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Exiting ions of Venus' ionosphere.

Exiting of Venus' ionosphere

Data from various spacecraft, primarily the Pioneer Venus Orbiter, have identified "holes" in Venus' ionosphere, indicating regions with reduced numbers of hydrogen and oxygen ions. Theorists have wondered whether the ions move either up or down through the holes, and they have proposed answers on both sides of the question. Now two researchers report evidence from Pioneer Venus data that the ions seem to move out.

The clue that led to this conclusion, strangely, came from Saturn's big moon Titan, says Richard E. Hartle of NASA's Goddard Space Flight Center in Greenbelt, Md. In 1980, the Voyager 2 spacecraft flew past Titan and detected ions flowing outward along the "tail" of its weak magnetic field. The masses of the ions indicated that "we could be seeing something of ionospheric origin," Hartle says. Hartle notes that the ions could have come from external sources, but he says the observation brought to mind the matter of inward and outward flow through ionospheric holes on Venus, whose magnetic field is also very weak.

Earth, too, has ionospheric holes, but Hartle says they are localized processes, dominated by variations in the ionosphere's chemistry, and seem to have no significant upward or downward ion motion. This is also true of artificially produced holes created by dumping clouds of water vapor or chemicals from satellites, of by the exhaust of ascending rockets.

Inspired by Titan, Hartle and Goddard colleague Joseph M. Grebowsky went back through the Pioneer Venus data in search of ionospheric holes that might show signs of ions flowing up or down. The search revealed two such holes, detected on May 18 and 19, 1980. Both times, the spacecraft's orbit carried it on a sloping path through the hole, entering at an altitude of about 300 kilometers and leaving at about 140 km. And both times, more ions were counted at the greater height.

This suggests the ions were indeed moving up, Grebowsky says, though he acknowledges that the conclusion, reported in the recently released Jan. 1 JOURNAL OF GEOPHYSICAL RESEARCH, is tentative. More research will be needed (as well as more data, perhaps from Japan's proposed Venus ionosphere satellite) to distinguish actual upward ion flow from mere differences in the numbers of ions at different altitudes.

Hartle and Grebowsky are now working on the question of whether the ions flow outward only through the holes or from much of Venus' ionosphere. Another question is whether the holes served as an escape route for some of the water believed to have existed in the planet's early history.

"There is not enough flow through the holes to account for much water," Hartle says, although he adds that the effect could have been larger if the escape hatch turns out to include most of the ionosphere.
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Publication:Science News
Date:Feb 17, 1990
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