Sodium found in Mercury's atmosphere.

Astronomers used to believe that the planet Mercury had no atmosphere. Earth-based observations could find no trace of one. About a decade ago, however, the space probe Mariner 10 sent back news that Mercury had a trace atmosphere that seemed to be mainly hydrogen and helium. So tenuous is this atmosphere that its pressure at the surface of the planet is estimated in ten-trillionths of the surface pressure on earth. Now, ground-based observations report that sodium is also present in Mercury's atmosphere.

Andrew Potter of NASA's Johnson Space Center in Houston and Thomas Morgan of Southerwestern University in Georgetown, Tex., report in the Aug. 16 SCIENCE that sodium is byfar the most abundant component now know to exist in Mercury's atmosphere, being about 33 times as abundant as helium, the next most abundant. Potter and Morgan estimate the density of sodium at Mercury's surface to be 150,000 atoms per cubic centimeter, compared with 4,500 atoms per cubic centimeter for helium and 8 atoms per cubic centimeter for hydrogen. The sodium exerts a pressure of about 1.2 X 10.sup.-11 millibars. (The surface presure of earth's atmosphere is 1 bar.)

The relative abundance of sodium in Mercury's atmosphere invites comparison with Jupiter's satellite Io, which also has a lot of sodium. In Io's case sodium appears to be sputtered off the satellite's surface by energetic particles in Jupiter's magnetosphere. In Mercury's case the solar wind probably does the sputtering. (The hydrogen and helium seem to come directly from the solar wind.) the solar wind can also take sodium from the atmosphere of Mercury, but bombardment of the planet by meteors could provide a replacement supply to maintain a steady amount. All in all, Potter and Morgan say, the atmosphere of Mercury resembles the coma of a comet more than it does the atmosphere of a planet like earth.

The figures for Mercury's helium and hydrogen come from work done with Mariner 10, which found no definite evidence for any other constituents except possibly atomic oxygen, for which the spacecraft got a signal at the extreme low limit of detection. Those observations also set upper limits for neon, argon and carbon, which would have been detected had they been present in amounts greater than those limits. The Mariner 10 work used an ultraviolet spectroscope to identify elements in Mercury's atmosphere.

Sodium does not have a prominent signature in the ultraviolet. However, it identifies itself in visible light by a pair of yellow lines, the Fraunhofer D lines. Potter and Morgan found these lines in light resonantly refleced off the atmosphere of Mercury.

"It was daylight work," Morgan says. Mercury is so close to the sun that it is rarely seen at night, and then only immediately after sunset or immediately before sunrise. Morgan points out that the managers of telescopes do not like to have their instruments pointed closer to the sun than 10[deg.] away -- direct sunlight can damage equipment engineered for much fainter starlight. So Potter and Morgan worked mostly when Mercury was at greatest elongation, that is, its farthest distance from the sun, which is 28[deg.] away. They used the 2.7-meter telescope, the largest belonging to the McDonald Observatory at Ft. Davis, Tex.

The density of sodium present can be determined from the ratio of the widths of the two Fraunhofer D lines, provided the temperature is known. The two lines are not single, precise wavelengths. Each has a certain small spread of wavelengths. The amount of spread depends on the density of sodium and its temperature. The figures quoted are calculated for the point on Mercury where the sun is at the zenith, assuming a temperature there of about 500 kelvins. (Earth's surface temperature on a warm spring day might be 300 K, which is about 75[deg.]F.

If sodium is present in the Mercurian atmosphere, potassium might not be far behind. Potter and Morgan plan to look for potassium next.