THE INNER PLANETS MERCURY VENUS AND MARS Telescopic observers have noted distinctive bright and dark features on Mars for hundreds of years One of the sharpest images taken from the vicinity of Earth photographed by the Hubble Space Telescope captures the red planet on the last day of spring in the northern hemisphere The large dark marking just below and to the east of center is Syrtis Major Beneath it is the giant impact basin Hellas covered with an oval of white clouds NASA JPL David Crisp and the WFPC2 Science Team
INTRODUCTION study the inner planets from afar The devel opment of space probes and satellites has allowed us to send spacecraft to fly by and even land on these planets Such missions have resulted in a number of discoveries For instance we now know that Mercury shrank considerably as it cooled early in its history and that Venus though sizzlingly hot and dry today probably once had oceans And though we have yet to find signs of life on Mars much evidence has been col lected suggesting that Mars was once more Earth like with warmer temperatures as well as flowing water Even now space mis sions continue In 2011 NASA's Messenger probe is thoroughly exploring Mercury for instance As you read this book you will get S a sense of what humans have learned so far about our three neighboring planets and what we still have to discover NS
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
