We stand to learn much about Earth’s evolution by comparing its properties to those of some of the other Terrestrial Planets. The interdisciplinary subject of comparative planetology has recently come into its own—the study of the broad and contrasting properties among the diverse worlds in our solar neighborhood. What makes Earth so different from the other planets? How is it that our home alone has blue skies, liquid water, and a gentle climate? And why is Earth the only world in the Solar System (as best we know) that is an abode for life?

Tectonics Consider the land. The high-standing continents on Earth, set slightly above the sea, owe their existence to the long history of plate-tectonic activity—activity probably absent on any other nearby planet. We take the land for granted, for that’s where humans live. We even tend to focus on the land areas in those magnificent photos taken by astronauts of a distant Earth in space. But most of Earth's surface (71% of it) is covered by water; a typical view from our planet’s surface would show exclusively water in all directions—which may be why the astronauts call Earth, when looking back from orbit, “the big blue marble.” In fact, the conditions needed to form the continents on Earth may be unmatched anywhere else in the Solar System. Those conditions led to active tectonics, the sign of a geologically lively planet, and life on the dry land is the beneficiary of it.

By contrast, Venus has recently expired, geologically speaking. Its closer proximity to the Sun might have shut down plate tectonics early, assuming it ever really got going. A difference of a few tens of millions of kilometers in the two planets’ distances from the Sun might have been enough to turn Earth’s nearest relative into a remote cousin. Extra solar heating seems to have thoroughly dehydrated Venus—its surface temperature today is a torrid ~750 K, nearly high enough to melt lead—making its crust and upper mantle too dry and especially too buoyant to sink back down into its interior. The great upwelling of lava, outgassing of chemicals, and jostling of crust that accompany tectonics on Earth probably never much affected the Venusian surface. Robotic radar observations of this totally enshrouded planet, especially those made by the Magellan spacecraft in the 1990s, show little evidence for recent faulting, ridges, or volcanism (though long past, now dormant, volcanoes are visible). Surface features comparable to Earth’s continents, such as the highland landmasses Ishtar and Aphrodite Terra, have apparently not wandered around much, if at all. Parched Venus seems to have been inactive for the past ~0.5 billion years (as dry rock is stronger than water-bearing rock), and to have encased itself in a single, thick shell—yet a shell that probably preserves a record of its last attempt at crustal deformation, such as that which produced its most dramatic topographic feature, the Maxwell Montes mountain chain that exceeds the height of Mount Everest on Earth. Planetologists surmise that Venus, being slightly smaller, is aging more quickly than Earth, and as such its recent past may portend our future. Ironically, the volcanic surface of Venus, repaved as recently as ~700 million years ago before the planet went dormant, most likely resembles a young Earth that began to solidify and therefore might also tell us something about our planet’s distant past.

As for Mars, the red planet has been geologically dead for a long time. Its store of internal heat ran down billions of years ago, shutting off all surface activity except at a few volcanic sites. The problem here is size; Mars is a good deal smaller than Earth. Most people regard Mars as comparable to Earth in size and scale, but it actually has ~10 times less mass. Venus is more properly labeled “Earth’s sister planet” than Mars. Consequently, Mars never did heat up enough to melt its whole interior, to generate global magnetism, or to drive much (if any) plate tectonics. It seems to have been a one-plate planet since the end of the heavy-bombardment period nearly 4 billion years ago. Mars’ topography has probably been locked (and maybe frozen) in place for >3 billion years—which is why some of its fixed lava sites that were previously active, such as Olympus Mons and the Tharsis rise, are so much more extensive (~4000 km across) than volcanoes on Earth. Hence, the Martian surface might also inform us about early planetary evolution of Earth—before our plates began moving—a time domain for which firm knowledge of our planet is sadly lacking.

Atmospherics Another good example of comparative planetology is provided by atmospheric gases—again of Venus, Earth, and Mars. Although almost certainly endowed at birth with similar amounts of hydrogen, carbon, and oxygen, each of these Terrestrial Planets has evolved differently. Their differences, sketched in Figure 4.29, derive largely from their varying masses and distances from the Sun. When it comes to real estate value—terrestrial or extraterrestrial—the bottom line is much the same: size, location, and timing.

FIGURE 4.29 — A comparison of the relative atmospheric gas compositions of Venus, Earth, and Mars, as we know these planets today. In absolute terms, Venus and Earth have nearly the same amount of CO2, emphasizing the delicate balance between a poisonous and a life-sustaining environment—since most of it remained in the atmosphere on Venus but was dissolved in the ocean on Earth.

Of these 3 neighbors, our inward sister Venus receives the most solar energy, in fact roughly twice as much as Earth. Although liquid water is nowhere to be found on this planetary hothouse today, early in its history, when the Sun shone less brightly (~²/₃ of its present luminosity 4 billion years ago), Venus might have had widespread oceans, lakes, and rivers. As the Sun slowly increased its output in the normal course of its stellar evolution (from stage 6 to stage 7, as noted in the STELLAR EPOCH), the planet gradually heated and its water boiled off. In the meantime, Venus’ volcanoes continued to vent much carbon dioxide into its atmosphere. And without the water to change carbon into rocky carbonates such as chalk, limestone, or coral (as it did with CaCO₂ on Earth), the CO₂ gas levels on Venus rose unchecked—in short, most of Venus’ carbon stayed in its atmosphere. The result was a “runaway” greenhouse effect, allowing solar energy to penetrate the thickening atmosphere yet blocking some of its outgoing infrared radiation, all the while making the surface of Venus too hot to support even primitive life.

By contrast, Earth is far enough from the Sun to have retained its liquid water. As water vapor rises in our atmosphere, it cools, condenses into droplets, forms clouds, and rains back down—the “water cycle.” Earth is furthermore able to recycle its carbon through plate tectonics, an action probably untenable on Venus. Even today, CO₂ outgases from Earth’s volcanoes such as along the Cascade Range in Oregon and Mount Etna in Sicily, but it weathers on land and dissolves in the sea, forming carbonic acid that eventually reacts with oceanic rocks to help form limestone crust that, in turn, releases carbon dioxide yet again some tens to hundreds of thousands of years later —the “carbon cycle”—all of which checks the buildup of this greenhouse gas. The prolonged presence of water enabled the evolution of marine organisms, which then, as now, served as an effective means to further remove CO₂ from the air by making shells and skeletons, which later fall to the seafloor and compress into yet more rock—the most famous such geological feature being England’s White Cliffs of Dover. Long ago, an atmospheric steady state—a chemical and thermal balance of sorts—was apparently reached: Volcanism regularly vents CO₂ to the atmosphere, whereupon it’s trapped in plants and rocks. A small percentage of carbon dioxide gas in our air does manage to drive a weak greenhouse effect, thereby raising our average surface temperature above the freezing point of water. Our climate is thus more moderate than Venus’, although humans are beginning to tinker with that delicate balance. We are industrially polluting our air as well as deforesting the land, in the process causing both the CO₂ content and the global temperature to rise; these are measured facts.

Mars, too, probably once had a moderate, wet climate, with liquid water on its surface. Ample photographic evidence strongly suggests that water flowed through several large channels and smaller tributaries, possibly even inundating as much as a third of the planet in huge lakes and Martian seas. The robotic spacecraft, Spirt and Opportunity, which landed on Mars in 2004, confirm the idea that the landscape was once likely flooded with shallow seas of saltwater. But with only 10% the mass of Earth, Mars had trouble holding onto its original atmosphere. And given that its tectonics never really got going (nor is it likely they ever will, again owing to its small mass), Mars couldn’t generate much of another atmosphere. The result, despite the high percentage of CO₂ gas, was slight greenhouse warming at best. Unable to “hold a lid” on its water (as does a covered pot of heated water), much of it dissipated to space. Water that didn’t escape early on is now completely frozen at the poles and in permafrost, as Mars seems in the grip of a perpetual ice age.

Jovian Moons Surprisingly, some of the Jovian moons might also grant added insight into early changes in the planetary evolution of Earth. For example, Jupiter’s moon Callisto has a thick icy shell deeply pitted with impact craters that date back ~4 billion years to the burgeoning days of the Solar System. Since it has no source of internal energy, nor is it close enough to Jupiter to be affected (cracked or heated) much by tides, Callisto hasn’t been repaved with fresh, upwelling matter. That makes this scarred and battered object the oldest known surface anywhere, and as such might tell us something about conditions shortly after the Solar System formed. By contrast, another of Jupiter’s famous Galilean moons, Io, orbits so closely to the planet as to induce huge tidal forces that cause unceasing volcanism that wipes clean its surface and thus any clues about its past.

The contrast between Earth’s early atmosphere and that on Titan today is also instructive. The largest moon of Saturn (in fact, bigger than the smallest planet Mercury) is rich in methane and nitrogen gas as well as in several carbon-based compounds. Under the action of sunlight, these gases undergo a complex series of chemical reactions, producing a hazy, hydrocarbon smog. Perhaps most notably, these chemical reactions and the organic matter they yield are thought to resemble those produced in Earth’s atmosphere billions of years ago, before the advent of living things and oxygen-rich air. Titan seems to be a chemical “factory” that might provide a wealth of information about the vital prebiological steps that led to life on our planet long ago. This is one such task for the premier planetary mission recently enroute: The multi-billion-dollar Cassini mission left Earth in 1997 and arrived at Saturn in 2004. While orbiting in and amongst Saturn’s moons for many years thereafter, the Cassini mother-craft dispatched a small probe called Huygens into Titan’s atmosphere, seeking to unlock secrets of its—and perhaps our—past. In one of the greatest engineering achievements in history, the craft landed safely on this alien moon and revealed a pale orange landscape interspersed with icy valleys laden with hydrocarbon sludge. Unfortunately, the surface temperature was measured to be a chilly 94 Kelvin, which is nearly 200 degrees below the freezing point of water.

Despite our inability to explore much of our own planet’s early history, we are in the ironic position of being able to study better the earliest phases of other, alien worlds. Some of the planets and moons have become virtual fossils, or relics, telling us things about our origins that our own planet cannot. As we acquire more data regarding the vast range of their physical and chemical properties, we gradually gain a better understanding of whatever did happen here ~5 billion years ago. This is the way the scientific method most commonly operates: groping unsurely and probing incrementally into uncharted territory—both real interplanetary turf as well as theoretical insight—as planetologists make real and steady progress in our quest to better approximate the reality that was once Earth’s primal history.