The Planets (6 page)

If ever two worlds invited comparison, the twin sisters Earth and Venus lay such a claim, for these planets are almost identical in size, and orbit the Sun at similar distances. Early discoveries about Venus from afar—especially the detection of her atmosphere by Russian astronomer and poet Mikhail Lomonosov in 1761—fanned widespread fantasies of a lush abode of Earth-like life.

Recent research, however, has exposed only the
most glaring contrasts between the two planets. Although at an earlier epoch Venus probably possessed many of the same attributes as Earth, including once-abundant seas, her water has all boiled away. Now Venus parches and bakes under an obscuring sky that blocks light but traps heat, and bears down upon her surface with heavy pressure.

The ten Russian
Venera
and
Vega
spacecraft that successfully landed on Venus between 1970 and 1984 barely had time to take a few pictures and measurements, or quickly sample the surroundings, before succumbing to the harsh conditions. Within an hour or so of arrival, each vehicle either melted in the heat or crumpled under atmospheric pressure comparable to that found underwater on Earth, nearly three thousand feet below sea level.

Discoveries of the drastic differences between Earth and Venus evoked surprise sometimes expressed in moral terms, as though one sister had chosen the right course while the other veered down an errant path. Nevertheless Venus, the wayward sister, preaches an important cautionary tale to careless humans, for her hostile environment proves how even small atmospheric effects can conspire over time to convert an earthly paradise into a
hellfire cauldron. Indeed, much current study of Venus aims to save humanity from itself by verifying, for example, the destruction that chlorine compounds wreak in high-altitude clouds.

Differences between Earth and Venus doubtless began in their youth, with the Sun beating hotter on the closer of the two sisters. The Sun warmed the waters of Venus until they rose in steam, until water vapor and the hot breath of volcanic eruptions enveloped the planet. These gases then did the work of greenhouse glass: They allowed the Sun’s heat to reach the surface of Venus, but
refused to let heat escape. Instead of dissipating into space, the heat rebounded back down to ground level and made the surface hundreds of degrees hotter still.

High over Venus, sunlight split the water vapor into its components, hydrogen and oxygen, and the lighter hydrogen escaped the planet’s hold. Oxygen remained behind; it recombined with the surface rocks on Venus, and with gases vented by volcanoes, to create an atmosphere consisting almost entirely (97 percent) of carbon dioxide, the most efficient and pernicious of all greenhouse gases. Today, although only a trickle of solar energy penetrates Venus’s cloud cover and arrives at the surface, the greenhouse effect keeps temperatures above eight hundred degrees Fahrenheit all around the planet, day side and night side, even at the poles. Ice on Venus? Liquid water? Impossible, although traces of water vapor do lace the sky.

The abundant carbon dioxide weighs on Venus’s hot terrain with ninety times the pressure of Earth’s atmosphere. On and just above the surface, where the Russian robot explorers conducted their brief surveys, the Venusian air is thick but transparent, enabling the spacecraft’s cameras to see clear to the horizon in the dim available light. All
the light was red. Since only the long red wavelengths of light survive the journey down through the cloud canopy, the landscape presents itself as a monochrome in the sepia tones of old photographs. When night takes even this low-level light away, the vista glows in the dark. Its red-hot rocks, cooked halfway to their melting point by the ambient heat and pressure, resemble the embers of a fire.

Some twenty miles above the surface, the clouds set in, in layers fifteen miles thick, admitting no breaks in their coverage. They bar the Sun from ever showing itself at all during the whole course of the long Venusian day. The planet turns so slowly that a single day takes what would be reckoned as two months on Earth just to get from Sunrise to Sunset. Diffuse signs of the Sun’s light spread slowly from horizon to horizon as the hours pass, but even the brightest hours of the day stay as dimly lit as vespertide. At night, no stars or other planets ever appear through the perpetual overcast.

Venusian clouds comprise large and small droplets of real vitriol—sulfuric acid along with caustic compounds of chlorine and fluorine. They precipitate a constant acid rain, called virga, that evaporates in Venus’s hot, arid air before it has a chance to strike the ground.

Scientists suspect that every several hundred million years the clouds may be remade by a fresh injection of sulfur from global tectonic upheaval on Venus, but failing that, they probably never part.

At their topmost layer, the Venusian clouds display dark swirls when imaged in ultraviolet light. These markings change rapidly, revealing the high velocity at which the clouds roll by—about 220 miles per hour—circling Venus every four Earth-days on fierce winds. Lower down in the atmosphere the winds slacken gradually until they reach the surface, where they don’t so much blow as creep across the planet at two to four miles an hour.

Fast or slow, the winds head ever westerly, the same way Venus turns. In contrast to all the other planets, Venus rotates to the west, even as she revolves eastward with them around the Sun. If you could see the Sun rise on Venus, it would come up in the west and set in the east. Astronomers attribute the backward spin to some violent collision that overturned Venus early in her history. The same presumed impact could explain Venus’s very slow rotation rate, or perhaps it is the Sun that impedes the planet’s spin by raising tides in the vast ocean of Venusian air.

After hiding for an eternity beneath her seething atmosphere, Venus’s surface has surrendered to radar examination by Earth-based telescopes and a series of orbiting spacecraft. The finest of these envoys,
Magellan,
circumnavigated Venus eight times a day for four years beginning in 1990.
*
Magellan
resolved the planet’s vague face into distinct features, most of which turned out to be volcanoes of every variety on plains paved with lava.

Magellan
’s sudden identification of millions of land forms fomented a crisis in nomenclature. The International Astronomical Union responded with an all-female naming scheme that evoked a goddess or giantess from every heritage and era, along with heroines real or invented. Thus the Venusian highlands, the counterparts to Earth’s continents, took the names of love goddesses—Aphrodite Terra, Ishtar Terra, Lada Terra, with hundreds of their hills and dales christened for fertility goddesses and sea goddesses. Large craters commemorate notable women (including American
astronomer Maria Mitchell, who photographed the 1882 transit of Venus from the Vassar College Observatory), while small craters bear common first names for girls. Venus’s scarps hail seven goddesses of the hearth, small hills the goddesses of the sea, ridges the goddesses of the sky, and so on across low plains named from myth and legend for the likes of Helen and Guinevere, down canyons called after Moon goddesses and huntresses.

The only male name on the map of Venus—the great mountain range Maxwell Montes—belongs to Scottish physicist James Clerk Maxwell, who performed pioneering work on electromagnetic radiation during the nineteenth century. When the five-mile-high peaks were detected in the 1960s via Earth-based radar studies made possible by Maxwell’s insights, it seemed fitting to attach his name to them. For several decades after discovery, Maxwell Montes stood as the sole eponymous feature on the planet, while the low regions on either side of the mountains were designated simply as Alpha Regio and Beta Regio (“A” region and “B” region). When
Magellan
arrived thirty years later, and its revelations gave rise to names derived from women’s history, no one wished to evict Maxwell from his rightful place on Venus.

Magellan
’s radar images look like nighttime aerial reconnaissance photos, except that instead of providing a visual record, their blacks and whites reflect the varying textures of Venus’s exposed beauty: Hundreds of thousands of small Venusian volcanoes pop out as bright (rough) bumps against the dark (smooth) background of the plains. On the flanks of giant volcanoes, bright (new) layers of lava drape themselves over the dark (old) flows. Mountainsides glittering in radar brightness seem to boast slopes coated with a veneer of reflective metal, perhaps fool’s gold, that adheres to Venusian rock at the cooler temperatures a few thousand feet up.

Etched in these images, Venus reveals her unique oddities, such as overlapping “pancake dome” volcanoes that rise from surprisingly round bases to flat or softly mounded tops, and her numerous “coronae,” or sets of concentric rings that ornately surround so many of her domes, depressions, and crowds of small volcanoes. Rushing streams of lava dug the long riverine channels that wind across her ample plains. On her high plateaus, tectonic folding and faulting have decorated several thousand square miles to look like crazy-tiled floors, now called “tesserae.” Evocative patterns in Venus’s extruded lava and cracked ground that reminded scientists of sea anemones and spider webs have become “anemone volcanoes” and “arachnoids.”

After amassing their gallery of radar portraits, Venus specialists enhanced many of the images with color for improved resolution. They chose a fire-and-brimstone palette, beginning with the russet hue of the first photos taken by the Russian
Venera
spacecraft, continuing the theme in ochre, umber, sienna, copper, pumpkin, and gold. The vibrant colors suit the seared scenery, the rock that spewed forth as lava and still retains its
near-plastic consistency, the massifs ascending to altitude without ever hardening harder than taffy. Bright shades befit the youthful visage of a planet that only recently (within the last half billion years) repaved itself in veritable floods of lava, which welled up and covered over almost every vestige (about 85 percent) of her ancient past.

Relatively few craters mar the new face of Venus, since the rate of cratering over these past 500,000 years is much reduced from the Solar System’s earliest days. Many small would-be intruders are vaporized on their way through the thick atmosphere, never to touch down, so that only the very largest impactors reach the surface intact. These collisions eject copious debris, yet all the rubble hugs close around the crater margins in neat festoons, as though contained there by the heavy air. The atmosphere likewise may have soothed the fury of Venusian volcanoes, compelling their expelled lava to seep and pour rather than erupt with explosive force.