Johns Hopkins Magazine - November 1996 Issue

Science and Technology: In Short

The noctural nature of the northern lights... a wisp of oxygen on Ganymede... inside the Golgi there's no standing still

The northern lights: only at night

Inspiration for songwriters and astronomers alike, the colorful arcs, streamers, and curtains of the northern lights are humbling to behold, say people who have observed them. The Inuit thought they were lighted torches held by heavenly spirits to guide new arrivals. Medieval Europeans believed they were the reflections of the breath of heavenly warriors. The lights, known as aurorae, have also served as omens of plague, death, illness, and war--ideas that have been debunked over the centuries.

Now another notion about the northern lights has been dashed. Astronomers have long held that even though you cannot see them during the day, they're still there, simply masked by the sun. But researchers at Hopkins's Applied Physics Laboratory (APL) conclude that the northern lights are a purely nighttime phenomenon. The light show occurs mainly between dusk and midnight and mostly during winter, report APL scientists Patrick Newell, Ching-I Meng, and Kevin Lyons.

Aurorae are thought to occur when accelerated, electrically charged electrons and protons from the sun flow along magnetic field lines. The particles excite atoms in their wake, giving them a boost of energy. The energized atoms relinquish this energy as photons of light, whose wavelength varies, depending on the atom. Excited oxygen atoms produce green light. Nitrogen yields red, and so on.

However, astronomers have not understood the fine details of this phenomenon.

The Hopkins scientists examined spectra of beams of accelerated electrons in near-Earth space, which can be studied in daylight as well as at night. The data were collected by five weather satellites that are maintained by the U.S. Air Force Defense Meteorological Satellite Program. In all, the researchers analyzed 152 million spectra collected between 1983 and 1992.

The spectra of electrons show intense aurorae during darkness, and barely a glimmer during daylight, the scientists report in the June 27 Nature.

The finding supports one of 22 competing theories on what causes aurorae. The theory has to do with the level of conductivity in the atmosphere. "Accelerated electrons carry current from space to Earth," explains Newell. "But you've got to complete the circuit." When the conductivity is high, as it is during sunlight, the conditions allow the circuit to be completed. During night, however, conductivity drops, and there is an instability, "which has to find a way out," says Newell. The result, according to the theory, is electrical discharge, or the aurora.
--Melissa Hendricks

And speaking of aurorae...

Hopkins astronomers report that the Jovian moon Ganymede has an oxygen atmosphere (albeit an extremely thin one). Moreover, oxygen emission spectra indicate that Ganymede has aurorae at its north and south poles, analogous to the Earth's northern and southern lights.

Associate research scientist Doyle Hall presented the findings late last month in Tucson, at a meeting of the American Astronomical Society. "I want to stress that it is a tenuous atmosphere," he says taking a nervous breath. "That means really, really thin--probably about one one-hundred billionth of the surface pressure on Earth."

Life could not exist in such an environment. Still, the finding is exciting, says Hall. "Discovering another aurora and atmosphere challenges our theories on these phenomena. The processes that cause Earth's aurorae affect our lives (for example, by interfering with power grids and satellite transmission). The more examples we have of these phenomena, the more we can refine our theories about them." Hall's collaborators were Paul Feldman, chair of Physics and Astronomy; Darrell Strobel, professor of earth and planetary sciences; and Melissa McGrath, of the Space Science Telescope Institute.

This is not the first time Hall has observed oxygen on a Jovian moon. Two years ago, he and his colleagues discovered oxygen on Europa, another of the four Jovian moons discovered by Galileo in 1610. The astronomers then turned to the largest of the Galilean moons, the frigid Ganymede. Larger than the planet Mercury, Ganymede's icy surface is -140 degrees Celsius.

Late last June, Hubble's Goddard High-Resolution Spectrograph took ultraviolet spectra of Ganymede. At the same time, the Galileo spacecraft observed the moon at close range (within 1,000 kilometers).

UV spectra reveal the chemical composition of celestial objects. When Hall's team had analyzed Europa's spectra, they saw a single peak within the wavelength range that is a signature for oxygen. They had hoped Ganymede's spectra would reveal the same. But the astronomers got more than they had hoped for. Instead of one emission line, the astronomers saw a double peak. "Since we see two peaks, we suspect two emission spots," says Hall. The pattern of oxygen emission appears to correspond to Ganymede's north and south poles, says Hall.

Meanwhile, the spacecraft's data revealed that Ganymede has a magnetic field and a population of charged particles in its immediate environment. Taken together, they suggest that Ganymede has aurorae at its north and south poles, similar to Earth's (though weaker than our spectacular light show).

The fact that the oxygen emission lines are concentrated at the poles indicates that electrically charged particles are flowing down magnetic field lines that converge at the poles, just as occurs on Earth. Atoms (which, in this case appear to be oxygen) that are excited by the charged particles emit photons of light as they return to their normal energy state.

The astronomers believe Ganymede's oxygen comes from ice on the moon's frozen surface. "There are three possibilities," says Hall. Oxygen may be produced through evaporation of the ice. Charged particles hitting the surface of the ice may knock off oxygen molecules, a process called sputtering. Or meteorites could have generated the oxygen gas, by releasing plumes of smoke as they crashed into Ganymede. "We don't know," says Hall. "I suspect the answer is sputtering."

"Incredibly mobile" Golgi proteins

It has been almost 100 years since Italian neuroanatomist Camillo Golgi discovered lacy structures in cells that scientists later named the Golgi complex. But only now are biologists figuring out the inner workings of this organelle. A recent study by cell biologists, led by Jennifer Lippincott-Schwartz (PhD '86), reveals Golgi mechanisms that are radically different from those biologists had originally imagined.

The Golgi, you may remember from high school biology, consists of layers of flattened, fluid-filled vesicles, or cisternae--a microscopic stack of pancakes. It works something like the shipping and receiving department of a factory, which, in this case, is a protein factory. The Golgi puts the finishing touches (read: sugar groups) onto proteins, sorts the proteins, and sends them on their way to be secreted by the cell or woven into membranes. All growth hormones, growth factors, and membrane proteins must file through this organelle.

There's no sitting down on the job. Traffic through the Golgi is like a raging river, with molecules barreling through on their way to future lives in other parts of the cell. But that fact also raises a paradox: With all the traffic rushing through, why would components of the Golgi itself not get sucked into the raging current and swept away like so much flotsam and jetsam?

Biologists had hypothesized that Golgi molecules were somehow anchored into place, strongly enough to withstand the rush of traffic, says Lippincott-Schwartz, a cell biologist at the National Institute of Child Health and Human Development. And that is what she and her colleagues expected to see when they examined the Golgi using a technique called fluorescent photobleaching.

Instead, "to our surprise, we found that these proteins are incredibly mobile," says Lippincott-Schwartz. "They move through the entire compartment over a short period of time."

If you look at the Golgi at any one time, it would appear that the structure is static. But Lippincott-Schwartz believes that Golgi proteins are actually rapidly recirculating, flowing through a neighboring organelle called the endoplasmic reticulum, and back again to the Golgi. The molecules become concentrated in certain regions, like cars waiting at a stop sign. "If you flew over a line of cars waiting at a stop sign, you might say the cars lived there," says Lippincott-Schwartz. "But actually there's flux." Likewise, the Golgi is incredibly dynamic, but remarkably cohesive.

Lippincott-Schwartz and her colleagues were the first to label the proteins with a fluorescent tag derived from the bioluminescent jellyfish Aequorea victoria. When a laser beam is shone on the tag, through a process known as fluorescence photobleaching recovery (FPR), the tag fluoresces green. If the laser beam is high-intensity, the tag gets bleached and does not fluoresce. Under the microscope, bleached areas show up as black regions while unbleached areas appear green.

Lippincott-Schwartz's team photobleached small regions of Golgi complexes, and then videotaped the Golgi for several minutes. Protein movement would show up as green dots diffusing into the blackened regions.

The biologists chose four Golgi proteins to label with the fluorescent tag. Two of the proteins are enzymes that add sugars to proteins, and two are receptors that are believed to bind molecules as they return to the Golgi from the neighboring endoplasmic reticulum.

Within just seconds of photobleaching,the photobleached areas were dotted with green. "We recorded one of the fastest diffusion coefficients ever recorded," says Hopkins professor of biology Michael Edidin, who collaborated on the project. The diffusion coefficient is the rate at which the tagged molecules diffuse through the Golgi. Biology doctoral student Nelson Cole also collaborated on the project.

The researchers report their findings in the August 9 Science. Quicktime movies from the experiment can be seen on the Web at

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