Far-out surface science: role of surface phenomena in the formation of
tenuous planetary atmospheres by B. V. Yakshinskiy, and T.E.Madey
Another of our projects deals with
surface processes that affect the formation of tenuous planetary atmospheres,
i.e., smaller bodies in the solar system (Mercury, the Moon, other planetary
satellites) whose atmospheres are essentially high vacuum. For example,
neutral sodium vapor with a column density of ~1011 - 1012
Na atoms/cm2 has been observed on the solar facing side of the
planet Mercury; a somewhat lower concentration of potassium has also been
found. Na vapor is also a component of the lunar atmosphere and its density
varies with latitude. Both the Moon and Mercury have tenuous atmospheres
(p=BE10-10 Torr) and
neither can retain gaseous species
for a considerable time; for example, neutral Na atoms are lost by adsorption,
by escape, or by photoionization. Thus, the atmospheric constituents must
be continuously resupplied. Various mechanisms have been proposed as the
source of supply of atmospheric sodium and potassium: these include charged
particle sputtering by the solar wind, impact vaporization from meteoroid
collision (with Na and K coming from either the rocky surface or the micrometeoroids),
and photon stimulated desorption. In addition, readsorbed Na or K may be
thermally desorbed, an atmospheric recycling process. Evidence for 'hot'
non-thermal Na has been reported.
Our work deals with the adsorption and desorption of alkalis from oxide surfaces: the focus is on the relevance of surface science studies to the origins of alkali vapors in tenuous planetary atmospheres. Emphasis is on desorption phenomena: thermal desorption, electron- and photon-stimulated desorption to produce non-thermal alkalis, and ion-induced desorption (sputtering). Recent measurements are on a model mineral system, Na on SiO2 thin films. We find strong evidence that non-thermal processes can play an important role in desorption of Na from mineral surfaces.
References:
| 1. | B. V. Yakshinskiy, and T.E.Madey | Desorption induced by electronic transitions of Na from SiO2: relevance to tenuous planetary atmospheres | Surf. Sci., 2000, V.451, P.160-165 |