Index:
PHYSICS
A Slippery Subject
Why is ice slippery? This seemed to be one question that
scientists had satisfactorily answered years ago: the
pressure of, say, a skate blade or the sole of a shoe melts
the very top layer of ice, making a wet surface for the
blade or shoe to glide over. When the pressure is gone, the
liquid surface freezes again. This explanation made so much
sense that until recently no one took the trouble to test
it carefully. Now some researchers finally have, and
they've found that the old explanation is not quite right.
Chemist Gabor Somorjai and physicist Michel Van Hove, both
at the University of California at Berkeley, believe they
now understand the slipperiness of ice on an atomic scale.
They bombarded a thin layer of ice with a stream of
electrons. Detectors recorded the angles and velocities of
the electrons reflecting off the ice. To the researchers'
surprise, the electrons didn't follow the simple
trajectories expected of an object bouncing off a solid
surface. Instead they seemed to be hitting a constantly
changing surface that scattered them.
The only way to explain the result, says Somorjai, is to
assume that the uppermost layers of the ice aren't really
frozen at all but consist of a thin film of water that
serves as a permanent lubricant. "When something pushes
down on the ice, like a shoe," Somorjai explains, "the
water molecules compact into underlying interstices and
create a smooth, flush surface, which means you can slide
over it more easily."
The researchers also found that ice drastically decreases
in slipperiness as temperatures fall below about -22
degrees. At that temperature, they believe, the number of
molecular layers that behave like a liquid diminishes,
reducing the slipperiness, so that the ice resembles any
other solid.
Van Hove suspects that ice is much more slippery than other
solids because the chemical bonds that hold water molecules
are much weaker than the bonds joining other solids. An icy
surface behaves as if it were boiling, says Van Hove, and
the weak bonds between neighboring molecules make it
unstable. "There's always something coming off and coming
back on, in the sense that evaporation and condensation are
occurring all the time."
THE BRAIN
The Color of Stress
During the Gulf War, one of the biggest fears of allied
troops was that Iraq would attack with chemical and
biological weapons. To protect themselves, Israeli soldiers
took a drug called pyridostigmine. Luckily the troops never
had to find out how well the drug might have worked; the
feared chemical attacks never materialized. But the drug
itself took a toll on the soldiers. Many complained of
headaches, nausea, and dizziness. The complaints surprised
doctors because those symptoms, which occur only if the
drug reaches the brain, had been reported only rarely when
the drug was tested before the war. Why had the side
effects increased during combat? Biochemist Hermona Soreq
of Hebrew University and Alon Friedman, a physician at
Soroka Hospital in Beersheva, have now found that stress
dramatically increases the ability of chemicals to pass
from the blood into the brain.
Pyridostigmine molecules generally can't get into the
brain. They're blocked by the same fatty sheath surrounding
the brain's blood vessels that also keeps infectious agents
from passing through the bloodstream. Yet pass into the
brain they did, in nearly one-quarter of the soldiers who
took the drug during the war.
Pyridostigmine was designed to protect against chemical
weapons by reacting with an enzyme--acetyl
cholinesterase--that is found in many cells and is crucial
to everything from breathing to memory to digestion. Many
chemical weapons react with the same enzyme. But unlike the
weapons, which kill the enzyme, pyridostigmine is slowly
broken down by the enzyme. The drug thus keeps the enzyme
temporarily occupied, preventing it from being permanently
taken out by the poisons in chemical weapons.
Soreq and Friedman wondered whether the stress of war might
somehow have increased the permeability of the blood-brain
barrier. To find out, they took a group of mice and
stressed some by dunking them in water. They then injected
into the rodents' hearts a chemical that turns blue when it
binds to albumin, a common protein in blood. From the
intensity of the blue visible in autopsied brains, the
researchers could tell how much of the dye had penetrated
the blood-brain barrier. They found that the dye had passed
much more readily into the brains of the stressed animals.
The results are medically promising but also portentous,
notes Soreq. "Now we know that there are certain conditions
under which we can get drugs to reach the brain," she says.
"But there are also drugs that people have developed under
the assumption that they will stay in the periphery. As of
now, you have to test those drugs also under stress
conditions."
The Cyclops Gene
Our limbs and other symmetrical body parts develop from
separate fetal structures. But what about our eyes? The
question is prompted in part by birth defects that produce
a cyclopean eye, suggesting that a single structure gives
rise to two eyes. Molecular neurobiologist Yi Rao now
argues that, indeed, our two eyes begin as one. Rao, who
works at Washington University School of Medicine, tracked
eye development in embryonic frogs of the genus Xenopus. In
particular, he isolated a gene that may control eye
development. He found that the two dark spots in the
21-hour-old embryo (second from top), which later form the
eyes (third from top), split from an earlier single band
(top). When he removed a part of the tadpoles' brains that
signals the shutdown of the gene in cells in the middle of
the single eye band, allowing two separate eyes to form, he
found that one-eyed tadpoles (bottom) were the frightful
result. "Although we did most of the studies in frogs, we
also followed with chick embryos and found cyclopean eyes,
which is important because amphibians and birds are very
different," says Rao. "This suggests that it is general to
all vertebrate species."
Images
Courtesy Yi Rao
SPACE
Ganymede's Missing Oxygen
When the Galileo spacecraft flew by Jupiter's moon Ganymede
last June, it detected a dense stream of atomic hydrogen
escaping from the moon's sparse atmosphere. That in itself
is not so unusual: atomic hydrogen pours off the top of the
atmospheres of Earth, Mars, and Venus too. In all those
cases, however, the hydrogen comes from water vapor in the
atmosphere. Energy from the sun breaks up the water vapor
molecules into hydrogen and oxygen atoms. While some of the
hydrogen escapes, the oxygen accumulates in the atmosphere.
But Ganymede doesn't have much of an atmosphere, far less
water vapor. So where, researchers have wondered, is all
that hydrogen coming from?
Charles Barth, a planetary scientist at the University of
Colorado in Boulder, believes he has identified the source
of Ganymede's escaping hydrogen: it comes from ice that
covers its surface. Yet the oxygen left behind doesn't form
an atmosphere. "We think that it's building up and being
incorporated into the icy surface," says Barth. "So there
is ice with extra oxygen--hydrogen peroxide. I think there
is a good possibility that all of the ice on the surface of
Ganymede may be covered with extra oxygen."
Barth and his colleagues have calculated that solar energy
breaks apart a nanometer (.00000004 inch) of ice from
Ganymede's surface each year. "That is both the amount of
water that disappears and, under our theory, the amount of
oxygen that is left behind." In a thousand years, then, a
layer of ice .00004 inch deep would disappear; in a billion
years, the moon would lose over three feet of ice--and gain
more than three feet of extra oxygen. "That's a substantial
amount of oxygen," Barth says. Observations from the Hubble
Space Telescope, which has spotted oxygen in Ganymede's
ice, support Barth's theory.
The oxygen, says Barth, would not necessarily stay on the
surface. It might, over time, have mixed down through the
ice to react with the rock below. "This is what happens on
Mars: there is extra oxygen that gets left behind, and it
has reacted with the rocks to turn the dull colors into red
iron oxides that give Mars its distinctive hue," says
Barth. That is not to say that Ganymede is also red under
its ice. "It depends on whether there is iron in the rock."
Sonorasaurus
On Thanksgiving weekend in 1995, Richard Thompson, a geology
student looking for petrified wood in the Sonoran Desert
about 40 miles southeast of Tucson, came upon bone
fragments exposed along a sandstone ridge. His chance find
turned out to be the first glimpse of a 51-foot-long,
35-ton dinosaur--probably a brachiosaur--that lived about
100 million years ago. The Sonoran fossil is the only
dinosaur skeleton ever found in southern Arizona. More
surprisingly, brachiosaurs--long-necked, long-tailed
herbivores--were thought to have died out some 125 million
years ago. Ronald Ratkevich, a paleontologist at the
Arizona Sonora Desert Museum, says that about half the
skeleton has been unearthed, including a complete hind
foot, numerous vertebrae, a humerus, the pelvis, and the
entire skull. Although the skull had been crushed, it
features notable brachiosaur traits, such as a nasal
opening on top of the head, as well as sockets for wide,
spoonlike teeth. On the off chance that the dinosaur turns
out to be a new genus of brachiosaurid, the museum plans to
name it Sonorasaurus thompsoni. Since the dinosaur was
found in a part of the Sonoran Desert called the Chihuahua
Desert, Ratkevich at first wanted to name the dinosaur
Chihuahuasaurus but felt it wasn't appropriate for a
51-foot-long animal.
TECHNOLOGY
Beacons for the Blind
Before a genetic disease damaged Michael Hancock's vision,
he was an avid pilot, an avocation that inspired his
current research. Hancock, a neuroscientist at the
University of Texas Medical Branch at Galveston, has
developed a system of infrared beacons that will help blind
people find their way inside unfamiliar buildings. He calls
his invention FIND, for friendly infrared navigation
device, and modeled it after aircraft navigation
technology.
The beacons in Hancock's system are placed over office
doors, rest rooms, water fountains, and elevators, or at
relay points like hallway intersections. Each beacon sends
a set of numbers, coded in infrared signals, down both
directions of a hallway. The beacons flash one after the
other, rather than simultaneously, to prevent interference
among the signals. The numbers indicate the room, floor,
and hallway where the beacons are located. To get to a room
marked by a beacon, you use a small receiver, about the
size of a television remote-control unit, which picks up
the infrared signals. Using a braille list, you punch in a
room number and then point the receiver around until it
finds the infrared signal of the closest beacon.
The receiver is programmed to know which beacons are
between you and the target beacon. If the room you want is
on a different floor, the receiver will automatically send
you to an elevator. But if the first beacon the receiver
picks up is on the way to the destination, the receiver
beeps (or if a person is also deaf, it vibrates). If that
beacon is not on the way, you sweep the receiver around
until it finds one that is. "As long as you're hearing
those beeps, you're on course," says Hancock.
Once you are in the right hallway and within 100 feet of
your destination, the receiver will pick up the target
beacon and lock out all other signals, giving you double
beeps to tell you that you've found the right beacon. When
you are within three feet of the room, a weaker beacon,
placed lower on the wall, takes over and guides you with
triple beeps to the doorknob.
Hancock is working on improving the system. He has just
installed a voice chip in the receiver to replace some of
the beeps with spoken directions. To date he has
successfully tested FIND at the Texas School for the Blind
and Visually Impaired in Austin. "For blind children that
go to school, this system would give them some
independence," says Hancock. "They wouldn't have to have a
buddy take them places."
An Indecent Beetle
"The ground beetle fauna in the Northeast is pretty well
studied by people who are well-known entomologists--as far
as entomologists go," says Kip Will, a graduate student at
Cornell. That's why Will and his adviser, entomologist
James Liebherr, were surprised to find a new beetle species
in Cornell's collection, one that had escaped the scrutiny
of bug experts for 85 years. As part of a study, Liebherr
and Will were sifting through thousands of beetles in
Cornell's collection when they came across an odd pair
taken from nearby McLean Bog. Scouting out other museum
collections and boggy areas, Will found that the new beetle
ranged from Maine to Maryland, and from Ontario to Ohio. It
had been misidentified as a common woodland beetle,
Platynus decentis, a species with a lot of natural
variation. Unlike the woodland beetle, the new beetle has a
series of hairs on its lower legs and wings long enough to
fly--a big help in getting from bog to bog, its exclusive
habitat. Liebherr and Will have dubbed their find Platynus
indecentis: the fact that it could happily be going about
its beetle business right under the noses of prominent
entomologists, says Will, "seemed positively indecent."
SPACE
Asteroids on the Loose
Astronomers know surprisingly little about the Trojan
asteroids, which travel along with Jupiter in two separate
clumps, one preceding the planet in its orbit and the other
following. Only recently, thanks to years of painstaking
research, have astronomers realized that the Trojan
asteroids are probably as numerous as those in the main
asteroid belt between Mars and Jupiter. "The reason they
aren't recognized as an important part of the asteroid
population is that they are so far away. So we see only the
bigger ones," says astronomer Hal Levison of the Southwest
Research Institute in Boulder, Colorado.
One of the things astronomers would like to know about the
Trojans is the stability of their orbits. Do any drift from
the swarm and travel to the inner solar system--and perhaps
intersect Earth's orbit? For most of the Trojans, Levison
says, this is not really a consideration.
Each of the two swarms clusters about a Lagrange
point--regions of gravitational stability where the
centrifugal force of the asteroids' orbits counterbalances
the gravitational pull of Jupiter and the sun. But some
Trojans travel in orbits quite a distance from these stable
points, and Levison wondered if over time these asteroids
could begin to wander. He selected 36 of these atypical
Trojans and projected their orbits over the next 4 billion
years. Twenty-one, he found, wound up leaving the swarm.
His computer model showed that once the asteroids move far
enough away from the Lagrange points, the subtle
gravitational influence of the outer planets eventually
provides enough pull to send the Trojans into completely
different orbits. Some 1,200 Trojans have probably left
their parent swarms in the last 100,000 years and are at
large in the solar system, says Levison. "When you do a
back-of-the-envelope calculation of how often one of these
guys should hit Earth, you get something like once every
500 million years," Levison says, adding that this hazard
is minor compared with the threat from comets and other
asteroids.
Do astronomers know of any objects that might be wandering
Trojans? Not yet, says Levison, and it would be hard to
distinguish them from comets, icy objects that the Trojans
probably resemble and that far outnumber everything else in
the solar system. "There are something like 10 million
comets that cross the paths of planets in the solar
system," Levison says, "and a million of them are inside
the orbit of Neptune."
The Surfball
Kentaro Toyama is very proud of this device--a racquetball
suspended in a wooden frame by rubber bands--a result of
his computer science research at Yale. Toyama, a graduate
student, calls the object of his seemingly unjustified
pride a surfball. What does it do? The surfball is
essentially a computer mouse that could control the motion
of, say, a robotic arm moving in three dimensions. A video
camera tracks the ball by following two colored dots on it
and feeds that information into a computer. The computer
calculates the exact motion of the ball and scales up the
movements for a robotic arm. Compared with a mouse, or even
a joystick, the surfball can move the objects it controls
through a greater range of motions. Says Toyama, "You can
move it forward and back, shift it left to right, up and
down, and then rotate around all of those axes."
Image
Courtesy Kentaro Toyama
BIOLOGY
Fake Cells
What separates living things from the inanimate world? On
the most fundamental level it is a barrier of fat and
protein that encloses a living cell. Until now, the
simplest model of a cell that biologists have been able to
create is a vesicle--a tiny balloon made up of a single
layer of fat molecules, or lipids, curled into a sphere.
"People like these because if you put something inside you
can separate it from everything on the outside," says
Joseph Zasadzinski, a chemical engineer at the University
of California at Santa Barbara. "So you can use them as a
drug delivery system and encapsulate a drug that might be
toxic, then have it released only very slowly into the
circulation or only at a particular site."
Vesicles are useful, says Zasadzinski, but a better drug
carrier would more closely mimic real cells. That is, it
would have two membranes instead of one. And it might
contain several vesicles within one larger package, to
allow the delivery of more than one drug at a time. Drugs
carried in vesicles within vesicles would have to diffuse
through both layers and would, theoretically, be released
more slowly, lengthening the time between treatments. "We
figured that if one membrane is good, two might be better,"
he says. Zasadzinski and his group recently succeeded in
making such a double-membraned vesicle.
It wasn't easy. "We had to figure out a nice gentle way to
wrap something around something else." To create his fake
cells, Zasadzinski first made conventional vesicles, each
with tiny molecules called biotins embedded in its lipid
membrane. To these vesicles he added a bacterial protein
called streptavidin. Each streptavidin protein binds to
biotin molecules on different vesicles, linking them
together.
He then mixed clusters of these linked vesicles with
another fatty brew made out of a molecule called
phosphatidylserine. The brew contained biotin and
streptavidin to make it stick to the vesicles. Zasadzinski
also added calcium to the mix, which made the fatty
molecules wrap around the clusters of vesicles, forming
enclosed, cell-like structures.
Zasadzinski hopes that his vesicle packages will provide a
way to put many different drugs into one-cell containers.
Before his artificial cells can be used to deliver drugs,
though, Zasadzinski will have to find a replacement for the
bacterial protein used to create his vesicles--it
invariably triggers an immune response in the body. "There
is a company that actually makes artificial biotin and
streptavidin, but they haven't given us any to try yet,"
Zasadzinski says. "We are just getting into the realm where
this might actually make some money, so everyone is
reluctant to share anything anymore."
