This month, Mercury makes one of its fleeting appearances in the evening sky. It swung around the far side of the sun on Feb. 7 and has been slowly rising into the sunset sky. It was farthest from the sun at 4 a.m. today, in a position known as "greatest eastward elongation." In astronomy, a body's elongation is its angular distance away from the sun. Mercury's maximum was only 18 degrees, so it will set only an hour and a half after the sun (about 7:30 p.m.).
After today, as it swings to pass between Earth and the sun on March 21, it will move down toward the horizon at sunset and be lost in the glare of dusk in another week or so.
Mercury is the smallest planet now that Pluto has been formally designated a dwarf planet. It is also closest to the sun and has a year only 88 Earth days long.
Its proximity to the sun means that the Hubble Space Telescope can't be aimed at it since even a small error in pointing could expose the sensitive instruments to full sunlight. So
this world, 40 percent the size of the Earth and 40 percent its distance from the sun, has been mostly ignored since Mariner 10 made three flybys in 1974 and 1975. The photos returned then gave evidence of a cratered world very much like our moon, but without the extensive lava flows forming the dark "seas" such as tranquility and serenity that appear as a man or a rabbit.
That changed in 2004 when the "Mercury, Surfaces, Space Environment, Geochemistry and Ranging" (MESSENGER) Mission was launched by NASA. After two
flybys of Venus and three of Mercury, on March 18, 2011, it settled into orbit to make a systematic study of its geology, surface material, gravitational and magnetic
fields, and solar-wind interactions. The first results were released last October at a meeting of European and American astronomical organizations in Nantes, France. The data show that Mercury, like all the worlds our probes have visited, is much more interesting than we imagined. Unlike Venus, Mercury has a planet-wide magnetic field. Planetary fields, like Earths, are created by the motion of liquid iron and nickel deep in the interiors and mimic the field of a bar magnet placed in the core. In Mercury's case, that bar magnet is displaced 20 percent of its radius to the north of the core.
This means the magnetic field is about 3.5 times weaker in the south than in the north. Thus the solar wind, charged particles flung off the sun at millions of miles per hour,
impact the southern regions of the planet more strongly than the northern. The impacts "sputter" material off the surface to form an extremely thin atmosphere, called an exosphere. Some complex features of the exosphere are proving a challenge
On the surface, scientists have identified broad volcanic plains covered in rocks very much like basalt, a common volcanic rock on Earth. They have also identified unexpected
features dubbed "hollows" where it appears that magma beneath the surface withdrew, allowing the surface to cave in. Thus Mercury has been much more geologically "active"
than the Mariner 10 images indicated. To explore the images created from Messenger data and read more about what we're learning about this illusive world, visit NASA's
Messenger website at www.nasa.gov/mission_pages/messenger/main.
Joining Mercury in the sunset sky are Jupiter and Venus that have been delighting us all winter with their brightness and motion. Today, Jupiter is only 7 degrees northeast of
Venus (less than the width of your fist held at arm's length), as shown in the diagram for sunset.
Of course, you'll have to wait for the sky to darken in order to see all the planets. But don't wait too long as Mercury is only 16 degrees above the horizon and will sink quickly, The circles show how all three planets will move between now and Monday, March 13. Mercury will head back toward the sun, more quickly than Jupiter's steady
sunward march. Venus, however, will continue to move farther from the sun until March 27 (see "The Wilderness Above" for December 27, 2011). Their paths bring the two
bright worlds within 3 degrees on March 13. If we're lucky enough to have clear skies on that day, grab a pair of binoculars and enjoy gazing at two worlds in a single field of view.
If you have questions about the natures and motions of the planets or any other astronomical topic, please visit the Adirondack Public Observatory web site at apobservatory.org or email Aileen at firstname.lastname@example.org.