped from smaller galaxies by the
gravitational forces generated by colliding galaxies. Cosmologists
think that the early Universe was full of small galaxies which led
short and violent lives, colliding with one another and leaving behind
debris which eventually settled into more familiar-looking galaxies
like the Milky Way.
UNRAVELLING THE NATURE OF HANNY'S VOORWERP
ASTRON, Dwingeloo, Netherlands
A group of researchers has made high-resolution radio observations
of the region of space around Hanny's Voorwerp (Hanny's Object), the
curious, greenish gas cloud discovered by Dutch schoolteacher Hanny
van Arkel. The astronomers undertook an observational campaign at
radio wavelengths in which several radio telescopes across Europe and
the United Kingdom were linked together in real time in order to
obtain a detailed picture of the central region of the adjacent galaxy
IC 2497. They observed a field a few arcseconds across, with a
spatial resolution of about 70 milliseconds. The observations show
two bright and very compact sources with broadband spectra that argue
for the existence of an active galactic nucleus (AGN) at the centre
of IC 2497. One of the sources appears to be identifiable with a
supposed black hole at the centre of the AGN itself, while the other
is likely to be the result of an energetic jet expelled by the black
hole and now interacting with dense gas that surrounds IC 2497. The
radiation from the AGN is believed to heat Hanny's Voorwerp to a
temperature above 10,000°.
It also appears that surrounding the AGN is a lot of extended radio
emission. The researchers argue that it is associated with a nuclear
starburst. Astronomers knew that IC 2497 is forming stars, but were
surprised to find that the star formation seems to be concentrated in
a very small central region, only 1000 parsecs across. It is fairly
unusual to find both vigorous star formation and AGN radio activity in
the same system and on similar scales. The radio observations
indicate that in the central region IC 2497 is producing stars with a
total mass of the order of 70 Suns every year -- a high rate, about 6
times higher than in the nearby starburst galaxy M82.
The observations support the group's earlier hypothesis that a hidden
AGN in the centre of IC 2497 is ionizing a distinct region of gas that
surrounds that galaxy. That distinct region is what is known as
Hanny's Voorwerp. Such phenomena must be rare in the local Universe
because they depend on a specific geometry of the observer, galaxy,
and gas, plus the interaction of several galaxies in the field in
order to fuel the AGN and the starburst, and to create the gas
reservoir that forms part of the Voorwerp.
VOYAGER SPACECRAFT AT 12,000 DAYS
Science Daily
On June 28, the Voyager 2 spacecraft had been operating continuously
for 12,000 days. For nearly 33 years, the venerable spacecraft has
been returning data about the outer planets, and the characteristics
and interaction of the solar wind between and beyond the planets.
Among its findings, Voyager 2 discovered Neptune's Great Dark Spot and
its 450-m/s winds. The two Voyager spacecraft have been the longest
continuously operating spacecraft in deep space. Voyager 2 was
launched on 1977 August 20; Voyager 1 was a little later, on
1977 September 5, so it reached its 12,0000 days on July 14. The two
spacecraft are the most distant man-made objects, out at the edge of
the heliosphere -- the bubble that the Sun creates around the Solar
System. Having travelled more than 21 billion kilometres on its
winding path among the planets towards interstellar space, Voyager 2
is now nearly 14 billion kilometres away; a signal from the ground,
travelling at the speed of light, takes about 12.8 hours one-way to
reach it. Voyager 1 is even further away, more than 17 billion
kilometres.
Topic: Mid July Astronomy Bulletin (Read 603 times)