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Mercury makes a rare appearance in the evening sky this week.
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February 21, 2006: It's not every day you get to
see a shrinking planet. Today could be the day.
Step
outside this evening at sunset and look west toward the glow
of the setting sun. As the sky fades to black, a bright planet
will emerge. It's Mercury, first planet from the sun, also
known as the "Incredible Shrinking Planet."
"This
is only the second time in my life I've seen Mercury,"
says sky watcher Jeffrey Beall who snapped this picture looking
west from his balcony in Denver, Colorado:
Above:
Mercury on Feb. 13, 2006. Credit: Jeffrey Beall.
Mercury
is the bright "star" just above the mountain ridge,
rivaling the city lights.
Mercury
is elusive because it spends most of its time hidden by the
glare of the sun. This week is different. From now until about
March 1st, Mercury moves out of the glare and into plain view.
It's not that Mercury is genuinely farther from the sun. It
just looks that way because of the Earth-sun-Mercury geometry
in late February. A picture is worth a thousand words: diagram.
Friday,
Feb. 24th, is the best day to look (sky
map); that's the date of greatest elongation or separation
from the sun. Other dates of note are Feb 28th (sky
map) and March 1st (sky
map) when the crescent moon glides by Mercury—very pretty.
When
you see Mercury popping out of the evening twilight, you're
looking at a very strange place. "Shrinking" is
a good example:
 In
1974, NASA's Mariner 10 spacecraft flew by Mercury and, for
the first time, photographed the planet from close range.
Cameras revealed a densely cratered world—with wrinkles. Planetary
geologists call them "lobate scarps" and, like wrinkles
on a raisin, they are thought to be a sign of shrinking. What
would make a planet shrink? One possibility: Mercury's oversized
iron core has been cooling for billions of years, and its
contraction may be the driving force behind the wrinkles.
No one knows for sure.
Right:
Discovery Rupes, a lobate scarp on Mercury. [More]
No
one knows because Mercury has hardly been explored. Only one
spacecraft has ever been there, and during its oh-so-brief
visit Mariner 10 managed to photograph less than half (45%)
of Mercury's surface: image.
The majority is terra incognita.
Another
puzzle is the mystery-substance at Mercury's poles. Radio
astronomers have pinged Mercury from afar using radars on
Earth, and they have found something very bright in Mercury's
polar craters. Again, no one knows what it is, although a
favorite possibility is ice. Frozen water is a good reflector
of radio waves and would explain the observations nicely.
Right:
Arecibo radar images of north polar craters on Mercury. The
bright features might be deposits of ice. [More]
 How
could frozen water exist on Mercury? The sun heats the planet's
surface to 400 °C (750 °F) or more, too hot for frozen anything.
Yet deep down in some polar craters, researchers believe,
the sun never shines. In permanent shadow, the temperature
drops below -212º C (-350° F). Suppose a piece of an icy comet
or meteorite landed in such a crater; some of the ice might
survive.
Or
it could be something else entirely.
What
does the unknown half of Mercury look like? Is the planet
really shrinking? Can ice stay frozen in an inferno?
Mercury poses many questions: list.
A new NASA probe named "MESSENGER" is en route to
find some answers, but it will not reach Mercury until 2008.
For
now, one can only peer into the twilight and wonder. Give
it a try, this evening.
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Author: Dr. Tony
Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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