Cloud Colonies on Venus
By: Leslie Mullen
Could there be life on Venus? The idea appealed to Carl Sagan, especially when he considered the mysterious radio emissions that emanated from the planet. But then Sagan learned the radio signals were due to the planet's searing hot average surface temperature of 453 degrees C (847 F).
The high temperatures, along with a crushing surface pressure of 90 atmospheres (equivalent to the pressure of being 3,000 feet below the sea), are due to Venus's thick cloud layers of sulfuric acid and the carbon dioxide "greenhouse" atmosphere. But 50 kilometers up from the surface, the temperature ranges between 30 and 80 C (86 to 176 F), and the pressure is only one atmosphere - the same as sea level on Earth. Sagan suggested that organisms with hydrogen gas bladders could float amid these more hospitable cloud layers.
David Grinspoon, Principal Investigator for NASA's Exobiology Research Program, carried on Sagan's vision in his 1997 book "Venus Revealed." Grinspoon suggested that microbes living in the planet's clouds could explain several unusual observations.
"When I originally suggested that unexplained signs of chemical disequilibrium in the atmosphere of Venus could be signs of life, I was extremely curious to see what the reaction of the scientific community would be," says Grinspoon. "Initially, it was the worst possible reaction - it was ignored!"
But now Dirk Schulze-Makuch of the University of Texas at El Paso has investigated whether these observations of Venus may point to signs of life. Schulze-Makuch and his colleague Louis Irwin looked at data gathered from the Russian Venera space missions and the US Pioneer and Magellan Venus probes.
He says one of the observations that could indicate life is the simultaneous existence of hydrogen sulfide and sulfur dioxide. These gasses react and destroy each other on short time scales, so they are rarely found together unless something is continuously producing them. On Earth, anaerobic bacteria can produce these gases. But so can volcanoes. Venus, like Earth, is a geologically active planet with many volcanoes. In fact, much of the surface of Venus is covered by lava flows. However, volcanoes are not necessarily the source of the hydrogen sulfide, says Schulze-Makuch.
"The interesting pattern that we noticed is that hydrogen sulfide concentrations around the 50 kilometer cloud level are higher than near the surface of Venus," says Schulze-Makuch. "The pattern you would expect, if hydrogen sulfide can be attributed to volcanic exhalations only, is a maximum concentration near the surface and a dramatic decrease as you go up in altitude, especially given the oxidizing atmosphere."
Yet Grinspoon says he doesn't completely believe the reported altitude profiles of hydrogen sulfide. He says we need to send new missions to Venus in order to measure this seeming anomaly.
"Also, many chemical reactions occur much faster in the lower atmosphere where it is hotter," says Grinspoon. "This could potentially include reactions that destroy hydrogen sulfide. However, we just don't have a clear enough picture of what is going on chemically near the surface to come to firm conclusions. I regard life as a plausible explanation at present."
The existence of carbonyl sulfide is another potential indicator of life. On Earth, trees and microorganisms produce carbonyl sulfide. But again, volcanoes also emit this gas.
Since hydrogen sulfide, sulfur dioxide, and carbonyl sulfide all can be produced by volcanic activity, it would seem that volcanism is the most likely explanation. But Grinspoon says volcanism on Venus would have to be very active for it to be the culprit, and Venus is considered less volcanically active than Earth.
And, stresses Schulze-Makuch, for hydrogen sulfide and carbonyl sulfide to form, both reactions need catalysts. Such catalysts can be other chemicals or metals, but it is unknown which specific catalysts are present and active in the Venusian environment. He points out that on Earth, the most efficient catalysts are microbes.
Perhaps the deciding clue rests on the lack of carbon monoxide in the Venusian atmosphere. Schulze-Makuch says solar radiation and lightning should be producing large quantities of carbon monoxide, yet that gas is scarce. The missing carbon monoxide, he suggests, could be due to organisms that use the gas in their metabolisms.
On Earth, many organisms use carbon monoxide in their metabolisms. For instance, at least 450 strains of photosynthetic bacteria use carbon monoxide as their sole carbon source. The lack of carbon monoxide on Venus can't be explained away by volcanic activity, either. However, non-living chemical reactions can remove carbon monoxide from an atmosphere.
"For example, there are cycles involving chlorine chemistry which recycle carbon monoxide and various oxygen compounds back into carbon dioxide," says Grinspoon.
But Grinspoon also thinks that life is another possible explanation for the missing carbon monoxide. Schulze-Makuch suggests that if microbes are living in the Venusian clouds, they could be combining the sulfur dioxide with carbon monoxide and possibly hydrogen. This could lead to the production of either hydrogen sulfide or carbonyl sulfide in a metabolism similar to that of some early Earth microbes.
The ancestors of such early Earth microbes can be found today in hydrothermal vents under the sea. These organisms are chemosynthetic, meaning they use chemicals instead of sunlight to produce their food. What makes these organisms especially interesting is that they live in anoxic, acidic environments at high temperatures, which is roughly comparable to the conditions in Venus's clouds.
Other Earth organisms that might give us clues to a Venusian metabolism are the green and purple sulfur bacteria. These are thought to be some of the earliest forms of photosynthetic bacteria on Earth, and they use sunlight to convert carbon dioxide and hydrogen sulfide to glucose, releasing sulfur as a waste product. It's tempting to think these organisms might feel at home on Venus, with its carbon dioxide atmosphere, clouds of sulfur, and traces of hydrogen sulfide.
Photosynthesis requires light, but it is unknown how much sunlight can penetrate Venus's thick cloud layer. Still, Schulze-Makuch suggests the microbes on Venus could be using ultraviolet (UV) light from the Sun as an energy source.
"UV light is harmful for all Earth microbes," says Schulze-Makuch. "What likely would have to happen is the UV light is broken down by a pigment to visible light or near-infrared light frequencies so it can be used for photosynthesis. We are currently working on what kind of pigment compound could be used."
He says the utilization of UV light by microbes would explain the dark patches on UV images of Venus. The current non-biological explanation for these dark markings is that they are regions of sulfur dioxide near the cloud tops.
One of the prerequisites for "life as we know it" is liquid water, yet water only makes up .003 percent of Venus's present atmosphere. Most of the water on Venus is in the form of misty droplets suspended in the clouds at the 50-km range. Venus may have had global oceans of water earlier in its history, however. If so, then life could have emerged.
As the Sun grew hotter, this water would have boiled away. Perhaps life made a transition to the atmosphere in order to survive. Nature is remarkable for its ability to adapt to different and changing conditions. On Earth, says Schulze-Makuch, carbon dioxide is scarce, but terrestrial organisms learned to assimilate it from the atmosphere.
"Thus, in a water-limited environment where carbon dioxide is common and water is scarce, microbes could have adapted to assimilate water vapor from the atmosphere," he says.
But Grinspoon says that life on Venus might not need the water droplets at all. He suggests that water may not even be necessary for life to arise.
"The requirement of liquid water is a current consensus, a fad you might say, but completely unsupported by any scientific theory or observation," says Grinspoon.
Other than the presence of liquid water, one of the few agreed upon universals for life is that it creates and thrives on chemical disequilibrium. Although each case of chemical disequilibrium on Venus has a potential non-living explanation, each instance stacks up toward a greater possibility for life in the clouds.
But according to Grinspoon, chemical disequilibrium may not be the strongest indication of life on other worlds.
"Confronting Venus makes us realize that it is not enough to simply state that life creates disequilibrium," says Grinspoon. "This is true, but there are many non-biological processes that also destroy equilibrium, such as ultraviolet light and lightning. As I pointed out in 'Venus Revealed,' not all life increases disequilibrium. You and I, for example, breathe in oxygen and exhale carbon dioxide, thereby eating up some of the disequilibrium provided for us by green plants. So an atmosphere mysteriously close to equilibrium might also be a sign of life."
The intriguing thing about Venus, says Grinspoon, is that it has too much equilibrium in the case of carbon monoxide - you would expect to see more carbon monoxide because it is always being made in the upper atmosphere by the photochemical destruction of carbon dioxide. But at the same time, there's too much disequilibrium in the case of the sulfur gasses. So the question we need to ask, he says, is how much disequilibrium is necessasary to be a sure sign of life?
"Must all living planets be blatantly out of chemical balance?" Grinspoon asks. "Venus presents us with an intermediate case. It is not obviously dead like Mars, from an equilibrium standpoint, nor is it obviously alive like Earth. It is somewhere in between."
Schulze-Makuch says Venus's chemistry is not the only indication of life. Non-spherical particles of unknown composition have been detected in the atmosphere. These particles could be microbes. In addition, the large, continuous, fast-moving clouds would make for a stable microbial environment, allowing them to remain suspended in the atmosphere for several months (as opposed to only a few days on Earth).
"It is gratifying to see these ideas finally enter the refereed scientific literature and to see research scientists entertain them without too much fear of embarrassment," says Grinspoon. "I have been talking about this for years, and have felt like a lone voice in the wilderness until very recently. Perhaps it is a long shot, but I regard present-day life on Venus as much more likely than present-day life on Mars."
Grinspoon says that only way to find out for sure if there is life on Venus is to send new missions. Such missions could give us a more complete picture of the various processes that shape the chemistry of the atmosphere.
The National Academy of Sciences recently released a report about solar system exploration goals over the next decade. One of the top priorities recommended was a mission to Venus to study the planet's atmosphere and surface. Grinspoon is currently working with other Venus scientists to develop this mission.
The next planned Venus mission is the European Space Agency's Venus Express, launching in November 2005. This mission will study the atmosphere, surface, and plasma environment of Venus. The spacecraft will arrive at Venus after a flight of about 150 days, and then brake into a highly elliptical 5-day orbit around the planet. It will then assume a polar orbit between 250 km and 45,000 km above the planet for 450 Earth days (2 full Venusian years).