Mercury's atmosphere: an inside source?

Mercury's atmosphere: An inside source?

Spectral measurements in 1985 revealed that the planet Mercury, whose closeness to the sun makes it difficult to study from Earth, has a thin atmosphere containing sodium and potassium. The researchers who made the discovery suggested that the rarefied atmosphere might result either from tiny meteorites vaporizing as they hit the planet or from atoms knocked loose from the surface by the stream of charged particles called the solar wind. Last week, however, an astronomer described evidence that the planet's atmosphere may come from gases diffusing up through its crust.

The most prominent feature on Mercury is a huge basin named Caloris, about 1,300 kilometers across, that apparently formed from the impact of a large meteorite. After four years of studying the planet with the 1.5-meter telescope at Catalina Observatory on Mount Bigelow in Arizona, Ann L. Sprague of the University of Arizona in Tucson last week told a meeting of the American Astronomical Society's Division for Planetary Sciences in Providence, R.I., that about ten times as much potassium showed up in Mercury's spectrum when Caloris was in view as when the big basin was out of sights. Also, she told SCIENCE NEWS, Mercury's atmosphere shows signs of a similar Caloris-related sodium enrichment.

Sprague's group, including colleagues Donald M. Hunten and Richard W.H. Kozlowski, does not envision Caloris as a volcano, spewing forth eruptions of potassium and sodium. Instead, Sprague says, the atoms probably just "diffuse out of the well-fractured crust."

The original discoverers of Mercury's atmosphere, Andrew E. Potter Jr. of NASA's Johnson Space Center in Houston, Tex., and Thomas H. Morgan, now with NASA in Washington, D.C., have described it as essentially evenly distributed, with some irregularities caused by photons of sunlight pushing the atmosphere's atoms around. At last week's meeting, Morgan presented images showing enhanced sodium emissions in localized areas, which he attributed to interaction of the sodium with Mercury's magnetic field.

The idea of the material diffusing upward through Mercury's crust, on the other hand, says Sprague, can explain not only why more sodium and potassium exist above Caloris than elsewhere on the planet, but also why Mercury has a higher sodium-to-potassium ratio than does Earth's moon, where both elements also have been detected.

The moon has about five times more sodium that potassium, while Mercury shows about 15 times as much. Mercury's magnetic field alone cannot explain the ratio difference, Sprague says. But if Mercury and the moon have similar compositions and similarly cracked surface rocks, differing temperatures of the rocks beneath the surfaces of the two bodies, due to such factors as the moon's greater distance from the sun, could account for the different sodium-to-potassium ratios, she says.

Potter told SCIENCE NEWS this week that reexamining their initial potassium data in fact shows that Caloris was in view, a detail not noted at the time, and that there is even slightly more potassium in the planet's northern hemisphere, where Caloris lies, though he says that the difference is within the uncertainty of the measurements.