Origins: Fourteen Billion Years of Cosmic Evolution (24 page)

• It has been found in tumors of terminal cancer patients.

Forty-three out of fifty people approached by Zohner signed the petition, six were undecided, and one was a great supporter of the molecule and refused to sign. Yes, 86 percent of the passersby voted to ban dihydrogen monoxide (H
2
O) from the environment.

Maybe that’s what really happened to the water on Mars.

Venus, Earth, and
Mars together provide an instructive tale about the pitfalls and payoffs from focusing on water (or possibly other solvents) as the key to life. When astronomers considered where they might find liquid water, they originally concentrated on planets that orbit at the proper distances from their host stars to maintain water in liquid form—not too close in and not too far out. Thus we begin with the tale of Goldilocks.

Once upon a time—somewhat more than 4 billion years ago—the formation of the solar system was nearly complete. Venus had formed sufficiently close to the Sun for the intense solar energy to vaporize what might have been its water supply. Mars formed so far away that its water supply became forever frozen. Only one planet, Earth, had a distance “just right” for water to remain a liquid, and whose surface would therefore become a haven for life. This region around the Sun where water can remain liquid came to be known as the habitable zone.

Goldilocks liked things “just right,” too. One of the bowls of porridge in the Three Bears’ cottage was too hot. Another was too cold. The third was just right, so she ate it. Upstairs, one bed was too hard. Another was too soft. The third was just right, so Goldilocks slept in it. When the Three Bears came home, they discovered not only missing porridge but also Goldilocks fast asleep in their bed. (Don’t remember how the story ends, but it remains a mystery to us why the Three Bears—omnivorous and occupying the top of the food chain—did not eat Goldilocks instead.)

The relative habitability of Venus, Earth, and Mars would intrigue Goldilocks, though the actual history of these planets is somewhat more complicated than three bowls of porridge. Four billion years ago, leftover water-rich comets and mineral-rich asteroids were still pelting the planetary surfaces, although at a much lower rate than before. During this game of cosmic billiards, some planets had migrated inward from where they had formed while others were kicked into larger orbits. And among the dozens of planets that had formed, some moved on unstable orbits and crashed into the Sun or Jupiter. Others were ejected from the solar system altogether. In the end, the few planets that remained had orbits that were “just right” to survive billions of years.

Earth settled into an orbit with an average distance of 93 million miles from the Sun. At this distance, Earth intersects a measly one two-billionth of the total energy radiated by the Sun. If you assume that Earth absorbs all the energy received from the Sun, then our home planet’s average temperature should be about 280 degrees Kelvin (45˚ F), which falls midway between winter and summer temperatures. At normal atmospheric pressures, water freezes at 273 degrees Kelvin and boils at 373 degrees, so we are well positioned with respect to the Sun for nearly all of Earth’s water to remain happily in its liquid state.

Not so fast. In science you can sometimes get the right answer for the wrong reasons. Earth actually absorbs only two thirds of the energy that reaches it from the Sun. The rest is reflected back into space by Earth’s surface (especially by the oceans) and by its clouds. If we factor this reflection into the equations, the average temperature for Earth drops to about 255 degrees Kelvin, well below the freezing point of water. Something must be operating to raise our average temperature to something a little more comfortable.

But wait once more. All theories of stellar evolution tell us that 4 billion years ago, when life was forming out of Earth’s primordial soup, the Sun was a third less luminous than it is today, which would have left Earth’s average temperature even farther below freezing. Perhaps Earth in the distant past was simply closer to the Sun. Once the early period of heavy bombardment had ended, however, no known mechanisms could have shifted stable orbits back and forth within the solar system. Perhaps the greenhouse effect from Earth’s atmosphere was stronger in the past. We don’t know for sure. What we do know is that habitable zones, as originally conceived, have only peripheral relevance to whether life may exist on a planet within them. This has become evident from the fact that we cannot explain Earth’s history on the basis of a simple habitable-zone model, and even more from the realization that water or other solvents need not depend on the heat from a star to remain liquid.

Our solar system contains two good reminders that the “habitable-zone approach” to looking for life has severe limitations. One of them lies outside the zone where the Sun can keep water liquid, yet nevertheless has a worldwide ocean of water. The other, far too cold for liquid water, offers the possibility of another liquid solvent, poison to us but potentially prime for other forms of life. Before long we should have the opportunity to investigate both of these objects with close-up robot explorers. Let’s check out what we know now about Europa and Titan.

Jupiter’s moon Europa
, which has about the same size as our Moon, shows crisscrossing cracks in the surface that change on time scales of weeks or months. To expert geologists and planetary scientists, this behavior implies that Europa has a surface made almost entirely of water ice, like a giant Antarctic ice sheet girdling an entire world. And the changing appearance of the rifts and rills in this icy surface leads to a startling conclusion: The ice apparently floats on a worldwide ocean. Only by invoking liquid beneath the icy surface can scientists satisfactorily explain what they have seen, thanks to the stunning successes of the
Voyager
and
Galileo
spacecraft. Since we observe changes on the surface all around Europa, we may conclude that a worldwide ocean of liquid must underlie that surface.

What liquid could this be, and why should that substance remain liquid? Impressively, planetary scientists have reached two fairly firm additional conclusions: The liquid is water, and it remains liquid because of tidal effects on Europa produced by the giant planet Jupiter. The fact that water molecules are more abundant than ammonia, ethane, or methyl alcohol makes it the likeliest substance to provide the liquid beneath Europa’s ice, and the existence of this frozen water likewise implies that more water exists in the immediate neighborhood. But how can water remain a liquid, when the solar-induced temperatures in Jupiter’s vicinity are only about 120
o
K (
–
150
o
Celsius)? Europa’s interior remains relatively warm because tidal forces from Jupiter and the two large moons nearby, Io and Ganymede, continuously flex the rocks within Europa as this moon changes its position with respect to neighboring objects. At all times, the sides of Io and Europa closest to Jupiter feel a stronger force of gravity from the giant planet than the sides farthest away. These differences in force slightly elongate the solid moons in the direction facing Jupiter. But as the moons’ distances from Jupiter change during their orbits, Jupiter’s tidal effect—the difference in force exerted on the near side and the far side—also changes, producing small pulses in their already distorted shapes. This changing distortion heats the moons’ interiors. Like a squash ball or a racquet ball continually being smashed by impact, any system that undergoes continuing structural stress will have its internal temperature rise.

With a distance from the Sun that would otherwise guarantee a forever-frozen ice world, Io’s stress level earns it the title of the most geologically active place in the entire solar system—complete with belching volcanoes, surface fissures, and plate tectonics. Some have analogized modern-day Io to the early Earth, when our planet was still piping hot from its episode of formation. Inside Io, the temperature rises to the point that volcanoes continually blast evil-smelling compounds of sulfur and sodium many miles above the satellite’s surface. Io in fact has too high a temperature for liquid water to survive, but Europa, which undergoes less tidal flexing than Io because it is farther from Jupiter, heats more modestly, though still significantly. In addition, Europa’s worldwide ice cap puts a pressure lid on the liquid below, preventing the water from evaporating and allowing it to exist for billions of years without freezing. So far as we can tell, Europa was born with its water ocean and ice above, and has maintained that ocean, close to the freezing point but still above it, through four and a half billion years of cosmic history.

Astrobiologists therefore view Europa’s worldwide ocean as a prime target for investigation. No one knows the ice cap’s thickness, which might range from a few dozen yards to half a mile or more. Given the fecundity of life within Earth’s oceans, Europa remains the most tantalizing place in the solar system to search for life outside of Earth. Imagine going ice-fishing there. Indeed, engineers and scientists at the Jet Propulsion Laboratory in California have begun to envision a space probe that lands, finds (or cuts) a hole in the ice, and drops a submersible camera to have a peek at primitive life that may swim or crawl below.

“Primitive” pretty much sums up our expectations, because any would-be forms of life would have only small amounts of energy at their disposal. Nevertheless, the discovery of enormous masses of organisms at depths a mile or more beneath the basalts of Washington State, living mainly on geothermal heat, suggests that we may someday find the Europan oceans alive with organisms unlike any on Earth. But one pressing question remains: Would we call the creatures Europans or Europeans?

Mars and Europa
offer targets numbers one and two in the search for extraterrestrial life within the solar system. A third great “Search Me” sign appears twice as far from the Sun as Jupiter and its moons. Saturn has one giant moon, Titan, which ties with Jupiter’s champion, Ganymede, as the largest moon in the solar system. Half again as large as our own Moon, Titan possesses a thick atmosphere, a quality unequaled by any other moon (or by the planet Mercury, not much larger than Titan but much closer to the Sun, whose heat evaporates any Mercurian gases). Unlike the atmospheres of Mars and Venus, Titan’s atmosphere, many dozen times thicker than Mars’, consists primarily of nitrogen molecules, just as Earth’s does. Floating within this transparent nitrogen gas are enormous numbers of aerosol particles, a permananet Titanian smog, that forever shrouds the moon’s surface from our gaze. As a result, speculation about life’s possibilities has enjoyed a field day on Titan. We have measured the moon’s temperature by bouncing radio waves (which penetrate the atmospheric gases and aerosols) from its surface. Titan’s surface temperature, close to 85
o
Kelvin (
–
188
o
Celsius), falls far below those that allow liquid water to exist, but provides just the right temperature for liquid ethane, a carbon-hydrogen compound familiar to those who refine petroleum products. For decades, astrobiologists have imagined ethane lakes on Titan, chockful of organisms that float, eat, meet, and reproduce.

Now, during the first decade of the twenty-first century, exploration has finally replaced speculation. The
Cassini-Huygens
mission to Saturn, a collaboration of NASA with the European Space Agency (ESA), left Earth in October 1997. Nearly seven years later, having received gravity boosts from Venus (twice) Earth (once), and Jupiter (once), the spacecraft reached the Saturn system, where it fired its rockets to achieve an orbit around the ringed planet.

The scientists who designed the mission arranged for the Huygens probe to detach itself from the
Cassini
spacecraft late in 2004, to make the first descent through Titan’s satellite’s opaque clouds, and to reach the moon’s surface, using a heat shield to avoid frictional burning from its rapid passage through the upper atmosphere and a series of parachutes to slow the probe down in the lower atmosphere. Six instruments aboard the
Huygens
probe were built to measure the temperature, density, and chemical composition of Titan’s atmosphere, and to send images back to Earth via the
Cassini
spacecraft. At this time, we can only await these data and images to see what they tell us about the enigma that lies beneath the clouds of Titan. We are unlikely to see life itself, should any exist on this faraway moon, but we can expect to determine whether or not conditions do favor the existence of life by providing liquid pools and ponds in which life might originate and flourish. At the very least, we may expect to learn the array of different types of molecules that exist on and near the surface of Titan, which may shed new light on how the precursors of life arose on Earth and throughout the solar system.

If we require
water for life, must we restrict ourselves to planets and their moons, on whose solid surfaces water can accumulate in quantity? Not at all. Water molecules, along with several other household chemicals such as ammonia and methane and ethyl alcohol, appear routinely in cool interstellar gas clouds. Under special conditions of low temperature and high density, an ensemble of water molecules can be induced to transform and to funnel energy from a nearby star into an amplified, high-intensity beam of microwaves. The atomic physics of this phenomenon resembles what a laser does with visible light. But in this case, the relevant acronym is maser, for
m
icrowave
a
mplification by the
s
timulated
e
mission of
r
adiation. Not only does water occur practically everywhere in the galaxy, but it also occasionally beams at you as well. The great problem faced by would-be life in interstellar clouds arises not from a lack of raw materials but from the extremely low densities of matter, which enormously reduce the rate at which particles collide and interact. If life takes millions of years to arise on a planet such as Earth, it might take trillions of years to do so at much lower densities—far more time than the universe has so far provided.