In the upper part of this figure, two histograms represent typical
observations in the tail region of Mercury’s exosphere from calcium (left)
and sodium (right) atoms. Known as “spectral lines,” these emissions have
been scaled to approximately the same peak level for ease of comparison;
however, the sodium emission is much brighter than that of calcium. Each
emission occurs at a unique wavelength, with that of sodium in the
“yellow” part of the visible spectrum and that of calcium in the “blue”
part. The sodium emission is actually two very closely spaced emissions
that are usually termed the D lines of sodium. The peaks of the two
emissions are just separated (indicated by the D2 and D1 labels) in the
figure. These are the same emissions that produce the yellow glow in
sodium vapor lamps often used in street lighting. Although both sodium and
calcium in Mercury’s exosphere have been observed with ground-based
telescopes on Earth, this is the first time that measurements of the two
species have been obtained simultaneously. Atoms in the exosphere heavier
than hydrogen and helium predominantly originate from the surface of
Mercury, and a number of processes contribute to their release from the
surface material. Differences in the spatial and temporal distributions of
the exospheric constituents therefore provide insight into the relative
importance of the processes that generate and maintain Mercury’s exosphere.
The middle image of this figure shows the spatial distribution of sodium
emission during MESSENGER’s second flyby in the tail region of Mercury,
which extends away from the planet in the anti-sunward direction. In the
image, north is up and the Sun is to the left. The color scale represents
the relative brightness of the sodium emission in the tail. Because the
observed emission intensity is related to the number of atoms along the
line of sight, images such as this one are a measure of the density of the
emitting species. The small-scale structures in these images may be
artifacts of the viewing geometry and should not be given too much weight.
More important are the broad-scale features that are composed of numerous
observations and are therefore a better representation of the overall
emission structure. The sodium emission shows two broad peaks that are
located close to the planet to the north and south, and there is less
emission near the equatorial region.
The bottom image shows the spatial distribution of calcium emission in the
tail region of Mercury during the second flyby. In contrast to the sodium
emission, the calcium emission is mostly symmetric about the equatorial
region and less bright near polar regions. The spatial variations between
the calcium and sodium distributions indicate that the processes
controlling these two species are likely different.
Date Acquired: October 6, 2008
Instrument: Mercury Atmospheric and Surface Composition Spectrometer (MASCS)
These images are from MESSENGER, a NASA Discovery mission to conduct the
first orbital study of the innermost planet, Mercury. For information
regarding the use of images, see the MESSENGER image use policy.