Johns Hopkins Magazine -- June 2000
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JUNE 2000

S C I E N C E    &    T E C H N O L O G Y


The serene screech of falling snowflakes
A new name for NEAR
A harmonious matter
Unlocking an ancient atmospheric riddle
Is "faster, better, cheaper" really best?

Illustration by Charles Beyl
The serene screech of falling snowflakes

It's not every day that scientists get all bubbly.

Lawrence A. Crum, a physics professor at the University of Mississippi, was trying to record the sound of snowflakes hitting the surface of water to determine why, counter to logic, these fluffy, air-filled assemblies of ice crystals seem to make a lot of noise underwater. Several years ago, Crum got his big chance during a visit to Yale University in Connecticut.

After a dinner of pizza and beer, Crum heard television forecasters predicting snow for Baltimore. So he and a colleague borrowed the Yale engineering dean's van and equipment and went south in pursuit, but not before he picked up the phone and called colleague Andrea Prosperetti, Hopkins professor of mechanical engineering.

"He threatened to wake me up at 6 a.m.," Prosperetti remembers. "Thank God it didn't snow here. I kept sleeping and he kept driving."

As Crum recalls: "We asked if we could use his office or lab to do the measurements. But the storm went south and we couldn't find it, so we were chasing."

What Crum and his Ole Miss colleague Ronald Roy found that trip after rigging up acoustic equipment in a motel pool in Roanoke, Virginia, led to a research paper published in October in the Journal of the Acoustical Society of America. Crum, now chair of the Acoustics and Electromagnetics Department at the University of Washington, is the study's lead author, in conjunction with snowchase colleague Roy, now at Boston University, and Prosperetti.

The journal article, also co-authored by Hugh Pumphrey of the University of Edinburgh in Scotland, reveals a bit of the scientist's joy of discovery: Early snowfall data was "so unique and contrary to our intuitions and expectations that it has inspired us to accumulate data from a number of storms," the authors wrote. After studying such data, researchers believe the high-pitched sound Crum and Roy recorded is not caused by the impact of the flakes but by vibrating bubbles created after the snow hits the water's surface.

They had already detected a similar phenomenon in rainfall back in the 1980s. Prosperetti, a wizard theorist on the relation between bubbles and sound fields, had teamed up with Crum to publish groundbreaking research on the role of rain-induced bubbles in underwater noise. Among other tests, they used a high-speed camera to capture the bubbles. This time around, Prosperetti analyzed the acoustic signature of the snowflake noise recorded in Virginia; he found a similar "footprint." In both cases, the signatures revealed the typical features of pulsating bubbles. "If it walks like a duck and quacks like a duck, it's a duck," says Prosperetti.

It's an odd duck at that. "We think it is a bubble, but how can a bubble be involved? A snowflake is mostly empty. It's 10 percent water and the rest is air," Prosperetti says of the mystery. "As a snowflake deposits itself, there is no impact essentially. Leisurely, bumm, bumm, bumm, it drops down." A serenely quiet scene by any standard.

"But what the layer of water engulfs is not solid ice, it is engulfing the air of which the snowflake is made," he adds. As water melts the ice, a bubble remains, researchers postulate. Water surface tension and pressure then would cause the bubble to pulsate. Those pulsations, or oscillations, create the sound. "It's like beating a drum," Crum says.

Prosperetti (pictured at right) adds, "It's a high-frequency sound. It would sound like a hissing noise if we could hear it, but we can't." The sound, ranging between 50 and 200 kilohertz, is too high for human ears (which can normally hear nothing higher than 20 kilohertz). Snowflake screeching, which was first recorded in the mid-1980s, adds 30 decibels to the underwater environment. "It's the difference between a private conversation and a rock band," Crum says. It's unclear just how much it disturbs underwater animals, though porpoises can hear sounds at high frequencies, he says.

However, the noise does wreak havoc with underwater sonar equipment.

Wildlife researchers using sonar devices to count salmon in the Pacific Northwest, and U.S. Naval submarine officers using sonar to detect enemy subs, dread storms because raindrops and snowflakes have the potential to create "background noise," which could interfere with a sub's torpedo detection. Concerns about sonar detection gaps led the Office of Naval Research to fund Prosperetti's and Crum's work on underwater rain noise. The researchers suggest one possible solution: change the frequency range of fish finders and similar sonar devices.

Their findings could lead to other applications. Scientists need to measure rainfall in the oceans, an important factor in studying worldwide climatology. But gathering such data is difficult; oceans are big. Yet researchers could analyze the signature of bubble noise picked up by remote sensors to determine rainfall (in short, louder sound means heavier rainfall).

The next research step in the snowflake study? Using high-speed cameras to visually record snow bubbles. "In principle, what one would like to see is a snowflake caught in the act," Prosperetti points out. Trouble is, there isn't much of a practical demand for that verification. "This stuff is nice, but it's not by accident that there's no money in it," the veteran researcher says.

In the end, the four-university study can't help but be a bit of science for science's sake. Says Crum (who among other things has climbed onto the roof of his lab on Christmas Eve to record snowfall sounds): "When scientists get around each other and talk about things, they don't talk about girls and cars. They talk about how to find the next data point."
--Joanne Cavanaugh Simpson

A new name for NEAR

The NEAR spacecraft now in a yearlong orbit around the asteroid 433 Eros more than 145 million miles from Earth has been named for a man who, though he never traveled in space, launched today's scientific passion for the study of asteroids and comets: Dr. Eugene M. Shoemaker.

Now called NEAR Shoemaker, the NASA spacecraft designed and operated by Hopkins's Applied Physics Lab is exploring the geological surface of the potato-shaped space rock in part to look at asteroids' links to the birth of the solar system. Shoemaker, a legendary geologist and influential researcher on the role of asteroids and comets in the formation of planets, died in a 1997 car accident while studying asteroid impact craters in the Australian outback. Among other accomplishments, Shoemaker, along with his wife and research partner, Carolyn, helped discover the comet Shoemaker-Levy 9 that broke up and collided with Jupiter in 1994. He taught the Apollo astronauts about craters and lunar geology. He developed the lunar geological time scale researchers have used to date the moon's features. In a touching tribute last year, NASA's Lunar Prospector spacecraft scattered his ashes on the moon.

When the NEAR mission was first being developed in the mid-1980s, Shoemaker was part of the team. "Eros looks really old and solid," says Carolyn Shoemaker, "and may well be a piece of a much larger asteroid."

Illustration by Sean Kane
A harmonious matter

Stop whatever you are doing and think about your left foot for a moment. Really focus on it.

Now, what's happened? Perhaps you've noticed a tickle in your little toe or an itch on your arch. Chances are, you were giving no thought to your foot a moment ago, though you had the same neurons in it then as now. So why did you fail to notice that itch on the arch?

According to a model developed by Hopkins theoretical neuroscientist Ernst Niebur and verified by experimentalists, paying attention is all a matter of neural synchrony. Niebur and his colleagues reported their results in the March 9 Nature. The brain is constantly flooded with sensory signals. To pay attention to one aspect of that sensory landscape, says Niebur, the neurons involved unite in a synchronized chorus of firing.

The senses are always "on," Niebur explains. The nervous system is constantly feeling, smelling, hearing, and seeing. While you are thinking about sensations in your left foot, the rest of your body and nervous system continues to receive sensory input--about the lighting in the room, the weight of your clothes on your body, the odors wafting through the air. However, not all of those sensations garner your full attention. In fact, almost all of it gets dumped.

While Niebur is a theorist--the only one at the Krieger Mind/Brain Institute, he collaborated with three experimental scientists, institute neuroscientists Steven Hsiao and Kenneth Johnson, and former postdoctoral fellow Peter Steinmetz, who is now at the California Institute of Technology. The scientists applied Niebur's model to data collected from testing on rhesus monkeys.

In those studies, Johnson and Hsiao used electrodes to monitor the activity of individual cells in a region of the monkeys' brains containing neurons that react to touch. While the recordings were taken, the monkeys performed tactile and visual tasks, and switched from one type of task to another upon cue.

Previous studies had indicated that when an animal pays close attention to a stimulus, certain neurons in the animal's brain fire at a faster rate, perhaps two or three times as fast.

But paying attention appears to be even more complex than that, says Niebur. The team's recent study shows that when monkeys were paying close attention to a tactile stimulus, certain neurons in the tactile region not only increased their firing rate, they also fired in synchrony. When the monkeys switched to a visual task, the degree of synchrony decreased.

While the model appears to apply to the tactile system, says Niebur, there is no reason to believe that it would not also hold for other sensations and even for cognition. So telling yourself to pay close attention to your left foot probably entails an increased degree of synchrony in a subset of your neurons.

Many questions remain. The model describes what goes on in the brain when you are paying attention. But what starts the process? How do we decide what to pay attention to at any given moment?

"That's a good question," says Niebur, "and we don't have all the answers." He and his colleagues plan to conduct follow-up studies that may provide more clues.
--Melissa Hendricks

Unlocking an ancient atmospheric riddle

Scientists have long pondered how to determine what was in the air millions of years ago. "To study the climate of the past Earth, we would like to know the composition of the past atmosphere," says A. Hope Jahren, Hopkins assistant professor of geobiology and climatology. "Unfortunately, atmosphere is not preserved in the fossil record."

But ancient plants are preserved as fossils, and they may unlock part of the riddle.

Jahren, along with lead author Nan Crystal Arens, assistant professor of integrative biology at Cal-Berkeley, examined data from 44 studies on 176 species of modern grasses and trees. They found that the composition of plant tissue closely correlates with the composition of the carbon in the atmosphere at the time of the plant's life.

Says Jahren, "We believe that this will allow us to use plant fossil composition as a proxy for ancient atmospheric composition.

"The composition of the atmosphere is widely thought to control the major climactic forces of the planet--that is why there is so much concern over human activities substantially changing the modern-day atmosphere," she continues. "By studying parts of the Earth's history where we suspect climate change, we may be able to better understand the link between the composition of the atmosphere and its effect on the weather at the surface of the planet." The study by Jahren and Arens was recently published in Paleobiology.

Through photosynthesis, plants take in carbon dioxide from the air. The carbon atoms in carbon dioxide vary; some are the isotope carbon-12, and some are carbon-13, with one more neutron. Physical and enzymatic processes within the plant differentiate the two isotopes, allowing scientists to apply mass spectrometry and measure the ratio of carbon-12 to carbon-13. In the 44 studies that they examined, Jahren and Arens found a strong correlation between the ratio of the isotopes in plant tissue and the corresponding ratio of the isotopes in the atmosphere.

These atmospheric ratios form a sort of signature, indicating the sources of carbon in the air. Carbon enters the air from various carbon "pools": in ancient times, these included volcanic emissions, the weathering of carbonate rock, and emissions of gas from the ocean floor. (Modern sources also include burning of the biomass, such as rain forest clearing, and fossil fuel combustion.) The carbon from each of these sources has a different ratio of carbon isotopes. Read the ratio--examine the handwriting, so to speak--and you can determine the source of the carbon.

"A change in the ratio of carbon isotopes in the atmosphere indicates a disruption in the global carbon cycle--things like volcanic eruptions, changes in the extent of forestation, or continental burning," says Jahren. Significant changes in the carbon cycle, in turn, indicate major changes in climate: "Once we can discuss changing atmosphere composition through time, we can begin to approach what periods of carbon-cycle disruption mean with respect to climate."
--Dale Keiger

Is "faster, better, cheaper" really best?

Recent NASA efforts to slim down space missions came under evaluation by scientists and engineers in May, at the Fourth International Academy of Astronautics Conference on Low-Cost Planetary Missions held at Hopkins's Applied Physics Laboratory.

As the space century comes to a close, the cost of space missions has climbed into the billions. NASA launched its Discovery Program in the 1990s to reshape the culture of space science into the lean, mean corporate model. Some of the stay-within-budget tactics discussed at the four-day conference included employing smaller and lighter spacecraft, satellites, and instruments; adapting current software and other technology instead of reinventing it; and saving travel and other costs by linking team scientists from around the world via the Internet and teleconferencing.

Proponents of the "leaner, meaner" philosophy touted the success of a current APL-led mission, NEAR Shoemaker, which is now in a yearlong orbit around the asteroid 433 Eros, at a cost of just $216 million. But the slim-and-trim concept has its critics. Among problems cited: a lack of thorough testing, cuts in staff and resources (scientists who regularly work 80-hour plus weeks), fewer risk assessment measures, and an atmosphere where speed is a prime factor, all of which speaks to another slogan--Haste Makes Waste.

"We can't afford as a nation to take risks with these missions," said Liam Sarsfield, senior policy analyst for RAND's Science and Technology Policy Institute. "We should consider them a national asset. I don't know what FBC [faster, better, cheaper] is. As a slogan, I hope it goes away--the faster the better."

Tony Spear, project manager for the successful Mars Pathfinder mission in 1997 and a NASA consultant, does not condone taking shortcuts. But he says the overall concept is "here to stay. The best definition, I believe, is that it's simply an attempt to continually improve performance."