|David Grinspoon, author of "Venus Revealed: A New Look Beyond the Clouds of our Mysterious Twin Planet" and "Lonely Planets: The Natural Philosophy of Alien Life"|
In warm and wet Key Largo, Florida, planetary scientists recently met to discuss a hot and dry place. The conference, “Exploring Venus as a Terrestrial Planet,” was sponsored by the American Geophysical Union. Participants who woke before dawn and drove a short way to the Atlantic side of the Key were treated to the stunning sight of Venus rising as a bright golden star over the ocean waters.
It was quite the Veneraphilic confab, the first international conference in ten years devoted fully to the planet next door. The theme of the meeting, expressed on a large banner behind the speaker’s podium, and also emblazoned on a celebratory cake at the conference banquet, was “Return to Venus!”
This was a celebration of new Venus missions, thanks to our colleagues in other countries. Many of the scientists and students attending were from Japan, a country that will be launching their first Venus orbiter in 2010. Also well represented were the various member states of the European Space Agency, whose Venus Express spacecraft was launched last November and will enter Venus orbit this April.
It was also a reminder that in current NASA planning, Venus remains “the forgotten planet.” Throughout the week it became clear that the importance of Venus in planetary science is not reflected in NASA’s current exploration plans. In fact, Venus has received visits from only two American missions in the last 30 years (not counting the fly-bys of Galileo and Cassini, which got a gravitational kick from Venus on their way to the outer solar system).
|Moon occulting Venus, the morning star, taken by the lunar probe, Clementine.|
Over four days, 50 scientific talks and 61 poster presentations gave a broad overview of the scientific fascination with “Earth’s Evil Twin,” as meeting organizer, Dr. Larry Esposito of the University of Colorado, refers to Venus. Topics ranged from surface geology, to the physics of planetary cores, to climate evolution, global atmospheric motions, cloud physics, the effects of the solar wind on the upper atmosphere, studies of Venus-like planets around other stars and, yes, even astrobiology.
Such breadth can be both a blessing and a curse. While it’s great to stretch one’s mind, who could possibly grasp all the knowledge, language and methods of each of the diverse sub-disciplines? It’s challenging for even the most broad-minded scientist to follow all these threads. For this reason, scientists attending planetary conferences have an understandable tendency of skipping sessions farthest from their own expertise. We’ve all been to meetings where the geologists fly in for the day of geology talks and then leave as a new crew arrives for the atmospheric science talks. Yet the most important breakthroughs often arise from communication and collaboration across disciplinary lines.
Recognizing this fact, the organizers started each session with a tutorial on the subject at hand. So we all got to go back to school, in addition to traveling to Venus in our minds (and sunny Florida in our bodies). This had a wonderful and surprising effect of making everyone stick around for the whole meeting. It was gratifying to see the conference room full to capacity the entire week, especially since my talk, “Astrobiology and Venus Exploration,” was the final one of the conference.
|Much of the surface of Venus is covered by lava flows.|
The meeting started with a bang, the talks devoted to the most contentious and divisive subject in recent Venus science: its geology. The focus was the history of volcanism and how that relates to the interior evolution of Venus. This in turn leads to considerations of differences between Venusian and terrestrial history. I was curious to see how this discussion would play out because of the huge controversy that erupted when the Magellan radar orbiter images were received in the early 1990s.
Why the hullabaloo? The Magellan images of the Venusian surface showed that impact craters are almost randomly distributed around the planet, with no notable areas of greater or lesser crater density. That’s weird. On most planets, older regions are densely cratered, while younger areas have few impact craters. Old craters tend to get erased by whatever geologic processes create the younger terrain.
So, the globally uniform crater density on Venus would seem to imply that the surface is all the same age. But why should a planet, especially an Earth-sized one which should have a hot interior and active geology, show a uniform surface age? It shouldn’t. Bad Venus.
When you look at the craters in detail, it gets even stranger. Only a small percentage of them have been modified by volcanic flooding, even though most of the planet is covered in volcanic plains which should have partially buried many craters.
Some scientists believe this is evidence that 700 million years ago, “catastrophic resurfacing” occurred on Venus, during which massive global floods of lava covered up the older craters. The flooding then rapidly ceased, leaving a surface of uniform age, which is now covered by a population of craters that were made after the event.
Other scientists didn’t buy it. They felt this radical picture was not necessary, that the volcanic plains could have been created gradually over much of Venusian history and still have left a surface randomly dotted with relatively pristine craters. Years of debate in the 1990s led to a somewhat acrimonious stand-down with no consensus and neither side very happy.
|The hot and dry surface of Venus is hidden from view by the thick carbon dioxide atmosphere.|
This became one of those strange impasses that sometimes paralyze our small communities, where normally very intelligent people can get entrenched and intransigent. Could it be inbreeding? It must sound strange to those outside the scientific community that people get so worked up over these things. But this is what we do for a living and we really care about getting the story right.
So I wondered how this issue, which created such rancor in the past, would play out at this meeting. I’m glad to report that the discussions of Venusian geological history were productive and enlightening. The skirmishes of the previous decade were jokingly referred to as “the food fight.” The mood was downright civil, and, because of the new Venus missions in the works, optimistic. New progress in mapping and analyzing Magellan data was reported, furthering debate. There are still disagreements over the details of Venus’s history (and surely will be for years to come – we need more missions!). Still, the community seems to be gradually converging on a story that is somewhere in between the two extreme views of the surface of Venus being mostly one age versus Venus having randomly distributed surface ages. There was some kind of a major global transition, and much volcanic flooding, around 700 million years ago. But there has also been much activity since that time, and smaller areas show a diversity of ages and histories.
|Nice and hot, but a little dry: a “Return to Venus” cake.|
Worries of a new food fight did not materialize, even at the conference banquet. The chicken could have been manufactured by Goodyear, but we were entertained after dinner by Andrew Nagy’s humorous and insightful reminiscences of the Pioneer Venus project. He took us back to an earlier age of planetary exploration, when we were still racing the Soviets and enjoying the funding that comes with perceived national security threats. (Hey, maybe Osama is hiding in the mountains around Ishtar Terra!) The key lime pie, with a Venus cake chaser, quickly erased memories of the rubber chicken.
On Tuesday we moved on to discussions of the atmosphere, clouds and climate. The Venusian atmosphere has a strange “super-rotation,” whipping around the globe many times faster than the slow rotation of the planet. New ground-based telescopic observations, combined with sophisticated computer modeling, are getting us closer to understanding the complex chemistry, clouds and motions of the atmosphere. But there are major gaps in our knowledge that can only be answered with new visits to the cloudy planet.
What’s clear is that the climate and geology of Venus are strongly linked together. Volcanoes add gasses to the atmosphere, especially sulfur dioxide and water vapor, that contribute to the sweltering greenhouse and help maintain the global sulfuric acid clouds. In turn, changes in climate seem to have affected surface geology and maybe even the interior evolution of the planet.
Wednesday’s discussions turned to the tenuous upper atmosphere and the ways it’s affected by the solar wind. These interactions are the key to understanding how gases are escaping into space. We need to understand them if we hope to extrapolate current conditions back into the past, when the atmosphere was thicker and full of more water, and the climate more Earth-like.
|Ultraviolet image of Venus obtained by Pioneer-12.|
Image Credit: BNSC
The final day was exciting, covering new and future missions to Venus. We heard reports from several scientists involved with Venus Express which, as we spoke, was closing in on Venus. We learned some details about the Japanese PLANET-C Venus Climate Orbiter, which will conduct long term monitoring of the clouds and atmosphere, as well as search for signs of active volcanism on the surface. We also heard about plans for observing Venus from the American MESSENGER spacecraft as it briefly encounters Venus on its way to Mercury, in June 2007.
The final session dealt with extrasolar planets and astrobiology. But, wait a minute, this meeting was supposed to be about Venus – why were we talking about other solar systems and the search for life? Because, as Dr. Victoria Meadows from JPL explained, many extrasolar planets are likely to be Venus-like. If we want to be prepared for the coming era of comparative planetology, when we have data about Earth-sized planets around other stars, we had better pay more attention to Venus.
In the final talk, I made the case for the relevance of Venus to astrobiology. I spoke of the importance of Venus to our general understanding of how Earth-like planets work. After all, Venus is the only other example of an Earth-sized planet that we can hope to study up-close within the foreseeable future. We still don’t understand how and when Venus diverged from a more Earth-like past, or how long Venus or Mars retained habitable conditions. If life begins easily on warm, wet planets (which seems to be the case, judging from Earth) then Venus probably once harbored Venusians. Did they die out as the climate changed? Venus can help us to understand the evolutionary paths that habitable planets can take.
|Venus up-close, as photographed by the Soviet Venera 13 lander, which parachuted to the Venusian surface on March 1, 1982|
Another point I made is that Venus is starting to seem like a very interesting analog to early Mars. The Mars Exploration Rovers are revealing that if life ever formed on Mars, it probably existed in a highly acidic, sulfur-rich place. Sounds kind of like Venus, where the clouds are acidic, and the surface rocks are red, layered and sulfur-rich. I also discussed the possibility that there just might even be life today in the clouds of Venus. When the oceans disappeared in a runaway greenhouse, did the Venusians all die out, or did they move to a new habitable niche? The clouds are highly acidic, but we have no reason to believe that life cannot adapt to such extreme conditions. There are energy sources and stable locations within the clouds. We don’t know enough about life as a universal phenomenon to rule out life there (in fact we know about only one biosphere – Earth’s – which is next to nothing…). These ideas received an appropriately skeptical but respectful hearing. Anyway, the room was full until the end, and nobody threw any food at me.
I ended with a summary of crucial questions and observations for future astrobiological exploration of Venus:
How and when was the surface water lost?
What is the history of the climate?
What minerals are on the surface?
How old are the surface rocks in different regions?
Are there isotopic clues to a past biosphere?
What is the gas composition of the near-surface atmosphere?
What do the “noble gases” tell us about the origin and history of the atmosphere?
How are gases escaping from the upper atmosphere and how does this change in response to changes in the sun?
What is the “unknown ultraviolet absorber” seen in the upper clouds?
What unknown trace chemicals exist in the clouds?
What is driving the rapid atmospheric circulation?
|Mars (left), Earth (middle), Venus (right) with relative sizes.|
This list of needed astrobiological observations from future missions is basically identical to those desired to understand the planet, and its evolutionary divergence from Earth, more fully. The point is that what is good for planetary exploration of such a promising target – so Earth-like in bulk qualities but so alien in environmental evolution – is also good for astrobiology.
The conference ended with widespread agreement that to really crack the unyielding mysteries of our sister planet, and thereby figure out how Earth-like planets work, we will need more ambitious missions after the current round of orbiters. We will need to again brave the challenging depths of Venus with entry probes and landers that study the still mysterious deep atmosphere, reveal the subtle gaseous details that carry traces of the vanished past, and report directly on the minerals and rock types inhabiting the searing surface.
Back to Venus. Let’s go!
Talks from this meeting are posted online at the Venus Exploration Analysis Group (VEXAG) web site. A monograph of papers summarizing ideas from this conference will be published later this year by the American Geophysical Union.
Related Web Pages
Venus: Hothouse Planet
Venus: Inhabited World?
Visit Lonely Planets, Parts A: Introduction * 1 * 2 * 3 * 4 * B: Encore
Soviet Exploration of Venus
Magellan Mission Home
Fact Sheet on Venus
Past Missions to Venus
Atmosphere and Weather on Venus
Chemical Weathering Reactions on Venus