Part 3 (1/2)

An optometrist from Saint Paul, Minnesota, named Sherman Schultz wrote a letter in response to an article in the July 2002 issue of Sky and Telescope Sky and Telescope magazine. Schultz pointed out that the optical setup Lowell preferred for viewing the Venutian surface was similar to the gizmo used to examine the interior of patients' eyes. After seeking a couple of second opinions, the author established that what Lowell saw on Venus was actually the network of shadows cast on Lowell's own retina by his ocular blood vessels. When you compare Lowell's diagram of the spokes with a diagram of the eye, the two match up, ca.n.a.l for blood vessel. And when you combine the unfortunate fact that Lowell suffered from hypertension-which shows up clearly in the vessels of the eyeb.a.l.l.s-with his will to believe, it's no surprise that he pegged Venus as well as Mars with teeming with intelligent, technologically capable inhabitants. magazine. Schultz pointed out that the optical setup Lowell preferred for viewing the Venutian surface was similar to the gizmo used to examine the interior of patients' eyes. After seeking a couple of second opinions, the author established that what Lowell saw on Venus was actually the network of shadows cast on Lowell's own retina by his ocular blood vessels. When you compare Lowell's diagram of the spokes with a diagram of the eye, the two match up, ca.n.a.l for blood vessel. And when you combine the unfortunate fact that Lowell suffered from hypertension-which shows up clearly in the vessels of the eyeb.a.l.l.s-with his will to believe, it's no surprise that he pegged Venus as well as Mars with teeming with intelligent, technologically capable inhabitants.

Alas, Lowell fared only slightly better with his search for Planet X, a planet thought to lie beyond Neptune. Planet X does not exist, as the astronomer E. Myles Standish Jr. demonstrated decisively in the mid-1990s. But Pluto, discovered at the Lowell Observatory in February 1930, some 13 years after Lowell's death, did serve as a fair approximation for a while. Within weeks of the observatory's big announcement, though, some astronomers had begun debating whether it should be cla.s.sified as the ninth planet. Given our decision to display Pluto as a comet rather than as a planet in the Rose Center for Earth and s.p.a.ce, I've become an unwitting part of that debate myself, and I can a.s.sure you, it hasn't let up yet. Asteroid, planetoid, planetesimal, large planetesimal, icy planetesimal, minor planet, dwarf planet, giant comet, Kuiper Belt object, trans-Neptunian object, methane s...o...b..ll, Mickey's dim-witted bloodhound-anything but number nine, we naysayers argue. Pluto is just too small, too lightweight, too icy, too eccentric in its...o...b..t, too misbehaved. And by the way, we say the same about the recent high-profile contenders including the three or four objects discovered beyond Pluto that rival Pluto in size and in table manners.

TIME AND TECHNOLOGY moved on. Come the 1950s, radio-wave observations and better photography revealed fascinating facts about the planets. By the 1960s, people and robots had left Earth to take family photos of the planets. And with each new fact and photograph the curtain of ignorance lifted a bit higher. moved on. Come the 1950s, radio-wave observations and better photography revealed fascinating facts about the planets. By the 1960s, people and robots had left Earth to take family photos of the planets. And with each new fact and photograph the curtain of ignorance lifted a bit higher.

Venus, named after the G.o.ddess of beauty and love, turns out to have a thick, almost opaque atmosphere, made up mostly of carbon dioxide, bearing down at nearly 100 times the sea level pressure on Earth. Worse yet, the surface air temperature nears 900 degrees Fahrenheit. On Venus you could cook a 16-inch pepperoni pizza in seven seconds, just by holding it out to the air. (Yes, I did the math.) Such extreme conditions pose great challenges to s.p.a.ce exploration, because practically anything you can imagine sending to Venus will, within a moment or two, get crushed, melted, or vaporized. So you must be heatproof or just plain quick if you're collecting data from the surface of this forsaken place.

It's no accident, by the way, that Venus is hot. It suffers from a runaway greenhouse effect, induced by the carbon dioxide in its atmosphere, which traps infrared energy. So even though the tops of Venus's clouds reflect most of the Sun's incoming visible light, rocks and soils on the ground absorb the little bit that makes its way through. This same terrain then reradiates the visible light as infrared, which builds and builds in the air, eventually creating-and now sustaining-a remarkable pizza oven.

By the way, were we to find life-forms on Venus, we would probably call them Venutians, just as people from Mars would be Martians. But according to rules of Latin genitives, to be ”of Venus” ought to make you a Venereal. Unfortunately, medical doctors reached that word before astronomers did. Can't blame them, I suppose. Venereal disease long predates astronomy, which itself stands as only the second second oldest profession. oldest profession.

The rest of the solar system continues to become more familiar by the day. The first s.p.a.cecraft to fly past Mars was Mariner 4, Mariner 4, in 1965, and it sent back the first-ever close-ups of the Red Planet. Lowell's lunacies notwithstanding, before 1965 n.o.body knew what the Martian surface looked like, other than that it was reddish, had polar ice caps, and showed darker and lighter patches. n.o.body knew it had mountains, or a canyon system vastly wider, deeper, and longer than the Grand Canyon. n.o.body knew it had volcanoes vastly bigger than the largest volcano on Earth-Mauna Kea in Hawaii-even when you measure its height from the bottom of the ocean. in 1965, and it sent back the first-ever close-ups of the Red Planet. Lowell's lunacies notwithstanding, before 1965 n.o.body knew what the Martian surface looked like, other than that it was reddish, had polar ice caps, and showed darker and lighter patches. n.o.body knew it had mountains, or a canyon system vastly wider, deeper, and longer than the Grand Canyon. n.o.body knew it had volcanoes vastly bigger than the largest volcano on Earth-Mauna Kea in Hawaii-even when you measure its height from the bottom of the ocean.

Nor is there any shortage of evidence that liquid water once flowed on the Martian surface: the planet has (dry) meandering riverbeds as long and wide as the Amazon, webs of (dry) tributaries, (dry) river deltas, and (dry) floodplains. The Mars exploration rovers, inching their way across the dusty rock-strewn surface, confirmed the presence of surface minerals that form only in the presence of water. Yes, signs of water everywhere, but not a drop to drink.

Something bad happened on both Mars and Venus. Could something bad happen on Earth too? Our species currently turns row upon row of environmental k.n.o.bs, without much regard to long-term consequences. Who even knew to ask these questions of Earth before the study of Mars and Venus, our nearest neighbors in s.p.a.ce, forced us to look back on ourselves?

TO GET A better view of the more distant planets requires s.p.a.ce probes. The first s.p.a.cecraft to leave the solar system were better view of the more distant planets requires s.p.a.ce probes. The first s.p.a.cecraft to leave the solar system were Pioneer 10 Pioneer 10, launched in 1972, and its twin Pioneer 11, Pioneer 11, launched in 1973. Both pa.s.sed by Jupiter two years later, executing a grand tour along the way. They'll soon pa.s.s 10 billion miles from Earth, more than twice the distance to Pluto. launched in 1973. Both pa.s.sed by Jupiter two years later, executing a grand tour along the way. They'll soon pa.s.s 10 billion miles from Earth, more than twice the distance to Pluto.

When they were launched, however, Pioneer 10 Pioneer 10 and and 11 11 weren't supplied with enough energy to go much beyond Jupiter. How do you get a s.p.a.cecraft to go farther than its energy supply will carry it? You aim it, fire the rockets, and then just let it coast to its destination, falling along the streams of gravitational forces set up by everything in the solar system. And because astrophysicists map trajectories with precision, probes can gain energy from multiple slingshot-style maneuvers that rob orbital energy from the planets they visit. Orbital dynamicists have gotten so good at these gravity a.s.sists that they make pool sharks jealous. weren't supplied with enough energy to go much beyond Jupiter. How do you get a s.p.a.cecraft to go farther than its energy supply will carry it? You aim it, fire the rockets, and then just let it coast to its destination, falling along the streams of gravitational forces set up by everything in the solar system. And because astrophysicists map trajectories with precision, probes can gain energy from multiple slingshot-style maneuvers that rob orbital energy from the planets they visit. Orbital dynamicists have gotten so good at these gravity a.s.sists that they make pool sharks jealous.

Pioneer 10 and and 11 11 sent back better pictures of Jupiter and Saturn than had ever been possible from Earth's surfce. But it was the twin s.p.a.cecraft sent back better pictures of Jupiter and Saturn than had ever been possible from Earth's surfce. But it was the twin s.p.a.cecraft Voyager 1 Voyager 1 and and 2 2-launched in 1977 and equipped with a suite of scientific experiments and imagers-that turned the outer planets into icons. Voyager 1 Voyager 1 and and 2 2 brought the solar system into the living rooms of an entire generation of world citizens. One of the windfalls of those journeys was the revelation that the moons of the outer planets are just as different from one another, and just as fascinating, as the planets themselves. Hence those planetary satellites graduated from boring points of light to worlds worthy of our attention and affection. brought the solar system into the living rooms of an entire generation of world citizens. One of the windfalls of those journeys was the revelation that the moons of the outer planets are just as different from one another, and just as fascinating, as the planets themselves. Hence those planetary satellites graduated from boring points of light to worlds worthy of our attention and affection.

As I write, NASA's Ca.s.sini Ca.s.sini orbiter continues to orbit Saturn, in deep study of the planet itself, its striking ring system, and its many moons. Having reached Saturn's neighborhood after a ”four-cus.h.i.+on” gravity a.s.sist, orbiter continues to orbit Saturn, in deep study of the planet itself, its striking ring system, and its many moons. Having reached Saturn's neighborhood after a ”four-cus.h.i.+on” gravity a.s.sist, Ca.s.sini Ca.s.sini successfully deployed a daughter probe named successfully deployed a daughter probe named Huygens Huygens, designed by the European s.p.a.ce Agency and named for Christiaan Huygens the Dutch astronomer who first identified Saturn's rings. The probe descended into the atmosphere of Saturn's largest satellite, t.i.tan-the only moon in the solar system known to have a dense atmosphere. t.i.tan's surface chemistry, rich in organic molecules, may be the best a.n.a.log we have for the early prebiotic Earth. Other complex NASA missions are now being planned that will do the same for Jupiter, allowing a sustained study of the planet and its 70-plus moons.

IN 1584, in his book 1584, in his book On the Infinite Universe and Worlds, On the Infinite Universe and Worlds, the Italian monk and philosopher Giordano Bruno proposed the existence of ”innumerable suns” and ”innumerable Earths [that] revolve about these suns.” Moreover, he claimed, working from the premise of a Creator both glorious and omnipotent, that each of those Earths has living inhabitants. For these and related blasphemous transgressions, the Catholic Church had Bruno burned at the stake. the Italian monk and philosopher Giordano Bruno proposed the existence of ”innumerable suns” and ”innumerable Earths [that] revolve about these suns.” Moreover, he claimed, working from the premise of a Creator both glorious and omnipotent, that each of those Earths has living inhabitants. For these and related blasphemous transgressions, the Catholic Church had Bruno burned at the stake.

Yet Bruno was neither the first nor the last person to posit some version of those ideas. His predecessors range from the fifth-century B.C B.C. Greek philosopher Democritus to the fifteenth-century cardinal Nicholas of Cusa. His successors include such personages as the eighteenth-century philosopher Immanuel Kant and the nineteenth-century novelist Honore de Balzac. Bruno was just unlucky to be born at a time when you could get executed for such thoughts.

During the twentieth century, astronomers figured that life could exist on other planets, as it does on Earth, only if those planets...o...b..ted their host star within the ”habitable zone”-a swath of s.p.a.ce neither too close, because water would evaporate, nor too far, because water would freeze. No doubt that life as we know it requires liquid water, but everyone had just a.s.sumed that life also required starlight as its ultimate source of energy.

Then came the discovery that Jupiter's moons Io and Europa, among other objects in the outer solar system, are heated by energy sources other than the Sun. Io is the most volcanically active place in the solar system, belching sulfurous gases into its atmosphere and spilling lava left and right. Europa almost surely has a deep billion-year-old ocean of liquid water beneath its icy crust. In both cases, the stress of Jupiter's tides on the solid moons pumps energy to their interiors, melting ice and giving rise to environments that might sustain life independent of solar energy.

Even right here on Earth, new categories of organisms, collectively called extremophiles, thrive in conditions inimical to human beings. The concept of a habitable zone incorporated an initial bias that room temperature is just right for life. But some organisms just love several-hundred-degree hot tubs and find room temperature downright hostile. To them, we are the extremophiles. Many places on Earth, previously presumed to be unlivable, such creatures call home: the bottom of Death Valley, the mouths of hot vents at the bottom of the ocean, and nuclear waste sites, to name just a few.

Armed with the knowledge that life can appear in places vastly more diverse than previously imagined, astrobiologists have broadened the earlier, and more restricted, concept of a habitable zone. Today we know that such a zone must encompa.s.s the newfound hardiness of microbial life as well as the range of energy sources that can sustain it. And, just as Bruno and others had suspected, the roster of confirmed exosolar planets continues to grow by leaps and bounds. That number has now risen past 150-all discovered in the past decade or so.

Once again we resurrect the idea that life might be everywhere, just as our ancestors had imagined. But today, we do so without risk of being immolated, and with the newfound knowledge that life is hardy and that the habitable zone may be as large as the universe itself.

EIGHT.

VAGABONDS OF THE SOLAR SYSTEM.

For hundreds of years, the inventory of our celestial neighborhood was quite stable. It included the Sun, the stars, the planets, a handful of planetary moons, and the comets. Even the addition of a planet or two to the roster didn't change the basic organization of the system.

But on New Year's Day of 1801 a new category arose: the asteroids, so named in 1802 by the English astronomer Sir John Herschel, son of Sir William, the discoverer of Ura.n.u.s. During the next two centuries, the family alb.u.m of the solar system became crammed with the data, photographs, and life histories of asteroids, as astronomers located vast numbers of these vagabonds, identified their home turf, a.s.sessed their ingredients, estimated their sizes, mapped their shapes, calculated their orbits, and crash-landed probes on them. Some investigators have also suggested that the asteroids are kinfolk to comets and even to planetary moons. And at this very moment, some astrophysicists and engineers are plotting methods to deflect any big ones that may be planning an uninvited visit.

TO UNDERSTAND THE small objects in our solar system, one should look first at the large ones, specifically the planets. One curious fact about the planets is captured in a fairly simple mathematical rule proposed in 1766 by a Prussian astronomer named Johann Daniel t.i.tius. A few years later, t.i.tius's colleague Johann Elert Bode, giving no credit to t.i.tius, began to spread the word about the rule, and to this day it's often called the t.i.tius-Bode law or even, erasing t.i.tius's contribution altogether, Bode's law. Their handy-dandy formula yielded pretty good estimates for the distances between the planets and the Sun, at least for the ones known at the time: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. In 1781, widespread knowledge of the t.i.tius-Bode law actually helped lead to the discovery of Neptune, the eighth planet from the Sun. Impressive. So either the law is just a coincidence, or it embodies some fundamental fact about how solar systems form. small objects in our solar system, one should look first at the large ones, specifically the planets. One curious fact about the planets is captured in a fairly simple mathematical rule proposed in 1766 by a Prussian astronomer named Johann Daniel t.i.tius. A few years later, t.i.tius's colleague Johann Elert Bode, giving no credit to t.i.tius, began to spread the word about the rule, and to this day it's often called the t.i.tius-Bode law or even, erasing t.i.tius's contribution altogether, Bode's law. Their handy-dandy formula yielded pretty good estimates for the distances between the planets and the Sun, at least for the ones known at the time: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. In 1781, widespread knowledge of the t.i.tius-Bode law actually helped lead to the discovery of Neptune, the eighth planet from the Sun. Impressive. So either the law is just a coincidence, or it embodies some fundamental fact about how solar systems form.

It's not quite perfect, though.

Problem number 1: You have to cheat a little to get the right distance for Mercury, by inserting a zero where the formula calls for 1.5. Problem number 2: Neptune, the eighth planet, turns out to be much farther out than the formula predicts, orbiting more or less where a ninth planet should be. Problem no. 3: Pluto, which some people persist in calling the ninth planet* falls way off the arithmetic scale, like so much else about the place. falls way off the arithmetic scale, like so much else about the place.

The law would also put a planet orbiting in the s.p.a.ce between Mars and Jupiter-at about 2.8 astronomical units from the Sun. Encouraged by the discovery of Ura.n.u.s at more or less the distance t.i.tius-Bode said it would be, astronomers in the late eighteenth century thought it would be a good idea to check out the zone around 2.8 AUs. And sure enough, on New Year's Day 1801, the Italian astronomer Giuseppe Piazzi, founder of the Observatory of Palermo, discovered something there. Subsequently it disappeared behind the glare of the Sun, but exactly one year later, with the help of brilliant computations by the German mathematician Carl Friedrich Gauss, the new object was rediscovered in a different part of the sky. Everybody was excited: a triumph of mathematics and a triumph of telescopes had led to the discovery of a new planet. Piazzi himself named it Ceres (as in ”cereal”), for the Roman G.o.ddess of agriculture, in keeping with the tradition of naming planets after ancient Roman deities. from the Sun. Encouraged by the discovery of Ura.n.u.s at more or less the distance t.i.tius-Bode said it would be, astronomers in the late eighteenth century thought it would be a good idea to check out the zone around 2.8 AUs. And sure enough, on New Year's Day 1801, the Italian astronomer Giuseppe Piazzi, founder of the Observatory of Palermo, discovered something there. Subsequently it disappeared behind the glare of the Sun, but exactly one year later, with the help of brilliant computations by the German mathematician Carl Friedrich Gauss, the new object was rediscovered in a different part of the sky. Everybody was excited: a triumph of mathematics and a triumph of telescopes had led to the discovery of a new planet. Piazzi himself named it Ceres (as in ”cereal”), for the Roman G.o.ddess of agriculture, in keeping with the tradition of naming planets after ancient Roman deities.

But when the astronomers looked a bit harder, and calculated an orbit and a distance and a brightness for Ceres, they discovered that their new ”planet” was teeny. Within a few more years three more teeny planets-Pallas, Juno, and Vesta-were discovered in the same zone. It took a few decades, but Herschel's term ”asteroids” (literally ”starlike” bodies) eventually caught on, because, unlike planets, which showed up in the telescopes of the day as disks, the newfound objects could not be distinguished from stars except by their motion. Further observations revealed a proliferation of asteroids, and by the end of the nineteenth century, 464 of them had been discovered in and around the swath of celestial real estate at 2.8 AU. And because the swath turned out to be a relatively flat band and did not scatter around the Sun in every direction, like bees around a hive, the zone became known as the asteroid belt.

By now, many tens of thousands of asteroids have been catalogued, with hundreds more discovered every year. Altogether, by some estimates, more than a million measure a half-mile across and up. As far as anyone can tell, even though Roman G.o.ds and G.o.ddesses did lead complicated social lives, they didn't have 10,000 friends, and so astronomers had to give up on that source of names long ago. So asteroids can now be named after actors, painters, philosophers, and playwrights; cities, countries, dinosaurs, flowers, seasons, and all manner of miscellany. Even regular people have asteroids named after them. Harriet, Jo-Ann, and Ralph each have one: they are called 1744 Harriet, 2316 Jo-Ann, and 5051 Ralph, with the number indicating the sequence in which each asteroid's...o...b..t became firmly established. David H. Levy, a Canadian-born amateur astronomer who is the patron saint of comet hunters but has discovered plenty of asteroids as well, was kind enough to pull an asteroid from his stash and name it after me, 13123 Tyson. He did this shortly after we opened our $240-million Rose Center for Earth and s.p.a.ce, designed solely to bring the universe down to Earth. I was deeply moved by David's gesture, and quickly learned from 13123 Tyson's...o...b..tal data that it travels among most of the others, in the main belt of asteroids, and does not cross Earth's...o...b..t, putting life on Earth at risk of extinction. It's just good to check this sort of thing.

ONLY CERES-the largest of the asteroids, at about 580 miles in diameter-is spherical. The others are much smaller, craggy fragments shaped like doggy bones or Idaho potatoes. Curiously, Ceres alone accounts for about a quarter of the total asteroidal ma.s.s. And if you add up the ma.s.ses of all the asteroids big enough to see, plus all the smaller asteroids whose existence can be extrapolated from the data, you don't get anywhere near a planet's worth of ma.s.s. You get about 5 percent the ma.s.s of Earth's moon. So the prediction from t.i.tius-Bode, that a red-blooded planet lurks at 2.8 AU, was a bit exaggerated.

Most asteroids are made entirely of rock, though some are entirely metal and some are a mixture of both; most inhabit what's often called the main belt, a zone between Mars and Jupiter. Asteroids are usually described as being formed of material left over from the earliest days of the solar system-material that never got incorporated into a planet. But that explanation is incomplete at best and does not account for the fact that some asteroids are pure metal. To understand what's going on, one should first consider how the larger objects in the solar system formed.

The planets coalesced from a cloud of gas and dust enriched by the scattered remains of element-rich exploding stars. The collapsing cloud forms a protoplanet-a solid blob that gets hot as it accretes more and more material. Two things happen with the larger protoplanets. One, the blob tends to take on the shape of a sphere. Two, its inner heat keeps the protoplanet molten long enough for the heavy stuff-primarily iron, with some nickel and a splash of such metals as cobalt, gold, and uranium mixed in-to sink to the center of the growing ma.s.s. Meanwhile, the much more common, light stuff-hydrogen, carbon, oxygen, and silicon-floats upward toward the surface. Geologists (who are fearless of sesquipedalian words) call the process ”differentiation.” Thus the core of a differentiated planet such as Earth, Mars, or Venus is metal; its mantle and crust are mostly rock, and occupy a far greater volume than the core.

Once it has cooled, if such a planet is then destroyed-say, by smas.h.i.+ng into one of its fellow planets-the fragments of both will continue orbiting the Sun in more or less the same trajectories that the original, intact objects had. Most of those fragments will be rocky, because they come from the thick, outer, rocky layers of the two differentiated objects, and a small fraction will be purely metallic. Indeed, that's exactly what's observed with real asteroids. Moreover, a hunk of iron could not have formed in the middle of interstellar s.p.a.ce, because the individual iron atoms of which it's made would have been scattered throughout the gas clouds that formed the planets, and gas clouds are mostly hydrogen and helium. To concentrate the iron atoms, a fluid body must first have differentiated.

BUT HOW DO solar system astronomers know that most main-belt asteroids are rocky? Or how do they know anything at all? The chief indicator is an asteroid's ability to reflect light, its albedo. Asteroids don't emit light of their own; they only absorb and reflect the Sun's rays. Does 1744 Harriet reflect or absorb infrared? What about visible light? Ultraviolet? Different materials absorb and reflect the various bands of light differently. If you're thoroughly familiar with the spectrum of sunlight (as astrophysicists are), and if you carefully observe the spectra of the sunlight reflected from an individual asteroid (as astrophysicists do), then you can figure out just how the original sunlight has been altered and thus identify the materials that comprise the asteroid's surface. And from the material, you can know how much light gets reflected. From that figure and from the distance, you can then estimate the asteroid's size. Ultimately you're trying to account for how bright an asteroid looks on the sky: it might be either really dull and big, or highly reflective and small, or something in between, and without knowing the composition, you can't know the answer simply by looking at how bright it is. solar system astronomers know that most main-belt asteroids are rocky? Or how do they know anything at all? The chief indicator is an asteroid's ability to reflect light, its albedo. Asteroids don't emit light of their own; they only absorb and reflect the Sun's rays. Does 1744 Harriet reflect or absorb infrared? What about visible light? Ultraviolet? Different materials absorb and reflect the various bands of light differently. If you're thoroughly familiar with the spectrum of sunlight (as astrophysicists are), and if you carefully observe the spectra of the sunlight reflected from an individual asteroid (as astrophysicists do), then you can figure out just how the original sunlight has been altered and thus identify the materials that comprise the asteroid's surface. And from the material, you can know how much light gets reflected. From that figure and from the distance, you can then estimate the asteroid's size. Ultimately you're trying to account for how bright an asteroid looks on the sky: it might be either really dull and big, or highly reflective and small, or something in between, and without knowing the composition, you can't know the answer simply by looking at how bright it is.

This method of spectral a.n.a.lysis led initially to a simplified three-way cla.s.sification scheme, with carbon-rich C-type asteroids, silicate-rich S-type asteroids, and metal-rich M-type asteroids. But higher precision measurements have since sp.a.w.ned an alphabet soup of a dozen cla.s.ses, each identifying an important nuance of the asteroid's composition and betraying multiple parent bodies rather than a single mother planet that had been smashed to smithereens.

If you know an asteroid's composition then you have some confidence that you know its density. Curiously, some measurements of the sizes of asteroids and their ma.s.ses yielded densities that were less than that of rock. One logical explanation was that those asteroids weren't solid. What else could be mixed in? Ice, perhaps? Not likely. The asteroid belt sits close enough to the Sun that any species of ice (water, ammonia, carbon dioxide)-all of whose density falls below that of rock-would have evaporated long ago due to the Sun's heat. Perhaps all that's mixed in is empty s.p.a.ce, with rocks and debris all moving in tandem.