Observations of Mercury’s Sodium Tail
Andrew Potter (NSO) & Rosemary Killen (University of Maryland)
W
hy does Mercury display a tail of escaping sodium atoms?
The velocity for escape from Mercury is 4.25 km/sec,
corresponding to an energy of about 4.3 eV for sodium
atoms. However, the energies provided to sodium atoms by the source
processes on Mercury (photosputtering, particle sputtering, and
meteoroid impact) are not sufficient to allow the atoms to escape from
the planet.
The answer must be that solar radiation accelerates the atoms, adding
enough energy to whatever source energy they initially possessed to
allow them to escape into the tail. Solar radiation acceleration can be
exceptionally large for sodium on Mercury, reaching 54% of surface
gravity at maximum radial velocity. The minimum radiation acceleration needed to boost sodium atoms into the tail is a measure of how
much energy must be added for escape. The difference between the
energy provided by this minimum value and the escape energy is a
measure of the initial source energy of the tail atoms.
pear. This is shown in figure 1, where radiation acceleration is plotted
against average source rate and extrapolated to zero source rate. For
the case that Mercury is moving away from the Sun, the limiting
value of radiation acceleration required to produce an observable
sodium tail was estimated to be 112 ± 24 cm/sec2. For observations
where Mercury was moving toward the Sun, the emission intensity
in the sodium tail decreased very rapidly with distance downstream,
disappearing entirely beyond 12,000 kilometers (6 Rm) for radiation
accelerations of 128.7 and 135.4 cm/sec2. For smaller radiation accelerations, the sodium tail was not detectable at all, yielding a limiting
value for tail generation of about 122 ± 2 cm/sec2.
Previous work (Potter, et al., Icarus 186, 571, 2007) has shown that
sodium atoms on Mercury are exposed to solar radiation acceleration for an average of about 1,700 seconds before they interact with
the surface, or are photoionized. We can use this time to calculate the
energy added to a sodium atom by solar radiation acceleration. The
difference between this energy and the energy required for escape represents the energy that must have been imparted to the sodium atom
by the source process in order for the atom to escape. These differences are plotted in figure 2, where the additional energy required for
escape is plotted against radiation acceleration.
Figure 1. The minimum values of radiation acceleration needed to
generate sodium atoms in the tail are represented by the intercepts.
To find the minimum radiation acceleration needed for tail formation,
we used the NSO/Kitt Peak McMath-Pierce Solar Telescope to measure cross-sections of the sodium tail intensity at different distances
downstream for a range of values of solar radiation acceleration, both
for Mercury approaching the Sun, and Mercury receding from the
Sun. These data were used to calculate the rate that sodium atoms
were projected from Mercury into the tail in terms of atoms/sec.
Over the range of radiation acceleration values studied, the source
rates ranged between 2 and 6×1023 atoms/sec, corresponding to only
10–20 grams/sec of sodium projected into the tail. This represents
somewhere in the range of 1 to 10% of the estimated total sodium
generation rate on the planet.
With these data, we were able to find the radiation acceleration value
at which the sodium source rate and, consequently, the tail disap-
Figure 2. The energy needed for escape from Mercury of sodium
atoms accelerated by solar radiation. Data points represent the
observed limits for tail generation.
The limiting energies derived from the observed limiting radiation
accelerations are plotted as individual points on this graph. Their
position shows that the source process for sodium atoms that enter
the tail must impart energies in the range 3.2 – 3.5 eV. This range is
consistent with the value of 2.9 eV calculated for sputtering of sodium
atoms by solar wind particles (Wurz et al., Icarus, 2007). We conclude
that sodium atoms in the tail have been generated by sputtering from
the surface of Mercury by the impact of solar wind particles. These
results have been accepted for publication in Icarus. NL