Sky & Telescope - May 2022 - 53

GREGG DINDERMAN / S&T, MERCURY: NASA / JOHNS HOPKINS UNIVERSITY
APPLIED PHYSICS LABORATORY / CARNEGIE INSTITUTION OF WASHINGTON
The ions in the solar wind are
electrically charged, so their trajectories
are influenced by magnetic fields.
Earth's magnetic field deflects the solar
wind and prevents our atmosphere
from being stripped away, but Mercury's
magnetic field has only 1% the
strength of our planet's and provides
scant protection. The ions from the
solar wind strike Mercury at velocities
of hundreds of kilometers per second,
transferring their energy and dislodging
atoms from surface minerals in a
process known as sputtering.
Micrometeorite bombardment
appears to be a second source of Mercury's
sodium. The flux of meteoric
material increases near the Sun and
is about eight times more intense at
the orbital distance of Mercury than
at Earth's. Instruments aboard NASA's
Messenger spacecraft, which orbited
Mercury between 2011 and 2015,
detected a transient plume of sodium
during a meteor shower on Mercury
associated with material shed by Comet
2P/Encke. No coronal mass ejection or
other energetic solar event occurred at
the time, so an impact origin for the
plume seems highly likely.
In addition to sodium, Messenger's
instruments also detected potassium,
calcium, and magnesium in Mercury's
exosphere, but these elements lack
sodium's strong spectral signature. The
same solar radiation that sputters atoms
of sodium and other metals off Mercury's
surface sweeps them away from the
Sun to form a comet-like tail, fulfilling
a 1986 prediction by the Chinese
planetary scientist Wing-Huen Ip. A tail
extending 41,000 km from Mercury
was detected in 2001 by Andrew Potter
of the National Solar Observatory using
the huge, 1.6-meter-aperture McMathPierce
Solar Telescope atop Kitt Peak in
Arizona.
Seven years later, Jeffrey Baumgardner
and his colleagues at Boston University's
Center for Space Physics tried
wide-angle imaging using a narrowband
filter that selectively transmitted
sodium's yellow emission lines. They
were amazed to record a tail extending
more than 2.6 million km from the
Sodium tail
Solar
wind
Space
weathering
weathering
Micrometeoroids
pMercury's sodium tail is produced when ions from the solar wind and a constant rain of
micrometeoroids vaporize Mercury's surface rocks. At high altitudes the released sodium atoms
overcome the planet's gravitational influence and are accelerated in the anti-sunward direction by
radiation pressure to form a tail.
planet. From an Earthbound vantage
point, this tail extended 1½°, three
times the apparent diameter of the full
Moon. Baumgardner's team determined
that an atom of sodium blasted from
Mercury's surface takes only 15 hours
to be driven to the end of its tail.
The Messenger spacecraft found
dramatic changes in the abundance
of sodium in Mercury's exosphere and
the intensity of sodium emission from
its tail. In 2020 Japanese investigators
presented evidence that increases in the
amount of sodium released by micrometeoroid
impacts during encounters
with cometary dust streams are responsible
for this variability.
Amateurs can record images of
Mercury's tail using surprisingly modest
equipment. A Wratten 12 or 15 yellow
filter will darken the background sky by
selectively transmitting light emitted
by ionized sodium while blocking other
wavelengths. Even higher contrast can
be achieved with a narrowband dielectric
interference filter centered at 589.2
nanometers like the one available at
https://is.gd/Sodiumna. Still, the tail is
exceedingly faint and requires an exposure
of several minutes. Stacking several
short exposures is also a viable option.
Mercury is notoriously difficult to
observe due to its close proximity to
the Sun. This spring, the planet has its
most favorable elongation for northern
observers, reaching greatest eastern
elongation on April 29th. At sunset on
that date the planet will be nearly 19°
above the western horizon for observers
at a latitude of 40°N. During the last
week of April and the first week of May
it should be possible to image Mercury
against the backdrop of a reasonably
dark sky. The planet sets as twilight
deepens, so the window of opportunity
will be brief. A transparent sky free of
light-scattering haze is essential. Careful
preparation and no small amount
of luck are required to record one of the
solar system's largest but most insubstantial
features.
¢Although Contributing Editor TOM
DOBBINS regards Mercury as an unrewarding
telescopic target, he's keen to try
to capture an image of its tail.
sk yand tele scope .o r g * MAY 2022 53
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https://www.is.gd/Sodiumna

Sky & Telescope - May 2022

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