ince ancient times humans have gazed in wonder at the vast blanket of stars covering the night sky Records going back 5 000 years show that ancient Sumerian astronomers recognized that some stars moved differently than other celestial bod ies Three special objects they tracked are the three closest planets to Earth Mercury Venus and Mars These objects still fascinate today This book will take you on a journey to learn about our neighbors in the solar system The first stop on our tour will be tiny Mercury At less than two fifths Earth s size it is the smallest of the eight planets Because it is the closest planet to the Sun it can complete an orbit around the Sun in only 88 Earth days But it takes almost 59 Earth days to complete one rotation about its axis Mercury has almost no atmosphere leaving its dry and rocky surface little protection from crater blasts by asteroids Mercury is the densest planet in the solar system Our second stop is Venus the solar sys tem s third smallest planet and the one most similar in size to Earth Venus's atmosphere is remarkably massive Made mostly of thick heat trapping layers of carbon dioxide the heavy atmosphere makes it the hottest planet
INTRODUCTION in the solar system This thick atmosphere has also made it difficult to study the planet s surface Venus's orbit and rotation are also unusual A year on Venus is 225 days But the planet spins about its axis very slowly com pleting one rotation about every 243 Earth days making Venus the only planet in the solar system that takes longer to rotate once about its axis than to travel once around the Sun Venus and Uranus are also the only two planets that rotate counterclockwise The third stop on our journey Mars the second smallest planet in the solar system is only about half Earth's size The planet com pletes one revolution around the Sun in about 687 Earth days A Martian day lasts a little longer than 24 hours It also has two small potato shaped moons Observers have long wondered if there was life on Mars because it appears as if there are channels that might once have contained water a necessary ingre dient for life In fact there still is water on Mars but most of it is frozen beneath the planet s surface The thin atmosphere doesn't shield the surface from harsh ultraviolet radi ation or from being hit by space junk Since the beginning of the space age humans have had the power to do more than
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
