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Physics News Update -- Number 658, October 21, 2003



 
 
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Old October 22nd 03, 09:35 PM
Rich
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Default Physics News Update -- Number 658, October 21, 2003


[ from sci.physics ]

Physics News Update -- Number 658, October 21, 2003
by Phillip F. Schewe, Ben Stein, and James Riordon


Direct Imaging of Extrasolar Planets

Direct imaging of extrasolar planets might be easier than astronomers
thought, a new study shows. Evidence for the existence of planets
around nearby stars comes mostly in the form of tiny Doppler shifts in
the star's spectra as one or more orbiting planets tug on the star. In
a few cases the transit of a planet across the face of a star can be
detected from a minute dimming of the star's emission. These approaches
are indirect. The problem of imaging extrasolar planets directly is
that the planet is far outshone by the nearby star. One proposed way of
getting around this glare problem is to use nulling interferometry. In
ordinary interferometry the light waves from two or more telescopes are
added together in such a way that the resulting observation is
equivalent to one made with a single telescope with a much wider
diameter than any of the component scopes. But instead of maximizing
the composite signal from the distant object, it can be minimized (see
past Update item). By doing this, a weaker nearby object, like a
planet, might suddenly emerge from what had been irrepressible glare.

In a new paper, William Danchi (Goddard Space Flight Center) and his
colleagues have performed extensive studies of the interferometry
nulling technique, especially the way in which increasing the precision
of component detectors increases the degree to which the star's image
is truly nulled, the better to see either smaller planets or planets
that are closer in toward their parent star. Both the smaller and
closer criteria are pertinent when searching for earth-like extrasolar
planets. Danchi (301-286-4586) says that the new study shows that with
the right configuration of detectors, the spatial resolution of the
overall interferometer (which is related to its size) can be less than
have been thought, an important consideration for what would be an
orbiting space-based observatory. Danchi envisions that a first-round
nulling interferometer using two half-meter-sized telescopes separated
by a 12-meter boom could observe already discovered extrasolar planets
(including spectroscopic studies of atmospheres). With a later, larger
version of the nulling interferometer one could hope to search for
earthlike planets harboring characteristic molecules such as ozone,
and/or oxygen, plus carbon dioxide, water, and methane. Detecting these
molecules could help determine the age of the planet and what life
processes might be occurring there. (Danchi, Deming, Kuchner, and
Seager, Astrophysical Journal Letters, 1 November 2003; preprint
astro-ph/0309361)


Evidence for an Unusually Active Sune

Evidence for an unusally active sun since the 1940s comes from a new
estimation of sunspots back to the ninth century. Many natural
phenomena such as solar radiance and sunspots vary according to natural
cycles. The variation is subject also to additional fluctuations
(arising from as yet unexplained effects) which complicate any study
which examines only a short time interval. The longer the baseline, the
more confident one can be in drawing out historical conclusions. In the
case of sunspots, the direct counting goes back to Galileo's time,
around 1610. But earlier sunspot activity can be deduced from
beryllium-10 traces in Greenland and Antarctic ice cores. The reasoning
is as follows: more sunspots imply a more magnetically active sun which
then more effectively repels the galactic cosmic rays, thus reducing
their production of Be-10 atoms in the Earth's atmosphere. Be-10 atoms
precipitate on Earth and can be traced in polar ice even after
centuries. Using this approach, scientists at the University of Oulu in
Finland (Ilya Usoskin, 358-8-553-1377) and the Max Planck Institute in
Katlenburg-Lindau in Germany have reconstructed the sunspot count back
to the year 850, nearly tripling the baseline for sunspot studies. They
conclude that over the whole 1150 year record available, the sun has
been most magnetically active (greatest number of sunspots) over the
recent 60 years. (Usoskin et al., Physical Review Letters, upcoming
article)


Can a Single Gas Bubble Sink a Ship?

Yes, according to an experimental and theoretical analysis performed by
researchers at Monash University in Australia (David May and Joseph
Monaghan). The ocean floor contains vast quantities of methane gas
hydrates, ice-like crystals of methane surrounded by cages of water
molecules. If disturbed, these methane gas hydrates can erupt from the
floor and rise to the surface as gas bubbles, some of which can be very
large. Copious amounts of methane hydrates exist in the North Sea,
which lies in between the United Kingdom and continental Europe. At a
large eruption site in the North Sea known as the Witches Hole off the
coast of Aberdeen, a sonar survey recently uncovered the presence of a
sunken vessel, but the cause of the wreck remains undetermined. Simple
experiments have previously shown that many small bubbles rising to the
surface could sink a cylinder of water (and conceivably a ship), by
causing a loss of buoyancy (Denardo et al., American Journal of
Physics, October 2001). But could a single large gas bubble do the
trick? The Monash researchers investigated this possibility in a
simple, roughly two-dimensional system. Trapping water between a pair
of vertical glass plates, and launching single gas bubbles from the
bottom, they used a video camera to observe a single large bubble's
effect on a small piece of acrylic shaped like the hull of a boat.
Along with numerical simulations of this scenario, the experiments
showed that the bubble could sink the ship, if the bubble's radius was
comparable to or greater than the ship's hull. Sinking would occur
because a mound of water formed above the bubble as it approached the
surface. As the bubble reached the surface, it would temporarily lift
the ship. However, water in the mound would then flow off the sides of
the bubble, forming deep troughs at either side, and the water flow
would carry the boat to one of the troughs. In addition, the eventual
rupture of the bubble would create high-velocity jets of fluid that
moved into the troughs, creating vortices that further pulled down the
boat. The researchers say that their numerical simulations could test
other scenarios, including those involving multiple large bubbles, more
realistic boats, and ultimately a full three-dimensional simulation.
(American Journal of Physics, September 2003).

Physics News Update is a digest of physics news items arising from
physics meetings, physics journals, newspapers and magazines, and other
news sources. Subscriptions are free as a way of broadly disseminating
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Physics News Update appears approximately once a week. Questions?
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