Pale Blue Dot - Carl Sagan [82]
TANTALIZING AND MAJESTIC, Mars is the world next door, the nearest planet on which an astronaut or cosmonaut could safely land. Although it is sometimes as warm as a New England October, Mars is a chilly place, so cold that some of its thin carbon dioxide atmosphere freezes out as dry ice at the winter pole.
It is the nearest planet whose surface we can see with a small telescope. In all the Solar System, it is the planet most like Earth. Apart from flybys, there have been only two fully successful missions to Mars: Mariner 9 in 1971, and Vikings 1 and 2 in 1976. They revealed a deep rift valley that would stretch from New York to San Francisco; immense volcanic mountains, the largest of which towers 80,000 feet above the average altitude of the Martian surface, almost three times the height of Mount Everest; an intricate layered structure in and among the polar ices, resembling a pile of discarded poker chips, and probably a record of past climatic change; bright and dark streaks painted down on the surface by windblown dust, providing high-speed wind maps of Mars over the past decades and centuries; vast globe-girdling dust storms; and enigmatic surface features.
Hundreds of sinuous channels and valley networks dating back several billion years can be found, mainly in the cratered southern highlands. They suggest a previous epoch of more benign and Earthlike conditions—very different from what we find beneath the tenuous and frigid atmosphere of our time. Some ancient channels seem to have been carved by rainfall, some by underground sapping and collapse, and some by great floods that gushed up out of the ground. Rivers were pouring into and filling great thousand-kilometer-diameter impact basins that today are dry as dust. Waterfalls dwarfing any on Earth today cascaded into the lakes of ancient Mars. Vast oceans, hundreds of meters, perhaps even a kilometer, deep may have gently lapped shorelines barely discernible today. That would have been a world to explore. We are four billion years late.*
On Earth in just the same period, the first microorganisms arose and evolved. Life on Earth is intimately connected, for the most basic chemical reasons, with liquid water. We humans are ourselves made of some three-quarters water. The same sorts of organic molecules that fell out of the sky and were generated in the air and seas of ancient Earth, should also have accumulated on ancient Mars. Is it plausible that life quickly came to be in the waters of early Earth, but was somehow restrained and inhibited in the waters of early Mars? Or might the Martian seas have been filled with life—floating, spawning, evolving? What strange beasts once swum there?
Whatever the dramas of those distant times, it all started to go wrong around 3.8 billion years ago. We can see that the erosion of ancient craters dramatically began to slow about then. As the atmosphere thinned, as the rivers flowed no more, as the oceans began to dry, as the temperatures plummeted, life would have retreated to the few remaining congenial habitats, perhaps huddling at the bottom of ice-covered lakes, until it too vanished and the dead bodies and fossil remains of exotic organisms—built, it might be, on principles very different from life on Earth—were deep-frozen, awaiting the explorers who might in some distant future arrive on Mars.
METEORITES ARE FRAGMENTS OF OTHER WORLDS recovered on Earth. Most originate in collisions among the numerous asteroids that orbit the Sun between the orbits of Mars and Jupiter. But a few are generated when a large meteorite impacts a planet or asteroid at high speed, gouges out a crater, and propels the excavated surface material into space.