Download Rocks point to existence of seas on the red planet Ultrasound on the

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Rocks point to existence
of seas on the red planet
New images from the Mars Global Surveyor
suggest that the red planet may have been
covered with shallow lakes over 3.5 billion
years ago. The geological evidence comes
from detailed photographs that show numerous layered outcrops, similar to sedimentary rocks found on Earth. Michael Malin
and Kenneth Edgett of Malin Space Science
Systems in San Diego believe that the layered
sediments were probably formed as water
carrying silt and sand flowed into the Martian craters {Science 2000 290 1927).
One of the most important questions in
space science is whether Mars was once
warm enough for water to exist as a liquid —
and perhaps for extraterrestrial life to exist.
On Earth, rocks formed from layers of compressed sediment provide a record of climate
change and contain the fossilized remains
of plants and animals. If life on Mars ever
existed, there is a chance that it could be preserved in these outcrops.
Malin and Edgett found that the layered
rocks are concentrated in low-lying areas
close to the Martian equator and appear to
have be formed from fine-grained material
nearly 4 billion years ago. Many of the newly
discovered outcrops are in ravines or basins
in the planet's crust. Indeed, several craters
up to 200 km in diameter have been completely filled withfinematerial.
The team has failed to establish the source
of the sediment or tofindevidence for rivers
that may have once carried silt and sand into
the craters. However, the geological clues
that would support this explanation may
have been eroded over time.
Dust storms, volcanic ash or thefinedebris thrown up by a meteor impact could also
have created the sedimentary rocks. However, Malin and Edgett believe that these
processes fail to explain many of the largescale features or why the sediment appears
to favour low-lying regions.
The team has still not ruled out an alternative theory. It is possible that dramatic changes in atmospheric pressure - in a period
when the Martian atmosphere was much
denser than it is now - mean that dust could
have been carried by the wind. Although the
origin of the sedimentary rocks remains uncertain, Malin and Edgett's results provide
evidence that early Mars was very different
to what was previously thought.
Ultrasound on the brain
Neurosurgeons of the future could use ultrasound to treat tumours rather than operate,
according to scientists in the US. Kullervo
Hynynen and co-workers at the Harvard
Medical School in the US have developed
a method of destroying brain tissue using
high-power sound waves that could lead to
non-invasive treatment (G T Clement et aL
rounds the top of the head, thus distributing
the energy over a large surface area. The
phase of each transducer is adjusted so that
all the acoustic waves arrive at the centre of
the array in step, thereby maximizing the
intensity of the ultrasound. After they applied this phase correction, Hynynen and
co-workers tested the device on human
2000 Phys. Med. BioL 45 3707).
skulls covered with a special gel that mimics
The idea of therapeutic ultrasound is not skin. Hydrophone measurements confirmed
new. In trials, prostate and breast tumours that the ultrasound signal remained tightly
have previously been treated with a focused focused at the optimal frequency.
Blasts of ultrasound lasting several sebeam of ultrasound that heats the surrounding tissue and kills the cancerous cells (see conds were then applied to a skull that conPhysics World August 1996 pp28-33). How-tained a "brain" simulated by a piece of
ever, the major obstacle for ultrasonic brain muscle tissue. When Hynynen and cosurgery is wave propagation through the workers inspected the tissue, they found
skull. Bone absorbs a large fraction of the areas of damaged cells several millimetres
wave energy, thus reducing the power of the in diameter at the focal point of the sound
ultrasound and heating the skull to such an waves. Moreover, a series of thermocouples
extent that the scalp burns. Moreover, the attached to the skull confirmed that the
varying thickness of the skull destroys the temperature rose by about 15 K, which is
considered safe.
focused beam.
The Harvard team has overcome these
While neurosurgeons will have to wait
problems by designing a hemispherical ar- several years before they can throw away
ray with 64 piezoelectric transducers that their scalpels, the Harvard study demonvibrate at ultrasonic frequencies when an strates that non-invasive ultrasound brain
electric current is applied. The array sur- surgery is possible and practical.
P S Y l l C f WOILD
J»H«*«»
2001
All in a flap
Flapping sails and fluttering flags have
intrigued physicists since die time of Lord
Rayleigh. Yet die complicated interaction
between a flexible object like a flag and its
surrounding fluid is not fully understood.
Now an experiment by Jun Zhang and coworkers at New York University and Rockefeller University in die US has helped
explain howflagsflapin die breeze (J Zhang
etaL 2000 Nature 408 835).
By suspending a fine silk diread in die
plane of a fast-flowing soap film, die team
has studied die simplified case of a onedimensional "flag" in a two-dimensional
"wind". Zhang and co-workers found that
die filaments - which are several centimetres
long- have two distinct stable states depending on dieir length. Below a certain direshold
die filament straightens and lines up widi
die fluid flow. Even when die filament is
disturbed in an attempt to make it flap, it
quickly returns to die stable stretched state.
The observations refute die common belief diat aflagalwaysflapsin die wind. Above
die critical lengdi, however, die direadjumps
to a stable flapping state and sheds vortices
diat alternate in sign (see figure). For very
long direads, only die stable flapping state
exists - but diis behaviour remains mysterious. Zhang and co-workers believe that tiiis
effect arises from die tension, mass and elasticity of diefilament- features diat are usually overlooked influiddynamics.