© 2008 by Donald F.
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This article originally appeared, in very different form, in Sky & Telescope.
'Follow the water.' As scientists look for conditions that might have permitted life to exist, that has long been the theme for exploring Mars. Now, the water trail is unlocking mysteries at Venus.
Venus? For a world with an atmosphere, our closest planetary neighbor is just about the driest place imaginable -- and that is very odd.
If Venus is not quite Earth's identical twin, she is certainly a close sibling. The planet has a diameter ninety-five percent of Earth's, a mass of 0.81 relative to our world, and a density nine-tenths the terrestrial value. She is known to be geologically active and her deep interior structure may be similar to Earth's. Venus hangs out in Earth's Solar System neighborhood, so why wouldn't she experience a similar childhood leading to oceans -- and possibly life?
Earth and Venus probably did start out with broadly similar inventories of water. Scientists also think both planets had thick carbon dioxide atmospheres.
Today, Earth's water sits pooled on and under the surface, while Venus has only minuscule amounts in the upper atmosphere. Most of Earth's carbon is locked up, in and under the oceans, and pushed deep underground by plate tectonics. That creates a thin and relatively cool atmosphere. Venus' carbon remains as free as the planet's remarkably fast winds. This results in an extremely heavy atmosphere, smothered with tall decks of sulphuric acid clouds that trap the sun's heat. The soupy surface environment is hot enough to melt lead.
What caused these siblings, probably similar under the skin, to lead such dramatically different surface lives? A European spacecraft called 'Venus Express' is using the first instruments designed to peer through two narrow infrared windows, deep into and under the clouds, in an attempt to answer this questions. Results were presented in Nature and at an American Geophysical Union conference in San Francisco.
The clues likely lie in the atmosphere. On terrestrial planets like Earth and Venus, atmospheres are heated by direct sunlight near the equators and cooled at the less-directly lit poles. The cold polar air sinks then flows toward the equator. There, it gets heated by sunlight, rises, and flies back to the poles in an endless cycle. These 'Hadley Cells' -- mirrored north and south of the equator -- stir the air, distributing heat around the globe.
Earth's rapid spin distorts these cells, creating the familiar spiral patterns of terrestrial clouds seen from space. Venus' ultra-slow 243 Earth-day rotation and lack of complicating seasons creates a relatively 'pure' Hadley circulation.
Venus Express observed heated clouds boiling up directly under the sun, and giant double-vortexes at each pole where cooling air sinks like water down a drain. The newly discovered southern vortex is a mirror image of the northern one, known since 1979. Wide, relatively cool 'rivers' of air flow around each vortex.
Venus Express showed the vortexes stir the atmosphere at least to the bottom of the clouds, and likely lower, but probably not to the surface. Landers have measured only the gentlest of surface winds and radar images show that signs of aeolian transport like dunes or wind streaks are rare -- although they do exist. It takes extraordinary energy to move the heavy, liquid-like air near the ground.
Three distinct cloud layers, topped by a thin haze, have a collective depth of more than ten kilometers. There is nothing comparable to this vast 'sea' of clouds on Earth. The top two decks are thin fogs of tiny sulphuric acid particles. Venus Express saw strong day-to-day variations in the cloud-tops, and waves and convective cells are clearly visible. The orbiter used its radio link with Earth to sound out a series of strong temperature inversions that changed over time, suggestive of complex layering. The lowest deck is more dense, patchy, and broken -- more like clouds on Earth. Larger drops probably rain toward the landscape below, but evaporate long before they get there.
Winds moving ninety to one-hundred meters every second (224 miles per hour) whip the tops of the clouds around the planet in just four Earth days. The source of the energy to drive this 'super-rotation' remains a mystery. However, some still-unknown chemical or particle absorbs ultraviolet light, giving the clouds their dramatic ultraviolet patterns, and this energy may wind up as wind.
Venus Express may have confirmed the theory that Venus' water inadequacy is related to the planet's lack of magnetic field. The planet's slow rotation may not be fast enough to stir the outer metallic core into a magnetism-generating dynamo. With no magnetic shields, Venus' atmosphere is subject to direct attack by the solar wind.
Venus' atmosphere was known to be depleted in the lighter isotope of hydrogen, which escapes the planet's gravitational grip more readily than the heavier isotope, deuterium. The concentration of heavy hydrogen high above the clouds, where Venus' air flows from day to night rather than toward the poles, proved much higher than lower down. In one of Venus Express' more dramatic discoveries, Dr. Jean-Loup Bertaux of France's Service d'Aeronomie du CNRS, told Sky & Telescope that hydrogen is still undergoing thermal escape today.
If hydrogen were only escaping by being bounced out of the atmosphere by heat, relatively little could have disappeared – even at the newly determined higher rate and over the lifetime of the Solar System. Thus, only a relatively small amount of surface water would have existed on early Venus, according to Dr. Bertaux. That is considered by many scientists to be unlikely, although the dry, unlubricated surface might help explain Venus' apparent lack of plate tectonics. Even so, it is very hard to explain how the early Earth and Venus, made of similar materials in similar parts of the Solar System, could have started out with greatly different inventories of water.
Nor can we explain where water might be hiding. 'Water vapor distributes itself naturally over the whole atmosphere,' says Dr. Bertaux, 'except if there is some cold temperature trap at some altitude -- like Earth's tropopause. [Even in that case] some water vapor will always go high enough to get photodissociated, releasing hydrogen atoms for escape.' So, we're back to our question: where is Venus' missing water?
The loss observed now could be from impacting comets, or possibly volcanic explosions punching up though the atmosphere from below. Or, Dr. Bertaux argued, 'there could be other . . . non-thermal escape processes. One example is ionization of hydrogen and deuterium atoms,' both of which are then swept up by the Solar Wind. In that case, 'the mass difference is not important and the escape flux of deuterium might be as large as hydrogen.' If both escape at the same rate, the higher concentration of deuterium in today's atmosphere tells you only that Venus' water inventory was higher in the past, not how much higher.
Ionized oxygen atoms also are flying the coop, apparently from the solar wind sweeping over the sunrise and sunset terminators. Suggestively, Venus Express found that hydrogen and oxygen ions may be lost in the same two-to-one ratio as the hydrogen and oxygen atoms in water molecules (although NASA's MESSENGER spacecraft flying by Venus on its way to Mercury measured a different ratio).
The differences between Earth and Venus now seem a little less mysterious. Results so far broadly support ideas that the two planets started out with similar surface environments. Venus, closer to the sun, had warmer oceans -- which may have been smaller than Earth's deeper and colder seas -- and they evaporated faster. The water vapor retained more of the sun's heat than the thick carbon dioxide atmosphere could by itself. Over the eons, water molecules that made it to the top of the atmosphere were torn apart and ionized by solar ultraviolet light -- and slipped away in the wind from the sun.
As Venus quickly ran out of water to lose, the mechanism would have continued with ongoing sources like impacting comets and, possibly, volcanism, ensuring the planet retained her extraordinary dryness.
The Venus Express mission has been extended until mid-2009. By then, scientists hope to better understand the rate and mechanisms of water loss, showing more definitively whether Venus could have had the steaming, primeval oceans of early Science Fiction novels.
As the rising heat caused oceans to slowly boil away, any life swimming those seas would have been doomed to a premature and terrible end.
Life on Venus today? Ridiculous! Temperatures at the surface are 475 degrees Centigrade -- enough to sterilize even a science writer's kitchen. Dirk Schulze-Makuch at Washington State University, and Louis N. Irwin of the University of Texas, and other scientists, argue that conditions may be more benign high in the clouds where temperatures and pressures are Earth-like.
It's a hard life, but microbes do survive and reproduce in the water drops of terrestrial clouds. How about a droplet of sulphuric acid on Venus?
The large 'Mode-3' particles in Venus' lowest cloud deck are extraordinarily dry by terrestrial standards -- but they do contain water 'at a few hundred parts per million.,' according to Drs. Schulz-Makuch and Irwin Where there is water, carbon-based life is at least conceivable.
The Mode-3 particles are irregularly shaped and appear to be made of an unknown substance coated in sulphuric acid. There are organisms on and under Earthly ground that exist in environments approaching the acidity of Venusian clouds. Carbon, nitrogen, and phosphorous, all are present. The acid could block solar ultra violet radiation -- probably the key difficulty faced by life high in Venus' atmosphere -- while also providing energy.
In fact, these scientists argue that Venus' clouds may be a better environment for life than Earth's atmosphere. 'The clouds of Venus are a much larger, more continuous, and stable environment. Cloud particles last several months' as opposed to a few days in Earthly clouds. On the other hand, Venus expert Dr. David H. Grinspoon points out that, to be a viable habitat, the clouds would need to exist for the lifetime of Venus' hot environment -- which may be unlikely.
Many scientists believe Venus had an early hot ocean, and some believe it survived two billion years or more. The change to current conditions may have happened slowly, giving organisms time to adapt to life in the clouds, or in any water reservoirs deep underground. 'Observed deposits of explosive [volcanic] eruptions' and the fluid nature of some lavas imply high concentrations of ground water. Other scientists evoke an entirely dry surface to explain Venus' apparent lack of plate tectonics, and it is hard to imagine life underground surviving the global resurfacing (see the other box). Or, life may ride to Venus' atmosphere on debris splashed up during impacts on Earth or Mars.
All this speculation is fine, but is there a shred of evidence that life actually exists at Venus? Not much, but something unknown in the clouds is absorbing a lot of ultraviolet light -- which Dr. Grinspoon says behaves a little like chlorophyll. Lightning discovered by Venus Express probably tears molecules apart, allowing them to combine into new and potentially more complex forms. Down on the ground, Venus' atmosphere is known to be on chemically intimate terms with the surface, exchanging complex molecules with rocks. The atmosphere also is out of chemical equilibrium which requires ongoing chemistry to keep it that way.
The only other atmosphere with all these characteristics is Earth's.
For all their similarities, there are important ways that Venus differs from Earth. Venus smaller and closer to the sun. With her slow backward revolution the planet is an exception to the general rotation of the Solar System. That is hard to explain without evoking a giant impact that knocked the planet out of kilter early in its history, similar to the Mars-sized body that apparently stuck Earth. If so, no large Venusian moon like Earth's was created by the ejected material. Venus has none of the tides that Earth enjoys that may have played an important role in the formation and rapid evolution of life.
Venus is positively covered in volcanos, dotting the smooth, low-lying plains like pimples on a teenager, but there is little sign of the global system of plate tectonics Earth uses to rid herself of heat -- so how does Venus avoid melting? Maybe she doesn't. Cratering records suggest that something unknown, some four to seven hundred million years ago, caused the volcanic resurfacing of much of the planet. If that were to happen on Earth, we would not be around to write and read about it!
Donald F. Robertson is a freelance space industry journalist based in San Francisco.