 September 2003 |
Bolts from Beyond
Some “shooting stars” come to Earth bearing secrets
from other planets, as well as clues about the makeup
of the solar system before the planets formed.
By Donald Goldsmith
For two centuries, astronomers and geologists have recognized that the Earth is continually bombarded by small extraterrestrial objects called meteoroids. Each piece of this cosmic debris has its own orbit around the Sun. Because some of those orbits cross the Earth’s, our planet and certain bits of the debris inevitably reach the same point at the same time and collide.
Every day, in fact, about a hundred tons of extraterrestrial material rain onto our planet, most in the form of grains of dust that float gently downward and land undetected. Some of that dust has been captured by collectors mounted on high-flying aircraft, but the great hope for obtaining significant amounts of it resides with the spacecraft Stardust, launched in 1999 and now on the other side of the Sun from Earth. Early in 2006 Stardust will return to Earth with samples of the interplanetary medium.
It is probably natural to think of meteorites—as the meteoroids that fall to Earth are called—as threatening, even dangerous, phenomena. The best-known meteorites, not surprisingly, are the ones that strike something important, perhaps one of us. Despite the impression left by Hollywood movies, however, people have been hit by meteorites only once or twice in recorded history, and those impacts led to only minor injuries. The only verified mammalian fatality from a meteorite impact in the past century was a dog unlucky enough to occupy the exact spot near Alexandria, Egypt, where a meteorite from Mars struck on a June day in 1911. Closer to home (and more typical), on October 9, 1992, a large meteorite that passed over the eastern United States in a mere forty seconds reached its ground zero in Peekskill, New York, where it demolished the rear end of an aged Chevrolet [see “nature.net,” by Robert Anderson, in “Natural Selections”].
Truly large meteorites, such as the thirty-four-ton iron monster that the Arctic explorer Robert Edwin Peary brought from Cape York, Greenland, to New York’s American Museum of Natural History in 1897, rank among the scarcest, and scariest, objects on Earth. Fifty thousand years ago, a meteorite the size of a house and the weight of a destroyer struck near what is now the town of Winslow, Arizona, excavating a mile-wide hole known as the Barringer Meteorite Crater. Several much larger, though highly eroded, terrestrial impact craters have also been discovered, stark reminders that an object many miles in diameter strikes the Earth every 50 million to 100 million years.
Sixty-five million years ago the best-known of those supermassive impactors blasted a crater more than a hundred miles across, centered near what is now the town of Chicxulub on the northwest coast of Mexico’s Yucatán Peninsula. The incoming object raised an immense cloud of grit and dust that rose high above the atmosphere, spread around the globe like syrup on ice cream, and took months to settle back down. Because the geologic record shows that the Chicxulub impact coincided with the extinction of the dinosaurs (as well as with that of many other earthly species), most paleontologists regard it as the cause of the dinosaurs’ demise. Their extinction made room for the subsequent radiation of mammals into newly vacant ecological niches.
Yet meteorites also play a much less sinister role. Sizable meteorites offer astronomers and geologists extraterrestrial fragments, free for the finding—“the poor man’s space probes.” In spite of the extensive alteration of their exteriors by their passage through Earth’s atmosphere, those fragments nonetheless provide highly valuable samples of the early matter in the solar system.
In recent years it has also become clear that the incoming rain of meteoroids has a flip side: the much smaller, but potentially immensely significant, outflow of debris kicked into space by large impacts. A monster meteorite that strikes the Earth can shoot fragments of itself, along with terrestrial matter loosened by the impact, far out into space, adding to the swarm of meteoritic grit that already orbits the Sun. Even more important, the same process takes place on other worlds as well: the Moon, Mars, and the asteroid Vesta have all lost identifiable chunks that have made their way to Earth. Although the mass of that debris is an insignificant part of the total mass of incoming meteoroids, the recognition that matter can, and does, travel from planet to planet raises the stunning possibility that life itself, encapsulated within those bits of rock, might also pass between worlds.
Long before their nature was understood, meteoroids no larger than a small pebble continually attracted attention. Earth’s atmosphere protects us well, however, so we have nothing to fear from colliding with a pebble. But the fact that each colliding meteoroid has an enormous velocity with respect to the Earth, typically between ten and forty miles per second, has noteworthy consequences. Unable to move out of the way as the meteoroid plunges toward Earth, atmospheric gases pile up ahead of it, just as they do at the front of the space shuttle as it re-enters our atmosphere. The pressure exerted by the swiftly accumulating head of atmospheric gas heats the meteoroid (and the shuttle) to 3,000 degrees Fahrenheit or higher. Even a pebble-size meteoroid heats enough of the surrounding gas, as well as itself, to create a bright “shooting star”—the transitory visible object astronomers call a meteor.
Although a typical shooting star may appear to land over the next hill, it actually flames out between twenty-five and eighty miles above the observer. During the meteoroid’s roaring trip through the atmosphere, most of its mass sloughs off as tiny shards of matter. To survive such a violent passage, and thus to reach Earth’s surface as even a small remnant meteorite, the original meteoroid must be larger than a chair. Most meteors never end up as meteorites. When they do, they can be identified soon after their fall by their still-warm surfaces. Identifying older meteorites on the ground usually takes a practiced eye and a good deal of luck. On rare occasions a fall of hundreds of meteorites spreads over a few square miles.
On the basis of their composition, meteorites are classified into three main groups: stony, stony-iron, and iron. Each group embodies, in the details of its chemical composition, the history of its formation far from the Earth. The oldest meteorites are the stony ones, and within that group the oldest of all are the chondrites, so named for their rounded, glassy inclusions called chondrules.
Henry Clifton Sorby, a nineteenth-century meteorite enthusiast, described chondrules as “droplets of fiery rain.” Dating of the chondrules, based primarily on the radioactive uranium they contain, has identified chondrites as old as 4.6 billion years, far older than any other rocks on the Earth or the Moon. This age dates the oldest chondrites to the epoch when the Sun and its planets began to form within a diffuse cloud of gas and dust. Within a few million years, many of those pieces had joined together to form the large objects that now orbit the Sun: the four inner, rocky planets; the Earth’s moon; and the solid cores and large moons of the four gas-giant planets.
Some material from the primordial solar system, however, never became part of a planet or a large moon. Instead, that debris continued to orbit the Sun, most of it between the orbits of Mars and Jupiter, a region known as the asteroid belt. Asteroids are just meteoroids large enough to be identified with a telescope as individual objects; the asteroid belt comprises not only thousands of asteroids but also millions of smaller objects.
Today, more than four and a half billion years after the Sun and its planets formed, most of the leftover debris continues to orbit outside the orbit of Mars. But gravitational forces from the other planets continually divert some of the debris into smaller orbits that cross the Earth’s. When our planet encounters a region particularly rich in debris, most notably in mid-August and in mid-November, everyone in the world gets the chance to see a “meteor shower.” On every clear night of the year, though, dozens of meteors can be seen by anyone with decent vision (or a good pair of glasses) and the patience to gaze steadily at the sky. And of all the meteoroids that reach the Earth’s surface, the vast majority are, in effect, minute asteroids.
But what of the meteoroids that come from other large objects in the solar system? To escape from Venus or the Earth, matter must be ejected at a velocity of at least seven miles a second; on Mars, three miles a second will suffice. No modest impact can ping matter off a surface at such speeds; the impactor must be more than 300 feet across, substantially larger than the one that excavated the Barringer crater.
Once blasted into space, a typical fragment traces an elongated trajectory around the Sun. On every orbit, it makes a close approach to the planet it came from, and the gravity of that planet either recaptures it or deflects it into a new orbit. If the new orbit becomes so elongated that it crosses the orbit of another planet, the second planet’s gravitational field may pull the fragment into its embrace. In some cases that second planet is Jupiter—by far the most massive planet, whose gravitational force can either capture the fragment or launch it entirely out of the solar system. But a sizable fraction of the material ejected from either Mars, Venus, or the Earth—more than one-third—actually ends up on the surface of one of the other two planets.
Of course, finding such interplanetary messengers depends a great deal on where they fall. Most of them, given the ratio of water to land on the Earth’s surface, plunge unseen into the oceans. But on the slowly flowing ice fields of Antarctica, where other rocks are scarce, meteorites are ripe for the plucking. Of the several-score meteorites that have been securely identified as hailing from the Moon or from Mars, two dozen or so have been antarctic finds.
The chemical composition of every meteorite identified as lunar or Martian differs, subtly but surely, from the composition of every terrestrial rock. The chemical profiles of the meteorites match those of rocks sampled on Mars and on the Moon several decades ago. Strangely, no meteorites from Venus have yet been identified, though some of them should have reached the Earth, and a chemical analysis of the Venusian surface has been available for a quarter century. The luck of the cosmic draw may have led to that negative result; better meteorite searches may soon change it.
By examining Martian meteorites for the effects of impacts from cosmic rays—fast-moving, highly energetic atomic nuclei that permeate space—physicists have determined that they spent between 12 million and 17 million years in interplanetary transit before colliding with the Earth. All but one of those meteorites are less than 1.3 billion years old. The lone exception is a meteorite designated ALH 84001, so named because it was the first meteorite discovered in the Allan Hills of Antarctica in 1984.
In 1996, however, ALH 84001 became the most famous meteorite on the planet. An interdisciplinary team of scientists announced that this rock from Mars bore intriguing clues that life had once flourished on another planet. Moreover, the radioactive decay of minerals within the meteorite showed that the rock had formed 4.5 billion years ago, a time early in the history of the solar system. In that distant epoch the surface of Mars apparently had abundant running water, and thus a far greater potential than it does today for harboring life on its surface.
What were the signs of life within ALH 84001? First, it contained compounds that often occur in organisms on Earth. Second, it included tiny, magnetized grains of iron oxide and iron sulfide much like the ones that certain bacteria produce to orient themselves in the Earth’s magnetic field (Mars, too, must once have had a substantial magnetic field). Finally, it held within it a number of submicroscopic ovoid shapes, similar in form to various tiny fossils on Earth but much smaller than any of them.
For a few months many investigators hoped ALH 84001 would demonstrate that ancient life on Mars had been brought to Earth by two cosmic collisions: one that blasted the rock loose from Mars in the first place, and a second, 15 million years later, that slammed it into our planet. Alas, the verdict has largely gone against the believers (though there are still holdouts). Some earthbound organisms may have contaminated the meteorite. The resemblance between its mineral inclusions and the magnetic grains made by bacteria is apparently just happenstance. And the ovoids, too small to hold the molecules needed to carry out the chemical reactions of life, are just chance deposits with interesting shapes.
Nevertheless, ALH 84001 is a striking reminder that whenever a giant impact dislodges a life-bearing fragment from an inhabited world, life from that world could travel to another. In principle, since Jupiter’s gravity expels some meandering meteoroids from the solar system, life might even be able to cross interstellar distances millions of times greater than the distance between Earth and Mars, eventually to find its way onto worlds that belong to other planetary systems.
Panspermia, the concept that all life in the universe had a common origin and has been carried from planet to planet with the passage of time, sprang from the mind of the Swedish chemist Svante Arrhenius at the beginning of the twentieth century. The demonstrated fact that material does travel from one planet to another lends credence to the hypothesis. But could any life-forms have survived the shock of the blastoff, the long, harsh cold and exposure to radiation in space, and the final trauma of passing through a planet’s atmosphere and colliding with its surface?
Apparently they could have. Calculations of the blast-off process, together with experiments on such hardy bacteria as Bacillus subtilis and Deinococcus radiodurans (the latter notable for surviving doses of radiation a few thousand times the lethal dose for a human being), imply that microorganisms can survive not only the shock of impacts like the ones required to eject matter into interplanetary space, but also millions of subsequent years of orbiting in the cold. Microorganisms in space can be protected against interplanetary ultraviolet radiation by a few microns of shielding, which even a small rock can provide. (Protection against cosmic-ray particles might require several feet of solid material, implying that only relatively large ejected rocks could ferry life safely through space.) Some forms of life can remain dormant for many centuries, and possibly even for the thousands of millennia it takes for a meteoroid to travel from planet to planet.
Passing through a planet’s atmosphere, even one as thin as the veil surrounding Mars, substantially slows down a meteoroid before it lands. During that ten- or twenty-second passage, as its surface becomes red-hot, much of the meteoroid breaks apart or flakes off. But the passage happens so quickly that the interior of any sizable meteoroid fragment, including any microorganisms along for the ride, could remain cool. H. Jay Melosh of the University of Arizona in Tucson, the leading expert on the exchange of matter between planets, puts it this way: “Earth’s atmosphere—and Mars’s to some extent—couldn’t have been better designed to let organisms down gently.”
How can one estimate the probability that life-forms do travel from world to world, as Arrhenius envisioned? One conclusion seems rock-solid: The distances between the planets within our solar system (or within other planetary systems) make such a transfer billions of times more likely within a single planetary system than between planetary systems. Thirty years ago Carl Sagan concluded that probably not a single meteorite from another planetary system could ever reach the surface of the Earth. Earlier this year Melosh undertook detailed calculations to demonstrate systematically that Sagan’s assertion remains valid. The vast distances between the stars make the interstellar-panspermia hypothesis—that life has been transferred not only within a planetary system but also between systems—mathematically almost impossible, no matter how well a life-form could survive an interstellar voyage.
But for travel between the planets within a particular system, panspermia seems entirely possible. The Martian meteorites demonstrate that much already. Life on Earth may yet prove to be descended from ancient life on Mars—and any ancient life on Mars may in turn have come from Earth.
Even if meteorites turn out not to have brought life to Earth from other planets (or vice versa), they still arrive here loaded with useful information. Geologists have noted, for instance, that every meteorite from Mars contains carbon-oxygen compounds, sulfur-oxygen compounds, and minerals common in terrestrial clay—all of which signal that water was present at the time they were formed.
Perhaps even more amazing is what geologists have deduced about the geologic history of Mars from what might seem meager evidence in eight Martian meteorites. The key to the deduction lies in what can be inferred from the measured ratios of various isotopes in various rocks. (A chemical element can occur in nature in several varieties called isotopes. The isotopes of any one element are identical in their chemical properties, but they differ in mass as well as in stability against radioactive decay.)
Geologists Der-Chuen Lee of the Academia Sinica in Taiwan and Alexander N. Halliday of the Swiss Federal Institute of Technology in Zurich measured the proportions of various isotopes of tungsten in the eight meteorites from Mars. Tungsten-182, a rare isotope, arises from the radioactive decay of hafnium-182. The measured quantity of tungsten-182 in a meteorite therefore shows how much hafnium-182 was present in the rock when it formed. From that measurement, it was straightforward to calculate how much hafnium of all isotopes was present in the original rock.
Lee and Halliday then measured the total amount of tungsten in the meteorites, almost all of which is tungsten-184. All tungsten combines readily with iron-rich material, which, because of its high density, tends to concentrate in the core of a planet. Hence tungsten, too, became concentrated in the core. As a consequence, the rocks that did not contain much iron became relatively depleted in tungsten. Those rocks were the ones that came to form the Martian crust and mantle.
In contrast with tungsten, hafnium does not interact readily with iron-rich material, but it does combine readily with the elements in rocks lacking in iron. Hence when a planet differentiates into an iron-rich core and an iron-poor crust and mantle, hafnium tends to end up in rocks outside the core. The ratio of tungsten-182 (which came from hafnium) to total tungsten in the Martian meteorites turned out to be relatively high, signaling that the region from which they originated had been part of the early crust.
Furthermore, Lee and Halliday concluded, Mars must have differentiated itself into core, mantle, and crust within a few tens of millions of years after it formed. Thereafter it has remained geologically quiet, its crust relatively intact for almost the entire history of the planet. If, during the past four billion years, Mars had instead undergone plate-tectonic activity similar to that on Earth, more material from the core would have found its way into the crust. In that case, the ratio of tungsten-182 to tungsten-184 would have been lower, because much more tungsten-184 from the original core would have been mixed into the crust.
This December the European Space Agency’s Mars Express lander Beagle 2 is scheduled to touch down on the Martian surface. The following month NASA’s Mars Exploration Rover Mission will put two robot rovers on opposite sides of the red planet to scrutinize the surface for signs of water and provocative rocks. Someday within the next decade or two, Martian materials may be brought to Earth for analysis. A detailed examination of them should yield geologic conclusions even more startling and fine-grained than the ones derived from the tungsten isotopes. For example, if sedimentary rocks exist on Mars, they may contain fossil evidence of life from the era when liquid water flowed on the planet’s surface.
Someday, too, well before this century ends, geologists will walk on Mars; one of their number has already walked on the Moon. Their explorations will enhance the findings of the robot investigators that preceded them. Perhaps they will find rocks containing evidence of life—or possibly life itself—hidden beneath the Martian surface. Until then, we earthlings can continue to look for microscopic visitors, or their fossil remnants, that might reside in meteorites from Mars or from other worlds. The full implications of those interplanetary transfers, which depend on a more complete knowledge of what those visitors from other worlds have carried to Earth, will be intriguing to sort out.
Trained as both a research astronomer and an attorney, Donald Goldsmith devoted himself to popularizing astronomy thirty years ago. Since then he has written more than twenty books and collaborated on many PBS television programs on astronomy, including the 1991 series The Astronomers. In 1995 he was awarded the American Astronomical Society’s Annenberg Foundation Prize for outstanding contributions to science education through astronomy. He lives in Berkeley, California.
Copyright © Natural History Magazine, Inc., 2003