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 an atmosphere. Our planet is blessed with liquid water on its surface only because it has an 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-trapping 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 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 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: Mars’ atmosphere, they suggest, was blasted away by a succession of asteroids and comets.
Their work is crucial to our understanding of just how important impacts have been in making the solar system what it is today. It was the crashing together of planetesimals that formed the planets and moons in the first place, and impacts may have killed off great swaths of life on Earth on several occasions. Now Vickery and Melosh, along with other researchers who have adopted their methods, have shown that impacts may have shaped the atmospheres of planets and moons throughout the solar system.
Vickery and Melosh were not the first to wonder whether Mars had been robbed of its atmosphere by extra-Martian bodies. Others had considered the idea a decade ago; but their equations, which focused on how a projectile would heat the atmosphere as it fell, showed that no appreciable amount of air would be removed. In 1988, however, Vickery and Melosh decided to take a more careful look at the full complexity of an impact on Mars. Their preliminary calculations showed that previous researchers had missed a key part of what happens after a projectile hits the ground.
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 eons. 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 an atmosphere by going backward would be the same as the time it took to lose an atmosphere, traveling forward.
Using today’s rate of one large impact about every 10 million years or so, Vickery and Melosh weren’t able, in the time allowed them by the age of the Martian valley channels, to grow an atmosphere significantly greater than the present one. But we live in a relatively peaceful time, comets crashing into Jupiter notwithstanding. As recently as 3.7 billion years ago, large impacts were peppering Mars not once every 10 million years but once every 10,000 years. Although at the time the planets were already 800 million years old, the solar system was still littered with rubble from its formation, which continued to generate more wreckage as it crashed into planets and moons. Taking into account the higher impact rate, Vickery and Melosh were able to start with a virtually dead planet 3.7 billion years ago and grow a thick atmosphere in only 600 or 700 million years.
Still, the researchers knew that their preliminary equations were very simple and the results they produced could be off by a factor of ten. We were well aware that there were a lot of approximations involved, says Melosh, and so in 1992, he and Vickery began running a step-by-step simulation of impacts on a supercomputer. With their program they could break up the atmosphere and ground into tiny cells and calculate what happens in each cell after an impact, balancing energy and momentum according to the basic principles of physics. Eventually they managed to get the more complex model to yield a prediction, one that other researchers would begin to take seriously. It turned out to be much like their earlier ones: a cataclysmic atmospheric devastation of Mars.
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. Penn State geoscientist Jim Kasting and Cornell space scientist Steven Squyres have calculated that given so little sunlight, even if an early Martian atmosphere was five times denser than Earth’s present one, it still wouldn’t be able to trap enough heat to keep water from freezing. With just carbon dioxide and water, you can’t get above about -45 degrees Fahrenheit, says Kasting. The carbon dioxide would form frozen clouds, and they would reflect light. What little sunlight Mars received 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, Squyres and Kasting have suggested that the planet might have been covered by large expanses of ice and that heat from Mars’ interior could have thawed out hidden channels. Other researchers have suggested that the valleys weren’t formed by water at all but by some other, unknown process.
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. This implies running water, and running water implies a thick atmosphere. Somehow a thick atmosphere had to be gotten rid of, because there is so little now.
Mike Carr, a geologist with the U.S. Geological Survey, also dislikes the idea of a cold early Mars. I think we’re missing something, he says. One possibility is that Kasting’s models are wrong and there could be other greenhouse gases like methane or ammonia. I’m truly puzzled by this, and I’m working on different aspects of the problem. Even Kasting, despite the questions he has raised, still prefers a warm early Mars to explain the features seen there today. The jury is still out as to what the early Martian climate may have been like, he says. In Vickery’s view, we may not discover the true answers until we get back to Mars.
The debate over Mars notwithstanding, Vickery and Melosh have helped lead other researchers to demonstrations that atmospheres throughout the solar system, both present and extinct, may have been profoundly shaped by impacts. In fact, impacts may not only destroy atmospheres, they may also build them--the key lies in the way something hits a planet. A few years ago at Cornell, planetary scientist Chris Chyba decided to see what effect the kinds of impacts that were destroying Mars’ air were having at the same time here on Earth. Some, he found, would have eroded Earth’s atmosphere as they did on Mars, but far more would not have had enough momentum. For an impactor to blow off atmosphere, it has to be big enough and fast enough to produce a vapor plume able to escape the gravitational pull of the planet. The escape velocity of Mars is 3.2 miles a second, which is not hard for impacts to produce. But to escape Earth, which is 9.3 times bigger than Mars, an object has to be moving 7 miles a second. Big planets, because of gravity, tend to retain what impacts them, says Chyba. On big worlds, impacts build atmospheres, while on small ones, erosion is more likely.
Those impacts deliver more than just air. Ice in the comets and asteroids hitting Earth vaporized into a gas that cloaked the planet, and eventually an ocean’s worth of water condensed out of the atmosphere. Chyba calculates that Venus too would have gained about the same amount of water. And even as Mars was losing its atmosphere, it would have trapped enough slow-moving comets to form a layer of water up to a few hundred feet deep.
Other researchers have begun reconstructing the history of impacts in the outer solar system. Kevin Zahnle of NASA wondered why Saturn’s moon Titan has a massive nitrogen atmosphere, whereas Jupiter’s moons Callisto and Ganymede, with almost the same mass and density as Titan, have none. Using estimates of the populations of comets swarming in the outer solar system, he calculated how much material crashed into the moons. Then he assumed, for the sake of simplicity, that the material arrived in an evenly distributed range of impactors, from small to large. His results showed the same atmospheres on the moons that we see today. The main factors are the same as the ones Vickery and Melosh encountered on Mars: mass and speed. Ganymede and Callisto orbit very fast in Jupiter’s deep gravitational well, whereas Titan moves much more slowly around smaller Saturn. An object destined to collide with one of Jupiter’s moons speeds up under the giant planet’s influence and then rams into the fast- orbiting moon. Under such conditions, a vapor plume has too much momentum to linger and add to an atmosphere; it simply heads out for space. But since Saturn’s gravitational field isn’t so strong, impactors don’t move so fast. Thus when they fall on the moons, they stay there.
Zahnle wanted to take another step closer to reality, though, by shedding some of the artificial assumptions of his first model and leaving some of the outcome to chance. He teamed up with Caitlin Griffith, a physicist at the University of Northern Arizona in Flagstaff, for a new approach. From Voyager 1 photographs, researchers have been able to estimate how much mass fell on Saturn’s heavily cratered moons Rhea and Iapetus. Griffith and Zahnle extrapolated from these results to calculate how much material fell on nearby Titan; extrapolating a little further, they also came up with figures for Jupiter’s Callisto and Ganymede. Guided by the ratio of large to small impacts on the Saturnian moons, they divided the appropriate mass into randomly sorted collections of comets, which they then hurled at 1,000 Titans, 1,000 Ganymedes, and 1,000 Callistos.
With their simulated collisions, Griffith and Zahnle found they could often bestow on Titan an atmosphere at least as big as the one it boasts now. Surprisingly, though, Callisto and Ganymede each got a small, early atmosphere. Those atmospheres may have been too small to survive ultraviolet radiation and Jupiter’s magnetic field, Griffith says. Interestingly, Callisto, which received a larger atmosphere than Ganymede in the simulations, now has a dark, uneven surface. It may be some kind of organic goo--a leftover processed atmosphere, says Griffith.
The next atmosphere Griffith will simulate is that of Mars. She can trade results with Vickery, who is inveigling her finicky computer program to a new level of complexity to find out what happens when impactors hit the planet at an angle. Vickery has recently found that an object crashing straight down to the ground doesn’t erode the atmosphere as much as earlier models had suggested. But, as she points out, the chances of such an impact are essentially nil. Slanted impacts are more likely, and Vickery suspects they would also destroy more of the atmosphere. An angled impact should blast its vapor plume forward along its path of entry, much like a long jumper drives sand forward when his feet hit the ground. Moving through the air at a low angle instead of straight up, the plume can snowplow more air ahead of it and into space. Just last year Vickery coaxed the first few seconds of such an impact out of the program and found the plume did indeed move sideways.
As Vickery and other researchers get closer to the reality of impacts, they are changing the way we view the planets and moons. The science of planetology has until now been a comparative one: researchers try to figure out why one planet is a scorching greenhouse, another a ball of gas, and another a haven for life by comparing intrinsic qualities of the worlds themselves, such as their primordial birthright of gases, minerals, and metals. Impacts, it now appears, may play a more important role in the birth and death of worlds. From the dead gardens of Mars, it seems, comes a new growth of understanding.
