Knowledge of Mercury's Exosphere from MESSENGER Observations

Abstract:

The Mercury Atmospheric and Surface Composition Spectrometer on the MESSENGER spacecraft has observed calcium, magnesium, and sodium emissions in Mercury's exosphere on a near-daily basis since March 2011. The dayside observations, made mainly at near-equatorial latitudes, have revealed different spatial morphologies and temperatures for each of these species. The Ca exosphere shows a persistent seasonal dependence on source rate, is concentrated in the dawn equatorial region, and is of extreme temperature (>50,000K), such that almost all of the calcium is above escape velocity. Observations of neutral Mg reveal that at least two processes are required to reconcile the distributions of Mg measured far from and near the planet: a hot ejection process at the equivalent temperature of several tens of thousands of degrees K, similar to that of Ca, and an additional source at lower temperatures (3000–5000 K). The cooler process is consistent with an impact vaporization source. The hotter source may be consistent with impact vaporization in the form of molecules that are subsequently dissociated by a process that imparts enough energy to accelerate the atoms to escape velocity. The bulk of the Na exosphere is cooler than either Ca or Mg, but still much hotter, at about 1200 K, than the surface temperature. We observe a persistent, repeatable seasonal pattern. The temperature and morphology of the Na exosphere are consistent with photon-stimulated desorption as the primary ejection process for this species. Although each atomic species has its distinct temperature and spatial morphology, the variation between species is such that the most refractory species, Ca, is the hottest, and the most volatile species, Na, is coolest. This result indicates that the processes acting to produce the exosphere act in concert with the binding energies on the surface. Other species expected to be present in the exosphere, notably O, S and Al, have emission probabilities that are extremely weak. We have not observed S or Al in the Mercury exosphere but have a global average value for oxygen that is much less than the stoichiometric value. This may indicate that oxygen is bound in molecules. Going to lower altitudes will put us deeper in the exosphere, increasing our chances of measuring weak species.