Faint Echoes, Distant Stars_The Science and Politics of Finding Life Beyond Earth

by Ben Bova

  NEPTUNE’S INTERIOR

  Neptune is denser than the other gas giants; its interior most likely contains less hydrogen and helium and more methane, ammonia, and water. The pressure at its core is so great because of the weight of thousands of kilometers of material pressing on it that core temperatures must be high enough to melt rock. A mantle of liquid hydrogen probably surrounds the core, with an ocean of water—mixed with ammonia and methane and heated from below—above that.

  Finding The Unseen

  TRITON

  Neptune has seven small moons, ranging in size from nearly 50 to about 400 kilometers in diameter. It also has Triton, which at 2,695 kilometers is roughly three-quarters the size of our own Moon. Triton orbits Neptune in retrograde fashion, backward compared to most of the other bodies of the solar system. It may be a captured body that once orbited the Sun but strayed close enough to Neptune to be caught by the planet’s gravitational pull.

  Triton is itself surprising. It has a thin atmosphere of molecular nitrogen and methane, with a high haze layer floating above it. At Triton’s distance from the Sun, temperatures are so low that even a smallish body can hold an atmosphere. The gas molecules are moving so sluggishly at temperatures of -225°C that they cannot achieve escape velocity.

  Triton has a geologically “young” surface; that is, there are relatively few craters in its methane and nitrogen-ice covered crust. This is also surprising: What geological forces can be at work on this frozen ice ball to erase old craters? Or has Triton somehow escaped the meteoric bombardment that has peppered the other moons and planets?

  Triton also has a polar cap, probably of nitrogen frost. Most surprising of all, Triton has volcanoes. Well, perhaps geysers would be a more accurate term. Voyager 2’s cameras spotted dark plumes of gases erupting, indicating that there must be some internal heating within Triton. Perhaps geysers resurface Triton by spewing out gases that settle on the ground and refreeze there.

  It is difficult to envision biochemical reactions taking place in frigid -225°C temperatures, but the internal heat that drives Triton’s geysers might also supply the energy for a habitable zone inside the moon. Could Triton harbor a “deep, hot biosphere”?

  16

  The Realm of Ice

  And now there came both mist and snow,

  And it grew wondrous cold:

  And ice, mast-high, came floating by,

  As green as emerald.

  And through the drifts the snowy clifts

  Did send a dismal sheen;

  Nor shapes of man or beast we ken—

  The ice was all between.

  —Samuel Taylor Coleridge

  The Rime of the Ancient Mariner

  IN FEBRUARY 2000 the distant world of Pluto got its fifteen minutes of fame from the news media.21 It was demoted from its status as a planet.

  With considerable fanfare, the world-renowned Hayden Planetarium in New York City reopened its doors after a total renovation. In its exhibit on the solar system, the Hayden classified Pluto not as a planet, but as a Trans-Neptunian Object (TNO), one of the dark and icy bodies that dwell beyond the orbit of the planet Neptune.

  The planetarium decided “to present the solar system the way it presents itself to us,” in the words of Hayden astrophysicist Charles Liu.

  Is Pluto a TNO or a planet? To understand what the fuss is about and what it means for the search for extraterrestrial life, we must look into the farthest reaches of the solar system, the realm of darkness and ice. Strangely enough, this is also a realm of enormous interest to astrobiologists because of the carbon-based prebiotic chemistry that is possible in the particles of amorphous ice out at the solar system’s fringes (see Chapter 8).

  THE SEARCH FOR PLANET X

  The planet Neptune was discovered in 1846 because its gravitational influence caused slight perturbations in the orbit of Uranus. Mathematicians figured out approximately where Neptune ought to be, and astronomers found it in their telescopes.

  But Neptune did not account for all the wobbles in Uranus’ orbit. And Neptune itself suffered perturbations in its own orbit. Astronomers reasoned that there must be an unseen body somewhere in the dark outer reaches of the solar system that caused these slight orbital perturbations, a “Planet X” waiting to be discovered.

  The hunt for Planet X intrigued Percival Lowell, and when he wasn’t observing Mars, he had the astronomers of his Flagstaff Observatory search for it. In 1930, nearly fifteen years after Lowell’s death, Clyde Tombaugh (1906–1997) spotted the dim, distant planet, more than 39.5 AUs from the Sun (a mean distance of 5.915 billion kilometers). He named it Pluto, after the Roman god of the dark and dismal underworld (not the Disney character). The first two letters of the name also honored Percival Lowell.

  CURIOUSER AND CURIOUSER

  At Pluto’s distance, the Sun is nothing more than a very bright star. Dim, cold, distant, and small, Pluto is hard to observe. It is the only planet in the solar system that has not yet been investigated by a spacecraft flyby probe or orbiter. Ground-based telescopes and the Hubble Space Telescope, however, have begun to peel back the veils that obscure Pluto from our view. The more they find, the stranger Pluto seems to be.

  Pluto is not only the farthest planet from the Sun, it is also the smallest, with a diameter of only 2,300 kilometers, less than half that of Mercury and smaller even than our Moon. Yet in 1978 Pluto was found to have a moon of its own, duly named Charon, that is 1,250 kilometers across, more than half Pluto’s size. Charon orbits tightly around Pluto at a distance of less than 20,000 kilometers, so large in comparison to the planet and so close that the two could be considered a double planet rather than a typical planet and satellite.

  Prior to Charon’s discovery, our own Moon, which is about one-quarter the size of Earth, was the largest satellite in proportion to its planet’s size. Saturn’s Triton and three of Jupiter’s Galilean moons are larger than our Moon, but in comparison to their gas giant planets they are minuscule.

  Pluto’s 248-year-long orbit around the Sun is highly eccentric, so much so that it actually crosses the orbit of Neptune. From 1979 to 1999, Pluto was inside Neptune’s orbit, making Neptune the farthest planet in the solar system for a twenty-year span. This led some astronomers to speculate that Pluto might once have been a satellite of Neptune’s that somehow broke free and established itself in a planetary orbit. The discovery that Pluto itself has a satellite of its own put a damper on that idea, however.

  Pluto’s atmosphere of nitrogen and methane is thinner than Mars’. At Pluto’s average surface temperature of -250°C methane freezes into crystalline flakes. Pluto’s surface appears to be covered with methane ice, while nearby Charon is apparently mantled in water ice. The two bodies orbit so tightly around each other that they probably share the thin atmosphere, its gases flowing from one body to the other.

  It is summertime on Pluto now. The planet passed its perihelion—the closest it gets to the Sun—in 1989. As it swings in its orbit away from perihelion, its thin nitrogen/methane atmosphere will crystallize into snow and freeze on the surface, waiting another two centuries before springtime arrives again and the temperature climbs above -200°C.

  WHEN IS A PLANET NOT A PLANET?

  Since Tombaugh’s discovery in 1930, Pluto had been regarded as the ninth planet of our solar system. Yet those nagging perturbations in the orbits of Uranus and Neptune were still unaccounted for: Pluto is simply too small to be their cause. For decades astronomers searched sporadically for the “real” Planet X. (“X” took on a double meaning, since they were looking for a tenth planet in our solar system, and X is the Roman numeral for 10.) Only gradually did they begin to realize that the pesky perturbations were most likely being caused not by a single body, but by the enormous swarm of cometary bodies now known as the Oort Cloud and its nearer component, the Kuiper Belt (see page 195).

  By 1999 astronomers were wondering if Pluto should be classified as a planet. Some thought it more likely that Pluto w
as an icy member of the Kuiper Belt, a Trans-Neptunian Object, more like the core of a large comet rather than a true planet. The question was discussed at a meeting of the American Astronomical Society in January 1999, where it was decided to keep Pluto’s classification as a planet. Part of the reasoning was that Pluto is large enough to pull itself into a sphere, whereas typical comets are irregular chunks of ice.

  However, when the Hayden Planetarium reopened its doors in February 2000, Pluto was classified as a Trans-Neptunian Object rather than a planet.

  Does it matter? As Galileo said in his Letter on Sunspots (1613), “Names and attributes must be accommodated to the essence of things, and not the essence to the name, since things come first and names afterward.”

  Whether Pluto is called a planet or a TNO doesn’t change the essence of the thing, does it?

  ICEBERGS IN SPACE

  The American astronomer Fred Whipple characterized comets as “dirty snowballs”: that is, bodies of frozen water laced with sooty carbon compounds. Considering that most comets are kilometers long, perhaps they should be considered as icebergs rather than mere snowballs.

  As we saw in Chapter 6, comets originate in the cold, dark outer regions of the solar system. As they come closer to the Sun, their ices start to boil away and they develop their characteristic tails of gases and dust.

  Comet orbits are elongated ellipses that apparently start in the farthest reaches of the solar system, out beyond Pluto. Some comets swing through the inner solar system once and are never seen again. Either they gained enough velocity to break free of the Sun forever, or their orbits are so elongated that it will be millennia before they return to our neighborhood. More than one comet has been observed to crash into the Sun, drawn to a fiery death by the Sun’s powerful gravitational field like a moth that fluttered too close to a candle flame.

  Some comets break up as they near the Sun. Comet 2001 A2 split into at least four pieces between January and April of 2001. Comet 1994 S4 broke into a dozen pieces as it approached the Sun the previous year. As we saw in Chapter 15, comet Shoemaker-Levy 9 broke up into twenty-one fragments during one pass by Jupiter, then plunged into the giant planet on its next approach to it.

  HALLEY’S COMET

  Many comets are regular visitors. They are called the short-period comets. The most famous of these is Halley’s Comet, the first whose return was predicted.

  In 1705, Edmund Halley used Newton’s spanking new theories of motion and gravity to predict that a comet that had been observed in 1531, 1607, and 1682 would return in 1758. It did indeed, and even though Halley had died seventeen years earlier, the comet has been called by his name ever since. A search of ancient records has shown observations of Halley’s Comet dating back to 240 B.C.; it was bright enough in the sky shortly before William the Conqueror’s successful invasion of England in 1066 to be woven into the famous Bayeaux Tapestry.

  When Halley’s Comet returned to Earth’s vicinity in 1985–1986, spacecraft from Europe, Japan, and the Soviet Union went out to study it. The European Space Agency’s Giotto probe photographed Halley’s lumpy-shaped nucleus, which proved to be a 16 ¥ 8–kilometer oblong of ice, covered with extremely dark dust, darker than coal.

  Bright jets of gas spurted from Halley’s body when sunlight struck it, blowing off more than 30 million tons of water vapor, carbon monoxide, carbon dioxide, and complex organic (carbon) compounds during the six months of its approach to the Sun. The particles in the tail included mixtures of hydrogen, carbon, nitrogen, oxygen, and silicates. Halley’s 16-kilometer-long nucleus of dust-covered ice shrank by some 6 to 9 meters during its last visit to the solar system. At that rate of loss, Halley still contains enough ice for thousands more orbits.

  In 2001, NASA’s Deep Space 1 probe photographed Comet Borrelly, which coasts through the inner solar system every 6.9 years. DS1 was diverted from its primary mission—testing a high-efficiency ion rocket engine and other new technologies—to fly to within 2,200 kilometers of the comet while it was between the orbits of Earth and Mars. Borrelly’s nucleus is 8 ¥ 4 kilometers long.

  The following year NASA launched the Comet Nucleus Tour (Contour) spacecraft, intended to study comets Encke and Schwassmann-Wachmann-3 at close range. Unfortunately, the spacecraft apparently exploded during a maneuvering burn of its upper-stage rocket engine and had to be written off.

  THE OORT CLOUD AND THE KUIPER BELT

  Where do comets come from?

  In 1950, the Dutch astronomer Jan Oort (1900–1992) postulated that there must be a vast reservoir of trillions of comets far beyond the orbit of Pluto, the icy remains of the original building blocks of the solar system. Oort envisioned a spherical cloud of cometary bodies—big chunks of ice—drifting out at the fringes of the Sun’s gravitational influence between 20,000 and 100,000 AUs from the Sun. Perhaps the Oort Cloud even extends halfway to the next nearest star, the Alpha Centauri system, which is about 275,000 AUs from the Sun.

  Every so often, the gravitational interactions among the comets in the Cloud, or the particular lineup of the planets, or even the faint gravitational influence of a distant star will nudge a comet into a trajectory that sends it plummeting “downhill” toward the Sun and the inner regions of the solar system.

  Oort’s original idea of a spherical cloud of comets orbiting beyond Pluto was refined by Gerard Kuiper, who pointed out that many short-period comets—those that return to the inner solar system in periods of less than 200 years (like Halley’s Comet)—must originate in the inner fringes of the Oort Cloud. That region, now called the Kuiper Belt, is thought to contain millions of comets that orbit around the Sun. The icy bodies of the Kuiper Belt orbit the Sun in much the same flattened plane as the planets, while the more distant icebergs of the Oort Cloud form a vast sphere around the Solar System.

  Oort’s and Kuiper’s hypotheses have been born out by observation. Since 1992, ground-based observatories and the Hubble Space Telescope have detected dozens of objects at the distance that the Kuiper Belt should be. Some of them are more than 90 kilometers across. In 2002, astronomers at the California Institute of Technology discovered a rocky asteroid at the inner fringe of the Kuiper Belt, near the orbit of Neptune, that is more than 1,200 kilometers across: about the same size as Pluto’s moon, Charon, and bigger than Ceres, the largest body in the Asteroid Belt. They named it Quaoar, after a deity of the Native American tribe that once inhabited the Los Angeles area. Quaoar’s rocky composition shows that not all the Trans-Neptunian Objects are icy cometary bodies.

  Most of the TNOs discovered so far range in size from 6 to 10 kilometers across, more like the size of Halley’s Comet and other comets that have looped through the inner solar system.

  COMETS AS LIFE-BEARERS

  The media flap over Pluto illustrates how planetary astronomers have taken a new and different interest in these icebergs floating through space.

  The comets of the Oort Cloud and Kuiper Belt were formed in the earliest stages of the solar system, condensing out of the accretion disk at such immense distances from the Sun that their ices have remained unmelted, undisturbed, for more than 4 billion years. They represent, then, the oldest undisturbed material in the solar system. Thanks to the recent investigations of ice chemistry discussed in Chapter 8, we know that the first steps in the prebiotic chemistry leading toward life can take place in the amorphous ice grains that compose the comets, driven by ultraviolet radiation from the Sun and stars.

  Astrobiologists desperately want to study the comets firsthand to see if the molecules of life—or perhaps even living organisms—exist in these celestial icebergs.

  Since the late 1960s, astronomers have known that vast clouds of gas and dust deep in interstellar space contain many types of organic molecules and even quite complex polycyclic aromatic hydrocarbons (PAHs), such as were found inside the Martian meteorite ALH84001. The basic building blocks of life exist in deep space.

  And in comets. Spectroscopic studies of comets such as Ha
lley have confirmed that they contain organic molecules. In 1999, NASA launched the Stardust spacecraft, which is gathering up samples of dust particles from interplanetary space. In 2004, when Comet Wild-2 nears the Earth, Stardust will pick up dust particles from its tail. These samples will be returned to Earth in a protected reentry capsule in 2006.

  A DELIVERY SYSTEM FOR ORGANICS—AND MORE?

  Astrobiologists reason that if organic chemicals exist in interstellar clouds, such as the cloud that gave birth to our solar system, then organic molecules have been part of the solar system from its very beginning. The comets of the Oort Cloud and Kuiper Belt must be very ancient bodies, unchanged in their composition since the solar system’s formation. The comets that visit the inner solar system have been ejected from the Oort/Kuiper region. They contain organic molecules in the dark, sooty material that coats their ices.

  In the early days of the solar system’s formation, when the planets were growing out of the solar accretion disk, the proto-Earth must have been bombarded by countless asteroids and comets. A good deal of Earth’s water was undoubtedly brought to our planet by comets, together with organic molecules, the basic building blocks of life.

  It has even been suggested that periodic comets, those whose orbits loop through the inner solar system regularly (such as Halley’s, with its seventy-six-year return) could be natural biology laboratories. As such comets approach the Sun and warm up, all the ingredients for life are present: carbon molecules, water, and energy from the Sun. What kinds of chemistry might be taking place during those months when the comet is close enough to the Sun for some of its water to be liquefied? Could living organisms arise? Could they survive the comet’s swing back into the icy cold outer regions of the solar system?