Faint Echoes, Distant Stars

by Ben Bova

  Mercury is hot. Daytime temperatures soar to more than 426°C, hot enough to melt zinc. Your spacesuit had better have plenty of thermal insulation and an active cooling system. At night, temperatures drop to -185°C because there is no atmosphere to retain the day’s heat; it radiates away into space.

  AN IRON WORLD

  Mercury is a dense planet, with a large iron core and a relatively thin overlying layer of silicon-based rock. This may be because the planet formed so close to the Sun that most of the silicate material in the accretion disk was too hot to condense and solidify; it remained gaseous and was eventually blown away on the solar wind, leaving little material for the planet to build on except iron and other heavy metals.

  Another possibility, though, was revealed by the Mariner 10 spacecraft when it flew by Mercury three times in 1974 and 1975. Mercury’s battered, airless surface is heavily cratered, very much as the Moon. Mariner’s cameras photographed a huge bull’s-eye of circular mountain ridges, some 3,700 kilometers wide. Named Caloris Basin, this impact crater is the center of faults that run hundreds of kilometers across the planet’s rocky surface.

  A 90-some-kilometer-wide meteoroid smashed into Mercury nearly 4 billion years ago, gouging out the Caloris Basin and perhaps blowing away most of the planet’s original rocky crust. Mercury was hit by a planetesimal, much as Earth was in the “impact event” that led to the formation of our Moon. But the planetesimal that blasted Mercury hit it straight on, not with a grazing blow, an impact of such violence that it probably stripped away most of the silicate crust overlaying Mercury’s iron core.

  Despite the high daytime temperatures, there may be ice on Mercury. Earth-based radar studies of the planet show unexpectedly bright returns from Mercury’s north and south polar areas, the kind of radar “signature” that ice would give. The rest of the planet looks bare and rocky in the microwave frequencies used by the radar probes. Can there be water ice cached in deep craters near Mercury’s poles, as there seems to be on the Moon? If sunlight never touches the interiors of those craters, the answer might be—surprisingly—yes.

  Liquid water, however, is totally absent from Mercury’s surface. The surface gets so hot during the long Mercurian day that it is even difficult to imagine liquid water remaining underground; it would probably all be boiled away by now, if it once existed. Thus the chances for a deep, hot biosphere appear to be quite slim. It’s just too hot.

  Could there be life-forms that use a solvent other than water? We will examine such possibilities in our discussion of Venus. At present we simply do not know enough to give a valid answer to that question, except to say that water is abundant in the universe, and life—based on what we know today—goes along the easiest pathways.

  EARTH’S SISTER PLANET

  In Earth’s night sky there is no more beautiful sight than Venus shining gloriously. Virtually every culture that has looked at the planet has named it after its goddess of love and beauty. It took many millennia before skywatchers realized that the beautiful evening star—outshining everything in the sky except the Sun and Moon—and the equally bright and lovely morning star are one and the same planet. Even the ancient Greeks originally had separate names for the morning star, Phosphoros, and the evening star, Hesperos.

  Venus is our nearest planetary neighbor; it is the second planet from the Sun while Earth is third. It sometimes gets as close to Earth as 40 million kilometers. Venus is also most like the Earth in size and mass, and only Mercury is closer in density.

  Because our “sister planet” is perpetually shrouded by thick clouds, however, for many generations planetary astronomers (and writers of speculative fiction) could allow their imaginations a virtually free rein in describing Venus.

  Nineteenth-century astronomers speculated that beneath its perpetual veil of clouds Venus might be quite like Earth, perhaps a planet-wide jungle, wet and dark and even populated by dinosaurs. A “Jurassic Venus” became the dominant image, although cosmologist Fred Hoyle speculated that Venus might by covered with oceans of petroleum.

  As space scientists learned more about the actual conditions beneath Venus’ veil of clouds, they were able to produce a more accurate description of the hot, still, baking, dry conditions on the surface of our sister world—very different from the earlier notions of a “Jurassic Venus.”

  THE VEILED GODDESS OF BEAUTY

  The clouds that cover Venus reflect about 76 percent of the incoming sunlight, which led astronomers to think the clouds to be a good sunscreen that probably kept the temperatures on the surface reasonably mild.

  Although the clouds made it difficult to measure Venus’ rotation, by the middle of the twentieth century it seemed evident that the planet turned on its axis very slowly. There was practically no water vapor in the atmosphere, which made a swampy world below the clouds quite implausible. Modern astronomical observations—including several space flyby probes, orbiters, and landers—have shown that Venus is nothing like the dinosaur jungles of Earth’s Jurassic Period.

  Our “sister planet” is as close to hell as anywhere that can be imagined. Its surface temperature of more than 500°C is hot enough to melt aluminum. Its atmosphere consists mainly of unbreathable carbon dioxide at a pressure nearly ninety times Earth’s normal sea-level atmospheric pressure, equivalent to the pressure more than 900 meters below the surface of the ocean on Earth, deeper than most submarines can go.

  Sister planet? Venus rotates on its axis once in 243 Earth days—backward. In astronomical parlance, the planet’s rotation is retrograde. Every other planet (except lopsided Uranus) rotates counterclockwise. Venus rotates clockwise. Why? Perhaps it was struck by a very massive planetesimal early in the solar system’s history.

  If you were standing on Venus’ oven-hot, bare, rocky surface and could see through the perpetual cloud cover you could watch the Sun rise—very slowly—in the west. Those clouds are laced with sulfuric acid, which apparently comes from volcanic eruptions that spew sulfur dioxide into the atmosphere.

  RUNAWAY GREENHOUSE

  When radio astronomers first turned their antennas to Venus, they were surprised to find that the planet was emitting so much microwave energy that its average temperature appeared to be more than 500°C, five times higher than the boiling point of water.

  How could an “Earth-like” planet completely covered by highly reflective clouds have a higher surface temperature than airless, Sun-baked Mercury? Even though the planet receives twice as much solar energy as Earth does, its cloud deck reflects about 76 percent of that energy away. How did the surface get hot enough to melt aluminum?

  Maybe the “hot” microwaves are not coming from the surface, argued some astronomers. Perhaps Venus has a very active ionosphere, and the high temperatures exist not on the surface, but high in Venus’ thick, cloud-laden atmosphere. For several years the “hot surface” vs. “hot ionosphere” was debated by planetary scientists—hotly.

  An ingenious experiment carried out by the Mariner 2 spacecraft settled the issue. On December 14, 1962, Mariner 2 flew to within 66,000 kilometers of Venus. The spacecraft carried a radiometer that measured the microwave emissions coming from the planet.

  The experiment was based on a phenomenon known as limb darkening. As the spacecraft flew past Venus, the radiometer looked down at the center of the planet’s disk, then shifted away from the center toward the edge of the disk (called the limb in astronomical parlance). When the instrument focused on the center of the disk, it was looking through the least amount of atmosphere; as it shifted its aim toward the limb, it saw thicker and thicker amounts of the atmosphere until finally, at the very edge of the limb, the instrument’s line of sight passed beyond the solid body of the planet and saw nothing but atmosphere.

  If the high microwave emissions came from an active ionosphere, the radiometer would have seen limb brightening: that is, the microwave emissions would have been lowest at the center of the disk and would have become stronger and brighter as the instrument
looked farther out toward the limb. The brightest microwave signal would have come from the edge of the limb.

  The microwave experiment showed a definite limb darkening: The emissions were highest when the instrument was pointed at the center of the planet’s disk and became dimmer as it looked farther out along the limb. Venus’ high temperature comes from a hot surface.

  Venus’ atmosphere is 96.5 percent carbon dioxide. This has led to a runaway greenhouse effect. That thick atmosphere of carbon dioxide traps incoming heat and holds it. The higher the temperature goes, the more heat the atmosphere can store. Earth escaped such a fate when the earliest chlorophyllic species began to take carbon dioxide out of our atmosphere and replace it with oxygen. Today, Earth’s atmosphere contains less than 1 percent carbon dioxide, constantly replenished by animal respiration.

  This is why environmentalists worry about our civilization’s outpouring of carbon dioxide and other greenhouse gases, such as methane. Venus is a powerful example of what happens when a greenhouse effect takes over.

  SPACECRAFT OBSERVATIONS

  Most of what we know about the surface of Venus has come from spacecraft. The Soviet Venera 7 was the first spacecraft to land successfully on another planet, on December 15, 1970.18 Several more Veneras have touched down on Venus and photographed a bleak landscape of flat volcanic rocks. Enough sunlight comes through the perpetual clouds for cameras to work normally. Russian spacecraft have also released instrument-laden balloons to ride through Venus’ dense, still atmosphere, making measurements and radioing them back to Earth.

  Orbiting spacecraft such as the American Pioneer Venus and Magellan have mapped the planet with cloud-piercing radar. Their images show that Venus has been cratered by massive meteoroids from time to time and that its mountains seem to be volcanic in origin. There is also evidence that plate tectonics, which is the driving force shaping the continents and seas of Earth, was active on Venus up until about 500 million years ago, but is not presently operative. The continent-sized plates seem to be locked, perhaps because there is no liquid water underground to lubricate them, as there is on Earth.

  There is no water on Venus’ surface, of course; not with an average temperature five times higher than water’s boiling point. But there is a “rainfall”—of sulfuric acid.

  Water vapor makes up about 0.5 percent of the atmosphere. Apparently it rises to the upper atmosphere, where ultraviolet light from the Sun dissociates the water molecules into hydrogen and oxygen. Sulfur dioxide from volcanic eruptions also rises and is similarly dissociated by solar ultraviolet. The elements recombine to form concentrated sulfuric acid, which gives Venus’ clouds their characteristic yellowish tinge. The sulfuric acid condenses into droplets and falls, but long before it reaches the ground, the intense heat breaks it down into sulfur dioxide and water vapor again, and the cycle continues.

  While the turgid Venerian19 atmosphere at ground level displays hardly any movement at all because it is so dense, at higher altitudes there are winds of more than 300 kilometers per hour. These Force 5 hurricane winds are driven by solar heating; Venus rotates so slowly that the upper atmosphere overheats at the subsolar region and gives rise to powerful, perpetual wave-like superrotation winds that cross the planet’s sunlit hemisphere.

  Such a thick, corrosive atmosphere must erode surface features very rapidly. Venus is not heavily cratered, most likely because craters are “weathered” even more quickly than on Earth. Yet Venus is mountainous, which indicates that mountain building was once active on our “sister planet.” Radar maps show many volcano-like features, and there is enough sulfur dioxide in the atmosphere to convince most planetologists that volcanism is quite active on Venus.

  LIFE IN THE HOTHOUSE?

  Can life exist on Venus?

  It would seem not. Liquid water cannot exist on the planet’s surface, nor underground. Organic molecules would quickly be destroyed in that searing heat.

  Thomas Gold, originator of the “deep, hot biosphere” concept, thinks Venus (and any other solid planet) could host extremophiles underground, but with neither liquid water nor carbon-chain molecules it is difficult to see how. Yet perhaps we should consider the possible subtleties: Could there be some other liquid to serve as a solvent in place of water? Liquid sodium, perhaps? Or liquid sulfur? Could there be elements other than carbon capable of forming long-chain molecules at high temperatures?

  Stephen L. Gillett, a geologist at the University of Nevada, Reno, points out that silicon-oxygen siloxanes can form long-chain molecules at temperatures of 800 to 900°C. Silicones, which are siloxane chains with organic molecules linked to them, are stable up to a few hundred degrees Celsius, but only in the absence of free oxygen. Could silicone creatures exist on Venus’ 500°C surface?

  Planetary scientist David Harry Grinspoon of the University of Colorado, Boulder, points out that life is a process that depends on a flow of matter and energy; as Lovelock observed in his Gaia hypothesis, life needs (or creates) a chemical disequilibrium. Grinspoon writes:

  Venus has an active surface and interior [volcanoes] and a lively atmosphere with complex chemicals that perpetuate gradients of matter and energy. In theory, this kind of disequilibrium environment could feed a steady supply of nutrients and energy to any creatures crafty enough to evolve in the Venusian environment.

  While the surface may be too hellish for any forms of life, even silicone creatures, conditions are different in the upper atmosphere. At an altitude of approximately 50 kilometers the atmospheric pressure is only 70 percent of sea-level pressure on Earth and the temperature averages 42°C, cool enough for water droplets to form. Grinspoon speculates that several of the puzzling aspects about Venus could be signs that some form of life exists high up in the clouds.

  The superrotation winds could be caused by photosynthetic organisms. “Dark ultraviolet markings whip around Venus in swift and colossal winds, circling the planet every four [Earth] days. From the point of view of any Venusian bugs that want to use sunlight for energy, the superrotation would be a major plus because the night is so long there. The planet may rotate too slowly for photosynthesis unless you have something like a superrotation. Perhaps Venusian organisms transform solar energy into mechanical energy to drive the winds.”

  IS VENUS HOTTER THAN HELL?

  Something in the upper atmosphere of Venus absorbs ultraviolet energy. Planetary astronomers call it the “unknown ultraviolet absorber.” Grinspoon suggests that the absorber might be a photosynthetic pigment, another possible sign of organisms in the atmosphere.

  The cloud deck of Venus might make a good place for life, Grinspoon conjectures. Unlike the ephemeral clouds of Earth, Venus’ cloud deck is permanent and worldwide; it contains some water vapor and offers a stable environment where life might exist. Unfortunately, he adds, whatever water droplets may exist there are mixed with a strong sulfuric acid solution, and the clouds are also laced with hydrochloric acid, which destroys organic molecules. Hydrochloric acid is stomach acid, the stuff that people take antacid pills to counteract. However, bacteria can live in the human stomach in strong concentrations of hydrochloric acid, so perhaps extremophile organisms have evolved in the clouds of Venus.

  Grinspoon also points out that radar reflections from the surface show that mountains give a different, smoother return above an altitude of roughly 4,000 meters, where the temperature is a “cool” 440°C. “Perhaps some non-carbon-based equivalent to lichen grows on Venusian rocks below a certain temperature, feeding off the disequilibrium sulfur gases. These creatures could be causing some chemical change in the ground which concentrates radar reflective materials.”

  Grinspoon emphasizes that these are speculations, unsupported by observational evidence. Yet it is necessary to remember that in the search for life in the universe, we have only one example to guide us: life here on Earth. As the extremophiles have shown, our definition of life and its possible limits may be much too parochial.

  Venus seems an unlikely
abode for life as we know it. But there is so much that we do not know—yet.

  15

  The Realm of the Giants

  Now, my own suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose.

  —J. B. S. Haldane

  OUT BEYOND THE ORBIT of Mars, beyond the scattered rocks and boulders of the Asteroid Belt, dwell the giant planets: Jupiter, Saturn, Uranus, and Neptune. They are totally unlike Earth and the other inner worlds of our solar system. Yet these gas giant worlds and their planet-sized moons may be the best places in the entire solar system to look for life.

  It is difficult to grasp how different the gas giants are from the small, rocky worlds of the inner solar system. For example, Jupiter is more massive than all the other planets of the solar system combined. Yet it is composed mainly of the lightest elements, hydrogen and helium, plus hydrogen compounds such as ammonia, methane, and water.

  All that we can see of Jupiter is the top of a planet-girdling deck of clouds that swirl colorfully as the huge planet spins on its axis in slightly less than ten hours. Storm systems the size of Earth roil those clouds. What lies beneath those bands and eddies can only be guessed at.

  Yet it seems evident that the planet’s powerful gravitational field (2.54 times Earth’s gravity at the cloud tops) must squeeze those gaseous materials into liquids at some depth below the clouds. Jupiter might well have a boundless planet-wide ocean more than ten times wider than the entire planet Earth, an ocean that has never seen sunlight nor felt the rough confining contours of land. Its waves have never crashed against a craggy shore, never thundered upon a sloping beach, for there is no land anywhere across Jupiter’s enormous girth—not even an island or a reef. The ocean’s billows sweep across the deeps without hindrance, eternally.