Current Understanding Of Source And Loss Processes

The simplest loss process is thermal release (Jeans escape), which is caused by heating of the surface due to solar radiation. Given the large surface temperature on the dayside (Chapter 1), very volatile molecules can evaporate into space with scale heights and column densities corresponding to the local surface temperature. Only H and He escape Mercury predominantly through this mechanism. Thermal vaporization is too rapid to be sustained at temperatures near the sub-solar point on Mercury...

Magnetopause Structure

Mariner 10 data indicated that the Mercurian magnetopause had a significant aberration in the ecliptic plane as well as equator-pole asymmetries. Ness et al (1975) computed the shape of the magnetopause for the case where the solar wind was incident on a Mercury-centered magnetic dipole orthogonal to the solar wind flow. The theoretical position of the bow shock was determined for the case of aligned flow, in which the upstream magnetic field and solar wind velocity were deemed to be parallel....

The Exosphere Concept

An exosphere is an ensemble of atoms or molecules above a planet's surface or atmosphere for which the mean free path is greater than the scale height (the e-folding height for density). Because collisions are rare in an exosphere, each constituent maintains its own distribution, defined by its unique combination of source energy and distribution, mass, radiation pressure and loss processes (e.g. Jeans escape, photo-dissociation adsorption ionization surface chemistry). Killen and Ip (1999)...

The Selective Accretion Model

Selective Accretion relies on differences between the aerodynamic and mechanical properties of metals in the inner Solar System (where the gas densities were relatively great and the time scale short) sufficient to support dynamic sorting leading to iron enrichment (Weidenschilling, 1978). Metal silicate fractionation might have resulted from the Selective Accretion of the cores of differentiated asteroids. This process would require the original composition of the asteroids to be at least as...

Understanding The Planet Mercury

Thirty years have elapsed since the one and only mission to Mercury, Mariner 10, performed three flybys of the planet, capturing moderate-resolution (100 m at best) images of one hemisphere (45 of the surface) and discovering that Mercury could be the only other terrestrial planet to have a global magnetic field and core dynamo analogous to the Earth's. At the time of this writing, the MESSENGER mission to Mercury has been launched. We are still a couple of years away from the first of the next...

Ground Based Observations Of Sodium And Potassium

Well after the Mariner 10 flybys, a fortuitous observation led to the identification of sodium and potassium exospheres at Mercury. These first observations were made using ground based instrumentation consisting originally of single-slit, spectrographs (Potter and Morgan, 1985, 1986). Later, an image slicer technique was introduced (Potter and Morgan, 1990). An account of this latter method is contained in Pierce (1965). (Briefly, an image slicer placed at the entrance to the Echelle...

Magnetosphere Structure

Mercury Plasma Propulsion

Many workers have modeled the dimensions of the dayside magnetosphere based on the Mariner 10 magnetopause and bow shock transit measurements (Ness et al, 1974 Ogilvie et al, 1977 Russell, 1977 and Slavin and Holzer, 1979a, 1979b). A straightforward way to understand Mercury's magnetosphere is to scale the Earth's magnetosphere to be equivalent to Mercury's. This is done by decreasing the radius of Earth's magnetosphere by a factor of 8 relative to the radius of the planet and then decreasing...

Direct Accretion Of Reduced Components

Ringwood (1966) and Wanke (1981) modeled Mercury's initial composition using two chemically distinct components, one highly reduced and moderately refractory, the other highly oxidized. Modeling based on chemistry when applied to the Earth and Mars, display a trend that predicts Mercury to be extremely reduced (Dreibus and Wanke, 1984, 1985). In the case of Mercury itself, the models generated are consistent with the implied reduced state of the planet's interior because of the absence of a...

Summary

Mercury is in a unique position to provide understanding of planetary magnetospheres. The planet has a simple magnetosphere formed by the solar wind, which shapes it the interplanetary magnetic field (IMF), which activates it an interior magnetic dipole, which structures it and the planet itself which anchors it. These are the basic ingredients. All other known magnetosphere-bearing planets have added complexities, such as additional sources of plasmas or current systems, that encumber the...

Mercurys Exosphere After Sodium And Potassium Detection

Following the ground based discovery of sodium and potassium in the Hermean atmosphere, Hunten et al (1988) produced an excellent review of the state of knowledge concerning Mercury's exosphere. These authors pointed out that, although Mercury's atmosphere is technically an exosphere, the gas-surface interaction is very different from the interaction of an exosphere in contact with an underlying atmosphere. Mercury's exosphere is the observable result of a dynamically coupled system, with the...