Spring 1997

Above: Europa's icy surface is covered by dark and light streaks, ridges, and lobate flows Photo courtesy of NASA/JPL.

Above: Scheduled passes of the Galileo orbiter for 1996 and 1997. Graphic courtesy of NASA/JPL.

Galileo Raises Questions about Life on Europa
   NASA recently released up-close images of Europa from the Galileo spacecraft, one of Jupiter's largest moons. Giving scientists the first detailed look at features of the moon's icy surface, the new images contain substantial evidence supporting the hypothesis that life could have developed on Europa and may still exist there.
   Scientists agree that a planetary body must have water, organic compounds, and sufficient heat in order for life to arise. "Europa obviously has substantial water ice, and organic compounds are known to be prevalent in the solar system," said Ronald Greeley of Arizona State University, a member of the Galileo imaging team. "The big question mark has been how much heat is generated in the interior. These new images demonstrate that there was enough heat to drive the flows on the surface."
   Slightly smaller than Earth's moon, Europa is the smoothest object in the solar system: the moon has no lumps or bumps exceeding one kilometer. One of the spacecraft's findings—dark and light river-like streaks coursing across the surface—suggests geologic activity which could generate heat within the moon.
   Ice on Europa's surface is fragmented into plates resembling those comprising Earth's crust. Planetary scientists speculate that the cracked surface ice may float on a mantle of liquid water or slushy ice. The dark material Galileo found may result from plates shifting to force water and silicates up between plate boundaries. The material might also emerge from volcanoes or geysers along ice plate borders which spew clean water, ice, and dark debris in riverlike flows. Although current observations have yet to yield hard evidence, Greeley said that the dark material suggests geologic activity.

—Craig Dunton and Janina Wilen

Researchers Study Molecular Ice Dance
   As simple as an ice cube may seem, scientists are still contemplating the reasons why ice is slippery. Researchers at Lawrence Berkeley National Laboratory have recently put forth a theory about ice's slick property that just might snow in all others.
   While many have attributed ice's slipperiness to "pressure-induced melting," suggesting that heat created from force melts the ice and causes sliding, Berkeley's Michael Van Hove contends that the pressure explanation just "doesn't work out." Van Hove says there is no physical evidence for such a process, which, he argues, does not even make sense mathematically.
   Instead, Berkeley scientists believe the answer to the elusive question may have to do with the molecular structure of an icy surface. In a paper soon to be published in Surface Science, the researchers suggest that molecules comprising ice's outermost layer vibrate three to four times faster than the molecules in the solid lower layers. When surface molecules vibrate this rapidly, they collide very frequently and generate enough kinetic energy to exist in a liquid state. Mimicking the slick conditions of water on concrete around a swimming pool, the partially liquid layer of water on solid ice creates a slippery surface.
   Gabor Somorjai, Van Hove's colleague at Berkeley Lab, notes that the "dynamic surface" of ice might explain the curious rates of chemical change responsible for ice crystals in the upper atmosphere and ozone destruction.
   Steven George of the University of Colorado speculates that the partial liquid layer exists like topsoil on the earth's crust: it is constantly being eroded and redeposited elsewhere. "These findings illustrate how we don't understand some of the simplest things we know about," said George.

—Rob Peterson

Evolutionary "Law" Fails Test
   In a recent study, David Jablonski of the University of Chicago suggested that a theory of evolution purporting that all organisms tend to become bigger as they evolve is not necessarily true. The theory in dispute, known as Cope's rule for its developer 19th century American biologist Edward Drinker Cope, has long been accepted by evolutionary biologists as law and still appears as such in biology textbooks.
   Jablonski's findings argue that an organism is equally likely to become smaller or larger as it evolves. Studying the last 16 million years of the Cretaceous period, which ended 65 million years ago, he found an approximately equal tendency of various mollusks to increase and decrease in size: 27 to 30 percent of the species he examined showed an increase in body size over the 16 million year span while 26 to 27 percent of the samples showed a net decrease in size. In the 1/16/97 issue of Nature, Jablonski stated that "directional net increase in body size (including the loss of small-sized species and thus representing Cope's rule in the strict sense) is no more frequent than an increase in size range among species or a net evolutionary size decrease."
   Although the evolution of the horse is often cited as a traditional example of the validity of Cope's rule, other recent research has indicated that the horse, too, showed diversity in size: small and large species of horses coexisted until the most recent jump in its evolution resulted in the horse we know today.
   Several paleontologists agree that proving Cope's rule false is a major accomplishment. Also in the recent Nature issue, Stephen Jay Gould attributed the rule to a general attitude in humans that "bigger is better." It is significant to note that Cope devised his theory in 1871, around the discoveries of fossils of the huge plant-eating dinosaurs such as Diplodocus and Brontosaurus.

—Sanjay Satagopan

Scientists Ask: Does Weight Trigger Puberty?
   University of California researchers have revealed the possibility that a chemical linked to fat control in humans might trigger puberty in adolescents.
   Although scientists have accurately described the physiological changes that accompany puberty, they, along with the rest of the population, have long been curious as to why teenagers must change into adults. Figuring out what it is that triggers the hormonal changes that characterize puberty is a current scientific puzzle. Thanks to Farid F. Chehab and his team of researchers at the University of California, the science world is much closer to cracking the case.
   The new study, published in Science (1/3/97), demonstrates biochemical evidence for a long-standing proposal that the onset of puberty, especially in girls, is linked to body fat. For a long time, researchers have been trying to find some evidence supporting what is known as the "critical fat hypothesis." According to this notion, when a girl reaches a critical weight her brain recognizes that she is capable of sustaining a pregnancy. Only then can the body release the hormones that cause the onset of puberty. Chehab's team's study produced results suggesting that the brain determines the critical weight by monitoring levels of leptin in the blood.
   In their experiments, Chehab and his colleagues injected normal young mice with an excess of leptin. Because the brain was misled into thinking that the bodies of the rodents had more fat than they really did, the mice matured early. Specifically, their ovaries and uteri swelled, their reproductive tracts opened, and they began copulating at younger ages. Additionally, the mice injected with leptin grew very lean because their brains responded to the fake "fat signal" and prompted them to eat less than usual. Chehab explained in The New York Times (1/7/97) that the researchers "tricked the brain into believing that the body was fatter than it was."
   So, what does this research have to do with humans? Assuming that the rodent results hold up in human studies, they could offer an explanation as to why chubby girls often go through puberty earlier than their leaner counterparts. Conversely, the results could explain why very thin or athletic young girls tend to have a delayed menarche. Absence of periods and delayed onset of puberty in female athletes has received a lot of attention in the media since this past summer's Olympic games. Perhaps this new research will be able to unravel these mysteries.

—Becky Orfinger

MIT, Columbia Professors KO Biology Obstacle
   Scientists from MIT and Columbia University have made progress in the development of "knock out" organisms, creatures genetically engineered to lack a specific gene.
   Until recently, technology confined scientists to studying the effects of missing genes in organisms by removing genes extremely early in organismal development. Eliminating a gene during early embryonic development proves problematic because it exaggerates the gene's role in the body. This sometimes prevents scientists from attributing abnormal characteristics to a particular group of cells.
   Columbia's Eric R. Kandel and MIT's Susumu Tonegawa have taken steps to eradicate these drawbacks by establishing an experimental procedure to eliminate gene function later in organismal development. Whereas previous procedures to remove genes tended to affect all cells in the body, the new technique allows for the knockout of a particular gene in the brain. In particular, the researchers concentrated their efforts on the pyramidal cells of the brain's hippocampus.
   The experimenters made use of a gene recombination system, whereby a specific enzyme, Cre recombinase, promoted recombination between DNA recognition sites engineered to flank a gene of interest. Bringing the two recognition sites together, the enzyme can splice out the in-between genetic material from that cell.
   The biologists used this technique to knock out a particular hippocampal nerve cell receptor in mice. A normal mouse's receptors can respond to signals within the cell's environment. In the experiment, Kandel and Tonegawa deactivated the gene encoding this receptor in the hippocampus. This stopped the receptor from working.
   Lacking properly functioning receptors, the modified mice could not recognize features in their environment that their normal counterparts could.

—Howard Moskowitz

Researcher Catches the Cheshire Cat
   Even within a vacuum, a system seemingly devoid of matter and energy, minute energetic fluctuations occur on a subatomic scale. Until recently, scientists could not find a way to measure these fluctuations. Interested in the mystery and challenge of unmasking this phantom force, Steven K. Lamoreaux of Los Alamos National Laboratory devised a method for measuring the energy fluctuations—called the Casimir force—these fluctuations exert.
   Evidence of activity within a vacuum dates back to the 1920s' work of physics pioneers Max Planck and Werner Heisenberg. The duo noted that even at a temperature of absolute zero, where one expects atomic motion to cease entirely, vacuums actually teem with action. Describing the hypothesis of Planck and Heisenberg, a recent Science article (1/10/97) stated that "this ‘zero-point energy' can be thought of as an infinite number of ‘virtual' photons that, like unobservable Cheshire cats, wink in and out of existence."
   Named for Hendrick B.G. Casimir, a Dutch physicist who in 1948 discovered some unexpected oscillations in the relatively weak van der Waals electric forces that attract neutral atoms to one another in gases, the Casimir effect remained an abstract theory until Lamoreaux conducted his work at the University of Washington in Seattle. His findings appear in the 1/6/97 issue of Physical Review Letters.
   By placing two gold-coated quartz surfaces at a distance of approximately one micron apart, he created a box-like enclosure which literally trapped virtual photons. Energy differences between the inside and outside of the "box" forced the gold-coated surfaces together. Fine-tuned piezoelectric transducers—devices which convert oscillating electric fields into mechanical vibrations—supplied a small force to counteract this attraction. Lamoreaux could thus monitor the voltage and force he needed to supply in order to separate the two surfaces. This force proved equal to the Casimir force.
   Lamoreaux's resulting value for the force—less than one billionth of a Newton (a standard measurement of force)—was within five percent of theoretical guesses on the force figure. As a result, the Casimir effect is one Cheshire cat that won't be able to hide any longer.

—Craig Dunton

Left: The Clementine probe, built from leftover missile defense parts. Courtesy of JPL/NASA.

Right: The moon's south pole, where Clementine has detected the radar signature of water. Photo courtesy of the Naval Research Laboratory.

Clementine Returns Evidence of Lunar Ice
   Could there really be ice on the moon? Preliminary findings of the lunar spacecraft Clementine seem to suggest that frozen water may indeed be locked on the surface of Earth's natural satellite.
   The discovery, announced in December by Paul Spudis of the Lunar and Planetary Institute in Houston, may seem amazing considering that surface temperatures on the moon often become hot enough to boil water during lunar daytime. If ice exists, Clementine has shown that it lies in areas near the lunar south pole which never see the sun. Temperatures in these areas would linger around -230°C, remaining cold enough that ice would never evaporate into space—even over the course of billions of years.
   Built with leftover parts from the Star Wars missile defense system, Clementine was never designed to search for ice, but its transmitter is capable of shining radio waves onto the surface of the moon and sampling the reflected radiation. Ice is highly reflective and has the odd property of changing the polarization of incident radiation only slightly upon reflection, making it distinguishable from rock, which generally reverses the incident polarization. Clementine received this telltale signature of ice as it passed over a darkened region near the moon's south pole.
   Some scientists are skeptical about the results, as radar signals from this single scan could have been a statistical fluke. Another scan performed 200 kilometers from the pole and similar scans over the moon's north pole have revealed nothing. But the fact that the alleged ice appears exactly where it should—in the polar darkness—is encouraging.
   If ice really does lie at the moon's south pole, there probably isn't much there. Stewart Nozette of the Air Force's Phillips Laboratory says that if collected in one spot, the ice would form a block with the length and width of a football field and a height of 10 to 20 meters.
   Despite the small quantity of ice predicted, confirmation of Clementine's findings would prove a monumental event for the scientific community. The ice layers might disclose many secrets about how often comets have bombarded the Earth-Moon system in the past, and what the comets were made of. Some scientists speculate that future explorers could use the ice for rocket fuel—or sustaining a lunar colony.
   Confirmation of Clementine's findings will have to wait until September when NASA launches a new moon-bound craft called Lunar Prospector. Prospector will be capable of sensing hydrogen, and thus will be able to help confirm or reject the existence of ice on the moon's surface. "We'll know one month after we arrive whether any ice is there," said William C. Feldman of Los Alamos National Laboratory, the mission's principal investigator.

—Sam LaRoque