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  Night Sky

Enjoy our

starry

night skies

Be a considerate neighbor:
Reduce nighttime glare.
Shield your outside
lights dowward,
so no glare goes up to
dull the night sky
(or into your neighbor’s windows)
and enjoy the beautiful stars above.

July 2011 Night Sky

Mysteries of Mercury

Charlie Christmann

The innermost planet in our solar system has many mysteries to reveal, and NASA’s Messenger spacecraft, now in orbit around Mercury, is starting to reveal a few answers. Mercury is the smallest planet (remember, Pluto is no longer considered a planet by most astronomers) and has been compared to our Moon. Since Mercury is normally lost in the glare of the Sun, it can be viewed from Earth’s Northern Hemisphere only in morning or evening twilight or during a solar eclipse. From ground-based telescope observations and a brief fly-by visit from the Mariner 10 spacecraft in the 1970s, little is known about this planet.

Hopefully, Messenger can answer some of the questions haunting scientists: Why is Mercury so dense? Mercury is smaller than the largest moons in the solar system, Ganymede and Titan. Mercury is the second-densest planet in the solar system, just a smidge under that of Earth. Scientists think Mercury must have a giant, iron core that makes up two-thirds of its mass. Chemical analyses of Mercury’s surface by Messenger’s instruments may help provide an answer.

Why does Mercury have a magnetic field? Besides Earth, Mercury is the only other inner solar system planet to have a significant magnetic field. Mercury’s field is only about one percent of Earth’s. Researchers believe Mercury’s magnetic field is generated the same way as the Earth’s field is made; however, Messenger seems to contradict those ideas. For starters, Mercury’s magnetic field lies significantly north of the planet’s geographic equator, with the southern field strength lower than the northern strength, allowing the southern hemisphere more exposure to solar wind.

You might think Mercury is a bad place to look for ice, but some earth-based radar observations seem to show there may be ice there. Sun-side temperatures can reach 840 degrees Fahrenheit, so where is the ice? The Messenger probe data indicates that some polar craters may be so deep that their floors are in permanent shadow. These polar craters could have temperatures that never rise above -280 degrees Fahrenheit. Whether they actually contain ice will have to be confirmed by different instruments just beginning to return data.

Even though Mercury has very low gravity, it seems to have a tenuous atmosphere. And Mercury is losing this atmosphere, contributing to the comet-like tail that trails the planet. Scientists think captured material from the solar wind contributes to some of the atmosphere, as well as dust kicked up by micrometeorite impacts.

Messenger has already provided about 20,000 photographs of the surface. Those show a northern volcanic plain that covers 1.54 million square miles, nearly half the size of the continental U.S. And observations help confirm that volcanism has substantially impacted Mercury’s crust and surface. X-ray spectrographic studies have shown feldspar is the dominant mineral on the surface. It also detected surprisingly high levels of sulfur at the planet’s surface. Observations are putting to rest the notion that Mercury is similar to the Moon.

Other early data also indicates that Mercury is very different from Venus, Earth, and Mars.

Messenger is only one quarter into its yearlong mission. Many questions about the inner planet will be answered, but many more questions will most likely be asked.

A Big Thanks

Thanks to my new friends in Cub Scout Pack 241, Den 16, for having me as a guest to talk about our night sky.

The Planets and Moon

  • Mercury can be found low on the western horizon after sunset. The day-old crescent Moon joins Mercury on the 2nd. Look for Mercury six degrees to the upper right of the Moon 30 minutes after sunset―binoculars will be a big help. Mercury will be highest above the horizon on the 19th.
  • Venus rises in the east from 5 a.m. early in the month to 6 a.m. by the end of the month.
  • Earth reaches aphelion, its farthest point from the Sun in its elliptical orbit, on the 4th. At this point, Earth is moving at its slowest speed, making the northern summer longer than the other seasons.
  • Mars rises in the east from 3:45 a.m. early in the month to 3 a.m. by the end of the month. Find Mars three degrees to the lower left of the Moon on the 27th. On the 5th, look for Aldebaran, an orange giant star, to the lower right of Mars.
  • Jupiter rises in the east from 2 a.m. early in the month to midnight by the end of the month. One hour before sunrise on the 23rd, look for the Moon in the southeast seven degrees to the lower left of Jupiter.
  • Look for Saturn low in the west southwest after sunset. Saturn will be nine degrees to the upper right of the crescent Moon on the 7th, two hours after sunset. To the upper left of the Moon will be the star Spica. On the 8th, the Moon, Spica, and Saturn almost form a straight line an hour after sunset in the southwest.
  • The Moon will be new on the 1st at 2:54 a.m. The full Moon occurs at 12:40 a.m. on the 15th.
   

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