This plot shows Lyman-alpha emission at 121.6 nm associated with neutral
hydrogen in the near vicinity of Mercury. This is the first detection of
hydrogen tail emission at Mercury and the first time that neutral hydrogen
and sodium atoms have been observed in the tail simultaneously. This
emission is about 100 times less intense than the sodium emission. As with
the sodium emission, discovering the true spatial distribution requires
more analysis. The similar asymmetries in hydrogen, derived from the solar
wind, and the much heavier sodium nonetheless suggest that solar-wind
interactions with Mercury's magnetosphere have played a strong role in
supplying tail material at the time of MESSENGER's flyby.
Observing the Lyman alpha emission line, deep in the ultraviolet, is
possible only from space. Such hydrogen emissions were also observed by
Mariner 10 but only on the subsolar limb.
Calcium was detected in the near-Mercury exosphere by MESSENGER and has
also been observed telescopically from Earth. Other species are expected
to be seen in Mercury's exosphere as well, but the orbital phase of the
mission offers better opportunities to observe them.
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.
