Venus: dragging out new findings.

In analyzing the effects of atmospheric drag on the Magellan spacecraft, researchers have discovered that the density of Venus' upper atmosphere fluctuates with a period of about four Earth days. The fluctuation may relate to the way cloud layers revolve around the planet. Astronomers previously determined that the clouds, which lie some 100 kilometers below the craft, cycle around the planet every 4.2 days.

Gerald M. Keating of NASA's Langley Research Center in Hampton, Va. and his colleagues suggest that the moving cloud tops transmit waves of energy to higher altitudes. Like ocean waves that dump water onto the shore, these waves would deliver heat into the Venusian atmosphere, causing particles such as oxygen atoms and carbon dioxide molecules to rise temporarily to greater heights. During such times, the upper reaches of the atmosphere would increase in density,

The researchers suspect that the entire upper atmosphere may revolve around the planet, since the same waves would carry with them angular momentum from the clouds below. Verification of these results would indicate that the middle atmosphere of Venus strongly influences the upper atmosphere, Keating says.

He notes that atmospheric studies with Magellan, which previously used radar to image Venus, have been possible only since last September, the start of Magellan's fourth 243-day cycle around the planet. During this cycle, the craft descended to about 180 km above the Venusian surface -- low enough to encounter significant drag from the pIanet's atmosphere.

To study the Venusian atmosphere, scientists send a radio signal from Earth to the craft, which beams the signal back to Earth. A change in atmospheric drag would speed or slow Magellan, causing a frequency shiR in the radio signal beamed back to Earth. Because denser regions generate more drag, this experiment effectively measures the density of the atmosphere.

Researchers are using the same technique to measure Venus' gravitational field, since the craft also speeds up or slows down when it passes over a region of the planet that has a higher or lower mass density, Magellan should begin taking highresolution gravity measurements in late August after NASA engineers complete an unprecedented set of maneuvers, using atmospheric drag rather than expending fuel, to further lower and circularize the craft's elliptical orbit. These maneuvers have already brought the craft to within 140 km of the surface, the nearest any orbiting craft has come to Venus while still transmitting data to Earth.

In another study, Keating and his co-workers found that the solar cycle, roughly 11 Earth years in length, has little effect on the density of Venus' upper atmosphere. The finding may have consequences for Earth's atmosphere, Keating notes. The team examined data gathered by the Pioneer Venus spacecraft in 1979, when the sun was near its peak of activity, and compared them with data collected by Magellan and the Pioneer Venus in 1992, when the sun had significantly less' activity

Keating suggests that in Venus' atmosphere, which is rich in carbon dioxide, the increase in heating at the peak of the solar cycle is counterbalanced by a cooling mechanism. When oxygen atoms collide with carbon dioxide molecules high above the surface of Venus, the molecules dramatically cool the upper atmosphere by emitting infrared light. Ironically, such an effect, known as radiative cooling, is enhanced during the peak of the solar cycle, when the high intensity of sunlight creates a high abundance of oxygen atoms.

If, as scientists expect, the carbon dioxide concentration in Earth's atmosphere doubles in the next century, the same radiative process will cool our planer's upper atmosphere, Keating says. This same gas, however, has the opposite effect lower in the atmosphere, acting to warm Earth's surface.