Science and Space

space and astronomy articles.

Mercury

A cosmic oddity.

Mercury is closer to the sun than any other planet, but its temperature 840°F is not quite the hottest. That distinction goes to nearby Venus, where the temperature climbs to 900°F, owing to the planet’s dense, heat-trapping atmosphere

Yet there may be water ice in permanently shadowed polar crate. At the poles, where the sun is low on the horizon, light may never shine inside some craters. That means water, delivered by a comet or left over from the formation of the planet, could remain frozen indefinitely.

Mercury spins so slowly and scoots around the sun so fast that a Mercury year — just 88 Earth days — is half as long as a Mercury day.

On the planet’s illuminated side — where the sun looks three times as big as it does from Earth and is 11 times as bright — temperatures climb to 840°F. When that side rotates into darkness, the thermometer plunges to —300°F. Reasons for Mercury’s wild temperature swings are its lack of any appreciable atmosphere and its reflective surface that bounces 90% of the sun’s heat back into space Eons of this rotisserie roll have cooked Mercury down to a nub with a metal core that represents three-quarters of its diameter.

High density

Because Mercury is extremely dense for its size — it’s comparable to Earth — researchers believe it has a large metallic, most likely iron core. But exactly how large the core is, whether its outer regions are molten, and whether it rotates to power the planet’s strong magnetic field, are still unknown.

Astronomers have put forward several theories to explain Mercury’s extreme density.

According to one version of events, denser elements, such as iron, were drawn closer to the sun in the region of space where Mercury formed when the solar system was created from a swirling cloud of gas and dust.

Other experts think that the younger and hotter sun vaporized a large part of the planet’s original rocky exterior, leaving behind the iron core.

And still others hypothesize that much of Mercury’s outer surface was blasted away in a collision with another planet-sized body at the dawn of the solar system.

Magnetic field

Scientists believe Earth’s magnetic field — which acts like a giant bar magnet at the poles — is generated by swirling motions of molten rock that surrounds the solid core. Mars, the moon and possibly Venus once had magnetic fields, but they apparently went dormant when the interiors of these celestial bodies cooled and solidified.

Mercury, from all appearances, also looks like a dormant planet. The half of the planet is pitted with ancient craters and there are no indications of recent volcanic activity. Huge escarpments stretch for hundreds of kilometres, giving the impression that Mercury cracked and shrank during a cooling process long ago. Even so, this odd little planet is somehow producing a magnetic field.

Under the shrinking surface theory, researchers believe that Mercury’s crust first formed over a gigantic molten core. As that core cooled, it led to a volume change causing the surface to buckle and break.

Unlike water, which actually expands as it cools, most materials tend to contract and the same goes for Mercury’s rocky crust. Based on observations of the planet’s known hemisphere, scientists estimate the planet’s surface has shrunk inward between less than one kilometer and three kilometers

Unlike Earth, Mercury does not spin on a tilted axis, which means a crater at its north or south pole would be in permanent shadow. And Mercury’s ultra-thin atmosphere does not transport heat from the equator to the poles, as Earth’s does.

The floor of a shadowed crater would never see the sun and would be cold enough — minus 300 degrees Fahrenheit or colder — to freeze water for the lifetime of the planet.

Previous mission - Mariner-10

Three decades ago, the NASA sent its Mariner 10 spacecraft to Mercury on a pioneering mission. Looping around Venus and the Sun, the craft made three swift flybys of Mercury during 1974 and 1975, sending back almost 1,000 pictures that mapped only 40 percent of the surface of the heavily cratered planet. Mariner found iron-laden Mercury to be the densest planet in the solar system and the only inner planet besides Earth with a global magnetic field, but left scientists wanting to know more.

Current mission - Messenger

Messenger is one of NASA’s lower-cost, rapidly executed Discovery Program robot missions designed to go from planning to flight in about three years. Pulling off the $427 million mission requires getting gravitational boosts from planetary flybys to get the spacecraft to Mercury and maneuver it into orbit.

Even though Mercury is 50 million miles from Earth at closest approach, Messenger will travel 5 billion miles to get there. It’s technologically infeasible to fly straight to Mercury and so the spacecraft must swing once past Earth, twice past Venus and thrice past Mercury before slowing down enough to slip into orbit around Mercury.

The technology for designing a spacecraft capable of withstanding harsh heat for prolonged periods was unavailable until recently. The main body of the spacecraft, made of a lightweight, heat-tolerant graphite composite material, is covered with multilayered insulation and peppered with radiators and heat pipes to channel heat away. The craft, powered by two electricity-generating solar panels, weighs 2,424 pounds at launching, half of that rocket fuel for its maneuvering engine and control thrusters.

The most distinctive feature of the spacecraft is a large, highly reflective, heat-resistant sunshade attached to the front on a titanium frame. Measuring eight feet tall and six feet across, the quarter-inch-thick shield is made of front and back layers of Nextel ceramic cloth surrounding inner layers of Kapton plastic insulation.

Temperatures on the front of the white shield could reach 700 degrees Fahrenheit when Mercury is closest to the Sun, engineers said, but the spacecraft on the shady side should operate at a room temperature of 68 degrees. Messenger is programmed to keep the shade between itself and the Sun at all times.

Messenger is to circle Mercury for one Earth year in an oval-shaped orbit that takes it 124 miles above the surface at the nearest point and 9,420 miles out at the farthest. Cameras will send back images showing features as small as 60 feet across; a battery of spectrometers will identify the elements and compounds on the surface; a laser altimeter will map landforms and distances; and other experiments will study variations in the planet’s gravity and magnetic field.

Once its mission is accomplished in 2012, Messenger will keep orbiting until it eventually crashes onto the surface.

posted by: kyawoo at 10:57 | link | comments |
mercury

Comments: