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October
20, 2006: Mark your calendar: On Wednesday, Nov 8th,
the planet Mercury will pass directly in front the Sun. The
transit begins at 2:12 pm EST (11:12 am PST) and lasts for
almost five hours. Good views can be had from the Americas,
Hawaii, Australia and all along the Pacific Rim: visibility
map.
What will
it look like? A picture is worth a thousand words:
Image
credit: Larry
Koehn. Click to view the Sun's full disk.
During
the transit, Mercury's tiny disk—jet black and perfectly round—will
glide slowly across the face of the Sun. Only a speck of the
Sun's surface is actually covered, so the Sun remains as dangerous
as ever to look at. But with a proper filter and a little
imagination, the Transit of Mercury can be a marvelous experience.
There
are many ways to safely observe the Sun, e.g., through
eclipse glasses or by means of a pinhole projector. In this
case, nothing beats a telescope equipped with a sun-safe H-alpha
filter. H-alpha filters are narrowly tuned to the red glow
of solar hydrogen. They reveal the Sun as a boiling inferno,
cross-crossed by dark magnetic filaments and peppered with
sunspots. Warning: The sight of Mercury navigating this starscape
could be mind blowing.
Teachers,
call your local astronomy club and ask if they have such a
solar telescope. Amateur astronomers love to show off the
heavens, and someone will probably volunteer to bring their
'scope to your classroom for the transit. (You can also view
the transit online at the SOHO
web site--no telescope required.)
 Right:
Mercury, photographed by Mariner 10. [More]
Here's
something to think about while watching the transit: Mercury
is fantastically mysterious. More than half of the planet
is unknown to us. When Mariner 10 flew by in the mid-70s,
it managed to photograph only 45% of Mercury's cratered surface.
What lies on the other side? More craters? Or something totally
unexpected? You're free to speculate, because the next spacecraft
to visit Mercury, NASA's MESSENGER probe, won't enter orbit
until 2011.
One
of Mercury's greatest secrets is the mystery-material at its
poles. Radars on Earth have pinged Mercury and received a
strong echo from polar craters. A favorite explanation is
ice. While Mercury's daylit surface heats up to 400o
C, the temperature in deep, dark polar craters dips below
-200o C. If an icy comet landed in one of those
craters (or made one of those craters), the comet's
ices, vaporized by impact, might re-freeze and stick around.
As skeptics like to say, however, "it's just a theory,"
one of many that MESSENGER will check.
Another
puzzle is Mercury's wrinkles. Geologists call them "lobate
scarps." Like wrinkles on a raisin, the scarps are
thought to be a sign of shrinkage. Mercury may actually be
collapsing in on itself as its massive iron core cools and
contracts. To check this idea, MESSENGER will map Mercury's
magnetic field, which springs from the core. If the core is
collapsing, the collapse may leave telltale signs in the planet's
magnetism. MESSENGER
will also look for lobate scarps on the uncharted side of
Mercury to see if this is truly a global phenomenon.
The answers are years away. Meanwhile, we watch and wonder,
and Nov. 8th is a good day for that.
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Author: Dr. Tony
Phillips | Editor:
Dr. Tony Phillips | Credit: Science@NASA
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