CHARACTERISTICS OF MERCURY he planet that orbits closest to the Sun is Mercury It is also the smallest of the eight planets in the solar system These features make Mercury difficult to view from Earth as the small planet rises and sets within about two hours of the Sun Observers on Earth can only ever see the planet during twi light when the Sun is just below the horizon Relatively little was known about Mercury until the Mariner 10 spacecraft visited it in 1974 75 It was more than 30 years before another spacecraft Messenger visited the planet Mercury's orbit lies between the Sun and the orbit of Venus Along with Venus Earth and Mars Mercury is one of the inner planets nearest to the Sun The inner planets are also known as the terrestrial or Earth like plan ets They are dense rocky bodies that are much smaller than the solar system s outer planets Mercury has no known moons SIZE Mass AND DENSITY Mercury is the smallest planet in both mass and diameter It is about 18 times less massive IO
CHARACTERISTICS OF MERCURY Each view of the planet Mercury is a mosaic of many images taken by the Mariner 10 spacecraft during its first flyby of the planet in March 1974 Each view shows about half of the hemisphere that was in sunlight at the time NASA JPLNASA JPL than Earth With a diameter of about 3 032 miles 4 879 kilometers Mercuryis not quite two fifths the size of Earth It is only about a third larger than Earth's Moon In fact two moons in the solar system Jupiter s moon Ganymede and Saturn's moon Titan are II
THE INNER PLANETS MERCURY Venus AND MARS larger than Mercury However Mercury is larger than the dwarf planet Pluto For some 75 years when Pluto was classified as a planet Mercury was considered the second smallest planet Mercury is the densest planet in the solar system followed by Earth if one takes into account the planets internal compression because of gravity Mercury is unusually dense because it is composed of a high per centage of metal The metal is concentrated in a comparatively huge core which accounts for nearly 75 percent of Mercury s diameter AppEARANCE FROM EARTH Mercury can be seen from Earth without a telescope It always appears close to the Sun within about 28 angular degrees For this reason the planet can only be seen near the horizon At certain times of the year it appears as a morning star just before sunrise while at other times it appears as an evening star just after sunset Because Mercury's orbit lies between Earth s orbit and the Sun Mercury displays phases like those of the Moon and the planet Venus These phases can only be seen with the aid of a telescope Mercury sometimes 12
CHARACTERISTICS OF MERCURY O SO Five separate images were combined to show Mercury crossing in front of the Sun during a type of eclipse called a transit on Nov 15 1999 The false color images were taken in ultraviolet light by the Transition Region and Coronal Explorer TRACE satellite in Earth's orbit The time between successive images was about seven minutes NASA TRACE SMEX looks like a crescent to observers on Earth At other times when Mercury and Earth are in different positions more of the sunlight reflected off Mercury can be seen from Earth Mercury appears as DEAN then a half or fuller disk About a dozen times each century Mercury passes directly between Earth and the Sun This event called a transit is a type of eclipse During a transit of Mercury the planet appears as a small black disk against the background of the bright Sun Binoculars 13
