See the Incredible Shrinking Planet
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:
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:.
Friday, Feb. 24th, is the best day to look (); that's the date of greatest elongation or separation from the sun. Other dates of note are Feb 28th ( ) and March 1st ( ) 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:
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]
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.
Sky maps:, , , , , , , and .
MESSENGER -- NASA's latest mission to Mercury, managed by the John Hopkins Applied Physics Laboratory
Mercury, a Cratered Inferno -- (APOD) the half of Mercury photographed by Mariner 10 looks eerily similar to Earth's moon.
Mercury -- (The Nine Planets) an overview of basic facts and recent discoveries
The Discovery of Water Ice on Mercury -- (NRAO) a brief account of the discovery; includes an early radar image of Mercury.
High-resolution radar images of Mercury's north pole -- (Arecibo)