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The Sun Astronomy - Planet Mercury planet Venus

Mercury, taken by Mariner 10. Mass: 3.303 x 1023 kg (0.0553 Earths)
Radius (equatorial): 2 439 km
Mean density: 5.42 g cm-3
Distance from Sun: 57 910 000 km
Rotational period: 58.6 days
Orbital period: 88 days
Escape velocity: 4.25 km s-1
Apparent magnitude: -1.90
Surface temperature: 179C (427deg;C, max. -173C, min.)
Atmospheric composition: Helium (42%), sodium (42%), oxygen (15%).
Number of satellites: None

Characteristics

Mercury is the second smallest planets (smaller than Saturn's satellite Titan). Its orbit is highly elliptical. It is only 46 million km from the Sun at its perihelion but 70 million km at its aphelion. There are only three 'Mercury days' in two 'Mercury years'. In orther words, it rotates three times as it completes two orbits round the Sun.

Mercury has a very thin atmosphere. Because Mercury is so close to the Sun, most of the atomsphere has boiled off and escape to the space. The atmospheric residuals are constantly replenished from materials blast off from the surface by the solar wind. The lack of atmosphere means that the surface temperature is very extreme: from the low -173C to boiling hot 427C. The hot temperature is only surpassed by Venus but the latter temperature is very stable due to the presence of its thick atmosphere.

The surface features on Mercury is very similar to the Moon: peppered with craters and it has no plate tectonic. Mercury is, however, the second densest planet, followed closely after Earth. Due to its small size, Mercury is thought to contain no magnetic field as the core would have solidified long ago. However, it was during the Mariner 10 mission in 1974-75 found that Mercury has a very small magnetic field. The strength is only 1% of Earth's. This led to the conclusion that Mercury may have a partially molten iron core. Magnetic fields are generated from the rotation of a conductive molten core and is known as the dynamo effect.

The high density of the planet indicates that the planet is consisted of up to 70% of metal by weight and the rest silicates. This gives a core radius of 75% of the planet radius, or around 1850 km. Mercury is often visible to naked eyes but is difficult to spot since the planet is located low in the twilight sky. When looking at the projected image through a telescope Mercury may sometimes be seen as a small black disk moving slowly across the Sun (WARNING: Looking at the Sun directly with a telescope or binoculars only invites irreversible blindness).



Images

Beside Pluto, Mercury is the least explored (and least understood) planet. Much of our understanding of the planet are owed largely due to the Mariner 10 mission, the only space craft that ever visits the planet. For safety precaution, the HST is not used to observe Mercury due to its relative closeness to the Sun. Strong Sun light that accidentally enters the space telescope will overload the onboard light-sensitive equipment. Neither the ground-base telescope is of any useful because the image is impaired by the hazy atmosphere near to the horizon.

Mercury's southern hemisphereImage taken by the Mariner 10 space craft the Mercury's southern hemisphere.

Credit: Mariner 10, NASA/JPL

Mercury at close approachOne of the image taken by the Mariner 10 at the close approach of about 5900 km. Craters as small as 150 m across can be seen. These craters show in various stages of degradation. The ridges and long narrow hills resemble that of Moon.

Credit: Mariner 10, NASA/JPL

crustal compressionTaken from 64 500 km, a scarp, or cliff, is found to extend more than 300 km across diagonally from upper left to lower right of the picture. These features were also observed in other parts of the planet, with some even cut thru the rings of craters. They are believed to be formed by the compressive forces due to crustal shortening as the planet is cooled.

Credit: Mariner 10, NASA/JPL/Northwestern University

Links


Mariner 10 - The only space scraft that ever visited Mercury. No longer in operation since 1975.

MESSENGER - Mercury Surface, Space Environment, Geochemistry and Ranging, NASA's mission to Mercury. Launched in August 2004.

Bepi-Colombo - ESA proposed mission to Mercury.

The Mercury Watch - Lists of observational sites and campaigns dedicated to the study of Mercury.

Sun Planet Venus

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