Passage IV

NATURAL SCIENCE: This passage is adapted from the article “The Mars Model” by Bridget Mintz Testa, which appears in Discover Magazine (©1995 by The Walt Disney Company).

The first close-up images of Mars, captured in 1972 by the probe Mariner 9, were a planetary scientist’s dream: they revealed networks of valleys that looked uncannily like drainage basins and streambeds back here on Earth and thus implied that there had once been water freely flowing over the surface of Mars. The images also implied that Mars had once had a thick atmosphere. Our planet is blessed with liquid water on its surface only because it has a thick atmosphere to maintain a high pressure and trap the sun’s heat. So planetary scientists proposed that when Mars formed 4.6 billion years ago, it too had a dowry of a heat-trap-ping atmosphere, composed of carbon dioxide and water vapor. With warmth, water, and air, they speculated, Mars might once have been a garden world, a paradise among planets.

But, as they also discovered, the garden didn’t last long. None of the streambeds were younger than 3.7 billion years. Something happened to Mars, something that stripped nearly all of its atmosphere, killed its streams, and froze the garden forever.

Researchers have suggested many scenarios for the Martian apocalypse. Some have proposed that the sun gradually whittled away Mars’ atmosphere with its wind of charged particles. Others have hypothesized that the planet itself absorbed most of its atmosphere, turning carbon dioxide into carbonate rocks. For the past seven years, however, Ann Vickery and Jay Melosh, two planetary scientists from the University of Arizona, have been exploring a far more spectacular ending: most of Mars’ atmosphere, they suggest, was blasted away by a succession of asteroids and comets.

“The basic idea,” Melosh says, “is that an impact doesn’t just open a crater. With high velocities, the projectile vaporizes and expands into the atmosphere.” This superheated expanding plume shoves the atmosphere above it like a snowplow pushing snow to the heavens. How high a vapor plume goes depends on the mass and velocity of the object that crashed into the planet. If it is big enough and fast enough, it can drive its plume straight back up into space. The portion of the atmosphere it plows away is then stripped from the planet forever.

To see if this process could account for Mars’ missing atmosphere, Vickery and Melosh essentially ran a film of the Red Planet in reverse, starting with today’s wispy atmosphere and adding back the air that might have been removed by impacts over the cons. First they derived a mathematical expression relating time to the rates of both impacts and atmosphere loss. Using this expression, they then ran the clock backward to find out how long it would take to “grow” an ancient, Earth-like atmosphere from Mars’ current tiny one. If their model was right, it would produce the original, early Mars. And the time it took to “grow” a thick atmosphere by going backward would be the same as the time it took to lose a thick atmosphere, traveling forward. Using an impact rate that prevailed 3.7 billion years ago—one impact every 10,000 years—Vickery and Melosh were able to start with a virtually dead planet and grow a thick atmosphere in only 600 or 700 million years.

However, now that an attractive explanation finally exists for how the young paradise of Mars was destroyed, some researchers are questioning whether that paradise ever existed in the first place. Studies of other stars suggest that the young sun was 25 to 30 percent dimmer than it is today. Mars, which is 49 million miles farther from the sun than is Earth, would have been receiving less than a third of the sunlight we now enjoy. Some scientists have calculated that given so little sunlight, Mars’s atmosphere wouldn’t be able to trap enough heat to keep water from freezing and that under such conditions, carbon dioxide would form frozen clouds. What little sunlight Mars receive would bounce off the clouds, and the planet would cool even further. As to how the Martian valleys we see today might have formed without a warm atmosphere, it is suggested that the planet might have been covered by large expanses of ice and that the heat from Mars’ interior could have thawed out hidden channels.

Vickery, however, is sticking by her original assumptions. “There exist on Mars valley networks that look like terrestrial river valley networks and don’t look like any other kind of feature found anywhere else in the solar system.” she points out. “The first, obvious interpretation is that these networks were formed more or less the same way as similar terrestrial networks.”