Life
Earth is the only planet with life
Mars and the moons Europa and Titan may have the ingredients for life
Comets may have delivered the ingredients for life on Earth
The most significant change since the solar system's formation is the development of life. Earth is the only planet known to support life. Our planet is at just the right distance from the Sun for life; the temperature is warm enough for liquid water, and we have a thick atmosphere. The first one-celled organisms appeared on Earth at least 3.5 billion years ago, and perhaps as early as 3.9 billion years ago. But larger, more complex life forms did not appear until about 600 million years ago.
Life may have evolved elsewhere in the solar system. Scientists reported that microscopic life might have inhabited Mars about 3.6 billion years ago, although additional tests seem to refute the claim. Recent observations by Mars Global Surveyor, however, hint at the presence of large underground reservoirs of water on Mars, which would strengthen the possibility that life evolved on the planet. Water once flowed across the Martian surface, and might still exist in underground reservoirs, raising a slight hope that Mars might yet be inhabited by simple organisms.
Liquid water also appears to exist on one of Jupiter's moons, Europa, beneath a mantle of ice. Complex organic molecules abound on Saturn's largest moon, Titan, although temperatures there are so cold that water would be frozen solid. And organic molecules exist in icy comets, leading some to suggest that comets brought the "seeds" for life to Earth -- and perhaps to other planets and moons, too.
On to The End »
|
Contact StarDate | About StarDate | Friends of McDonald | Sign up for Sky Tips
