People tend to think of mountain building as a process in which mountains rise continually, until the tectonic forces building them cease. But actually, it's a tug-of-war between uplift and erosion, with some interesting feedbacks between the two processes. Bigger mountains, for example, generate more rain and more glaciers. And taller mountains are drained by steeper streams, with greater erosive power. The same process also affects the width of a mountain belt.
One result is a feedback loop in which mountains tend to grow to the size at which they're eroding just as fast as they're rising. Another is that the size of a mountain range can change if something alters the balance between growth and erosion.[5]
[FOOTNOTE 5: For an elegant, detailed description of this process, see John McPhee's classic geology book, Basin and Range .]
In the case of the Alps, the Mediterranean salinity crisis may have produced a huge change to the balance between mountain building and erosion.
Six million years ago, the Alps were at their maximum extent, perhaps 100 miles wider and 5,000 feet taller than at present. Then they began to shrink. It's possible, of course, that this could simply reflect a change in tectonic forces. But in a study of erosional deposits in Italy's Po River Valley, Willett's team found signs of a large increase in the rate at which the mountains were eroding.
In an article in the August 2006 issue of the journal Geology, Willett and coworkers attribute this to the change in sea level in the Mediterranean. Although the coincidence in timing is only circumstantial evidence, the theory makes sense because the enormous drop in sea level would force the rivers to drop thousands of extra feet to reach the sea. It wouldn't take them long to start carving big canyons that would soon begin stripping away the mountains at their headwaters.[6]
[FOOTNOTE 6: Several of Italy's large lakes now lie in these canyons. Elsewhere around the Mediterranean, seabed mapping reveals enormous, now-submarine canyons carved by such rivers as the Rhone and the Nile.]
When the Mediterranean refilled, however, Willett's team found that the Alps continued to erode more quickly than before. In this case, the cause appears to have been a climate change that had nothing to do with the Mediterranean. About five and a half million years ago, the Earth moved out of an extensive series of ice ages. This would have warmed the Atlantic Ocean, changing Europe's climate and increasing rainfall. At the time, the Mediterranean was still an inland sea far below sea level, but the increased rainfall would have dealt the Alps a second blow—one that continued, even when the Mediterranean regained its connection to the Atlantic and rose back to its present level.
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CAT Scanning a Volcano
Erosional and plate-tectonic processes, of course, work on an extremely long time horizon. To the extent that humans (or aliens) of the future will be interested in harnessing them, it would have to be in the realm of very long-term terraforming, where they would have to be aware that changing the climate might have unexpected geological consequences.
Other geological processes occur much more quickly.
The 2005 Discovery Channel feature “Supervolcano” involved a hypothetical eruption of the volcanic plume beneath Yellowstone National Park. Science-fictionally, one of the most interesting aspects of the feature was the way that volcanologists of the future were shown using a powerful computer to create three-dimensional holographic displays of magma movements, beneath the Earth.
The movie was set in 2025. But in a paper in the August 11, 2006, issue of Science, a team of seismologists was able to do a stripped-down version of what the movie depicted, tracking magma movements beneath Italy's Mt. Etna, which has been intermittently active for many years.
The Italian scientists used an array of forty-five seismic stations positioned on the slopes of the 11,000-foot volcano. These stations recorded more than 2,500 earthquakes during an eighteen-month interval, including one unusually violent eruption. Before-and-after maps of the mountain's interior were then made by comparing the speed with which the seismic waves reached each of the seismometers. Different speeds meant that they were passing through different types of rock, including magma. By assembling this data in a manner similar to that used for medical CAT scans, the team, led by Domenico Patane of the Istituto Nazionale di Geofisica e Vulcanologia, in Catania, Italy, was able to map the mountain's interior to a depth of five kilometers. Not only were the scientists able to spot the arrival of a new batch of magma beneath the mountain, but they were able to determine, from the speed with which the seismic waves passed through it, that it was rich in carbon dioxide, a major cause of explosive eruptions.
In essence, the scientists were making a movie of the magma motions, says Gillian Foulger of Britain's Durham University. “They only have two snapshots, but we're moving toward a situation where we can have many snapshots.”
Foulger has used a similar methodology to track the effect of geothermal power generation on underground steam reservoirs at The Geysers hydrothermal site in northern California. Her results indicated that excessive power generation was depleting the steam, leading to changes in how the power plants were operated.
She dreams of a future in which dangerous volcanoes will be instrumented with networks of seismometers capable of keeping an eye on lava movements. “This is what the human race should be doing,” she says, “rather than waiting until there is a disaster and then putting in a network. We have the technologies, but there is often a huge reluctance to implement them until too late.”
* * * *
Moon-Watching for Climate Change...
Finally, let's look at a new application of old knowledge.
Any sky watcher knows that if you look at the crescent moon on a dark night, you see not only its sunlit portion, but also a ghostly shadow image of the rest of its disk. That's because the dark portion is illuminated by “earthshine,” the lunar equivalent of bright moonlight.
Italian Renaissance thinker Leonardo da Vinci figured this out centuries ago. But until recently it was merely a curiosity. Now, astronomers have discovered that by watching the moon for changes in the faint glow of earthshine, they can monitor an important, previously ignored aspect of global climate change.
The Earth's climate depends on three basic factors: the amount of energy radiated by the Sun, the level of planet-warming “greenhouse gases” in the atmosphere, and the Earth's reflectivity, or albedo, which determines the fraction of incoming solar energy that reaches the surface.
Philip Goode, director of California's Big Bear Solar Observatory, believes that the last of these is the “undiscovered country” of climate-change research. Factors affecting albedo include changes both in average cloudiness and the bright haze produced by certain types of air pollution. Climate models haven't paid much attention to these, but even minor changes can have enormous effects.
“The Earth reflects about thirty percent of the sunlight that comes in,” Goode said at a 2006 meeting of the American Geophysical Union. “If it were twenty-nine percent, the change in energy balance would be larger than [that] of all the greenhouse gases since the start of the Industrial Revolution.”
One way to measure the amount of sunlight being reflected by the Earth is with satellite photos. But a simpler, less expensive method is by careful observations of earthshine. If the amount of earthlight reaching the moon increases, that means that the Earth is becoming brighter, reducing the amount of sunlight reaching its surface. If earthshine becomes dimmer, haze and clouds are diminishing and more planet-warming sunlight is reaching through them.
From the middle 1980s to about 2000, Goode's team has found, there was a drop in the Earth's reflectance—enough to have increased the average amount of absorbed solar energy by 3.5 watts per square meter—a 1.7 percent increase. By comparison, human emissions of greenhouse gases have added only 2.5 watts per square meter to the amount of energy retained by the Earth.
Then in 2000 the trend reversed. Earthshine increased, and the amount of sunlight reaching the surf
ace began declining. Nobody really understands why, but Goode believes it is probably part of a previously unrecognized natural cycle.
Unfortunately, the reversal may not have contributed to climate cooling. That's because, while there are more clouds, there appears to have been a change in the proportions of low (climate-cooling) clouds and high (warming) ones. The result: the Earth's albedo has increased, but the clouds have changed in such a way that the Earth may continue warming, even with decreased sunlight.
Nobody knows the cause of this natural cycle, nor do we know how long it will last. There are hints in weather data, though, that a similar increase in cloudiness occurred in the 1960s and ‘70s. If so, it would appear to represent a forty-year cycle that will reverse again in a few years, adding a double whammy to greenhouse-gas-caused global warming.
Meanwhile, Goode wants more funding for the study of earthshine. That's because the Big Bear observatory can only see the moon when it's above the horizon, which means it only sees the earthshine from our half of the globe.
“Observing from California, we see Southeast Asia and South America, but not all of the eastern Pacific, Indian Ocean, [and] Africa,” Goode says. What's needed is a network of remote-operated telescopes. “Six would be ideal,” he says. “They aren't expensive. It would cost less than $100,000 worth of hardware.”
* * * *
The Signs of Distant Earths
Other astronomers are using earthshine as a way of determining how Earth might appear from interstellar space. This, they hope, will improve their efforts to find earthlike planets circling other stars.
Earthshine can be used in this manner because it is more than just sunlight reflected by clouds. It also includes light reflected from the Earth's surface: light that carries the distinctive spectral signature of chlorophyll, the green pigment used by plants for photosynthesis.
Unfortunately, the chlorophyll signal is difficult to see because the bright light reflected by clouds easily obscures it. “For a typical day, the signal of the vegetation is very weak,” says Pilar Montanes-Rodriguez, of New Jersey Institute of Technology (which operates the Big Bear observatory).
But on some days—those on which the Earth is aligned so that it would appear as little but a thin crescent from the moon—the vegetation signal is very prominent. This means you would need a similar angle to see signs of chlorophyll on a planet orbiting a distant star: something that only happens when the planet is almost in line with its star.
That will make it a lot harder for a telescope to see the planet, but astronomers don't think it's an insurmountable task. Currently, NASA is designing an orbiting telescope capable of distinguishing an earthlike planet from its star. “The goal is to eliminate the star's light, which is from a million to ten million times brighter than the light from the Earth, then isolate the light from the planet,” says Wesley Traub, chief scientist for NASA's Navigator Program (a search for extrasolar planets) at the Jet Propulsion Laboratory in Pasadena.
One of the instruments that can do this is a coronagraph, which blocks out the light from the star, allowing you to see the planet. This should make it possible to spot planets of nearby stars if they are as much as thirty percent closer to their stars than the Earth is to the Sun.
Like Montanes-Rodriguez, Traub finds it useful to look at earthshine to test theories of what the coronagraph might be able to detect.
In addition to chlorophyll, he says, earthshine shows the spectral signature of the Earth's oxygen, a prime indicator of life. “The only way we know of that you can generate a lot of oxygen on a planet is life,” he says. Earthshine also reveals the spectral signatures of ozone and water vapor—also important indicators of life.
Similar observations can tell whether an atmosphere contains water vapor or ice—a hint to its climate. They might also allow astronomers to make educated guesses about the planet's surface. For example, comparing the strength of chlorophyll signatures (if any) to other signatures might reveal how cloudy the planet is, what fraction of it is vegetated, and what fraction is covered by oceans.
Traub has also calculated what earthlight would have looked like at various times in the Earth's evolution. “You could see the stage of evolution of a planet by comparing its spectrum with what we know of Earth,” he says.
And that is yet another idea that, decades ago, would have made great background for a first-contact story—if only someone had managed to think of it before “fiction” became “science.”
Copyright (c) 2008 Richard A. Lovett
[Back to Table of Contents]
* * *
Novelette: WATERBOT
by Ben Bova
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Illustration by Vincent Di Fate
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Sometimes motives matter less than results...
* * * *
“Wake up, dumbbutt. Jerky's ventin’ off.”
I'd been asleep in my bunk. I blinked awake, kind of groggy, but even on the little screen set into the bulkhead at the foot of the bunk I could see the smirk on Donahoo's ugly face. He always called JRK49N “Jerky” and seemed to enjoy it when something went wrong with the vessel—which was all too often.
I sat up in the bunk and called up the diagnostics display. Rats! Donahoo was right. A steady spray of steam was spurting out of the main water tank. The attitude jets were puffing away, trying to compensate for the thrust.
“You didn't even get an alarm, didja?” Donahoo said. “Jerky's so old and feeble your safety systems are breakin’ down. You'll be lucky if you make it back to base.”
He said it like he enjoyed it. I thought that if he wasn't so much bigger than me I'd enjoy socking him square in his nasty mouth. But I had to admit he was right; Forty-niner was ready for the scrap heap.
“I'll take care of it,” I muttered to Donahoo's image, glad that it'd take more than five minutes for my words to reach him back at Vesta—and the same amount of time for his next wise-ass crack to get to me. He was snug and comfortable back at the corporation's base at Vesta while I was more than ninety million kilometers away, dragging through the Belt on JRK49N.
I wasn't supposed to be out here. With my brand-new diploma in my eager little hand I'd signed up for a logistical engineer's job, a cushy safe posting at Vesta, the second biggest asteroid in the Belt. But once I got there Donahoo jiggered the assignment list and got me stuck on this pile of junk for a six month tour of boredom and aggravation.
It's awful lonely out in the Belt. Flatlanders back Earthside picture the Asteroid Belt as swarming with rocks so thick a ship's in danger of getting smashed. Reality is the Belt's mostly empty space, dark and cold and bleak. A man runs more risk of going nutty out there all by himself than getting hit by a ‘roid big enough to do any damage.
JRK49N was a waterbot. Water's the most important commodity you can find in the Belt. Back in those days the news nets tried to make mining the asteroids seem glamorous. They liked to run stories about prospector families striking it rich with a nickel-iron asteroid, the kind that has a few hundred tons of gold and platinum in it as impurities. So much gold and silver and such had been found in the Belt that the market for precious metals back on Earth had gone down the toilet.
But the really precious stuff was water. Still is. Plain old H2O. Basic for life support. More valuable than gold, off-Earth. The cities on the Moon needed water. So did the colonies they were building in cislunar space, and the rock rats’ habitat at Ceres, and the research station orbiting Jupiter, and the construction crews at Mercury.
Water was also the best fuel for chemical rockets, too. Break it down into hydrogen and oxygen and you got damned good specific impulse.
You get the picture. Finding icy asteroids wasn't glamorous, like striking a ten-kilometer-wide rock studded with gold, but it was important. The corporations wouldn't send waterbots out through the Belt if there wasn't a helluva profit involved. People paid for water: paid plenty.
So waterbots like weary old Forty-niner c
rawled through the Belt, looking for ice chunks. Once in a while a comet would come whizzing by, but they usually had too much delta vee for a waterbot to catch up to ‘em. We cozied up to icy asteroids, melted the ice to liquid water, and filled our tanks with it.
The corporation had fifty waterbots combing the Belt. They were built to be completely automated, capable of finding ice-bearing asteroids and carrying the water back to the corporate base at Vesta.
But there were two problems about having the waterbots go out on their own: First, the lawyers and politicians had this silly rule that a human being had to be present on the scene before any company could start mining anything from an asteroid. So it wasn't enough to send out waterbots, you had to have at least one human being riding along on them to make the claim legal.
The second reason was maintenance and repair. The ‘bots were old enough so's something was always breaking down on them and they needed somebody to fix it. They carried little turtle-sized repair robots, of course, but those suckers broke down just like everything else. So I was more or less a glorified repairman on JRK49N. And almost glad of it, in a way. If the ship's systems worked perfectly I would've gone bonzo with nothing to do for months on end.
And there was a bloody war going on in the Belt. The history discs call it the Asteroid Wars, but it mostly boiled down to a fight between Humphries Space Systems and Astro Corporation for control of all the resources in the Belt. Both corporations hired mercenary troops, and there were plenty of freebooters out in the Belt, too. People got killed. Some of my best friends got killed, and I came as close to death as I ever want to be.
The mercenaries usually left waterbots alone. There was a kind of unwritten agreement between the corporations that water was too important to mess around with. But some of the freebooters jumped waterbots, killed the poor dumbjohns riding on them, and sold the water at a cut-rate price wherever they could.
Analog SFF, June 2008 Page 8
