Science Fact Hell Is Other Planets How our image of Venus went from paradise to hell— and possibly back Julie Novakova Venus today seems like an embodiment of Hell. Its surface temperatures and pressures would quickly kill even the most resilient Earth microbes. It’s scorched dry and inhospitable. The thick sulfurous clouds never part to allow a glimpse of the Sun from the surface. This is the image Mariner 2 and other probes brought us since 1962. Before, though, our im - age of Venus could not have been more different. Back in 1918, the Swedish physicist, chemist, and Nobel laureate Svante Arrhenius wrote about Venus: “A great part of the surface of Venus is no doubt covered with swamps . The constantly uniform climatic conditions which exist everywhere result in an entire absence of adaptation to changing exterior conditions. Only low forms of life are therefore represented . .” He was by no means alone in thinking so. Many scientists of that time considered Venus a po - tentially habitable place. It was similar to Earth in size, its surface could not be seen, and the ever-present clouds could have easily been water vapor. The popular image of Venus consisted of hot humid rainforests, swamps, and oceans teeming with life. Science fiction stories of that era reflected this picture (and happily added lustful ama - zons, dangerous dinosaurs, and other proven tropes). But the scientific consensus was slowly shifting toward a less hospitable place. In the 1930s, after detection of carbon dioxide as the pri - mary component of the Venusian atmosphere, many expected much hotter surface conditions than thought before. But hope was not lost: Even if present-day Venus was uninhabitable, per - haps we could make it habitable. In his 1961 article in Science , Carl Sagan not only summarized the current state of knowledge and some conjectures about the planet (which included “oil seas” hidden beneath a smog layer, or a “soda ocean,” besides the more likely possibility of a desert), but also hinted at terraformation possibilities. “. if, indeed, Venus proves to be without life, there will exist the prospect of microbiologi - cal planetary engineering,” Sagan wrote. “To prepare Venus for comfortable human habitation, it is necessary to lower the surface temperature and to increase the partial pressure of molecular oxygen. Before such a scheme can be seriously considered, much more information must be acquired about the composition and meteorology of the Cytherean [Venusian] atmosphere, and extensive laboratory biological investigations must be performed. Nevertheless, some tentative 60 JANUARY /F EBRUARY 2018 Mariner 2, artistic depiction. https://commons.m.wikimedia.org/wiki/Mariner_2#/media/File%3AMariner_2.jpg specifications can be entertained at the present time. In order to have appreciable photosyn - thesis before thermal dissociation, the life form deposited must be a microorganism. Since there is no liquid water anywhere on Venus, the organism must be able to utilize water vapor (from the atmosphere) or ice crystals (from the cloud layer). The only photosynthetic, nitrogen- fixing, oxygen-evolving, temperature-resistant aerial microorganisms are the blue-green algae, primarily of the Nostocaceae family. But it is conceivable that these problems can be solved, and that the microbiological re-engineering of Venus will become possible. With a few cen - timeters of precipitable water in the air, surface temperatures somewhere near room tempera - ture, a breathable atmosphere, and terrestrial microflora awaiting the next ecological succession, Venus will have become a much less forbidding environment than it appears to be at present. Hopefully, by that time we will know with more certainty whether to send a pale - obotanist, a mineralogist, a petroleum geologist, or a deep-sea diver.” A beautiful vision, isn’t it? In December 1962, Mariner 2 made the first flyby of Venus—and put the last nail into the cof - fin of lush, teeming-with-life Venus. Surface temperature measurements above 400°C hardly fit that picture. Although the probe did not feature any cameras and only made a brief flyby, it de - livered a mortal blow to the fantasies of a pleasant tropical world. But even then, we did not know the full extent of Venus’ hellishness. Later observations by the Venera, Pioneer Venus, and Vega spacecraft specified the conditions, and also delivered a mortal blow to Sagan’s terraformation vision, which depended upon a thinner atmosphere. No algae could possibly convert that much carbon dioxide, even if they managed to survive the HELL IS OTHER PLANETS 61 ANALOG acidic, low-water conditions of Venus’ cloud deck; and even supposing the scheme worked, the result would be nothing like habitable: a thick layer of fine graphite on the surface, and thick pure-oxygen atmosphere. One of the first, and in its simplicity quite ingenious, terraforming proposals was therefore off the table. In 1968, Brian Aldiss edited a science fiction anthology Farewell, Fantastic Venus . It honored the old view of Venus, while the world was coming to terms with the new one. The dream Venus of our past was irrevocably gone. But perhaps . we should look into Venus’ own past and see what we find there. * * * Through the looking glass into the land of models Reconstructing planetary history is a difficult job. We barely know how exactly Earth started out. Yes, we have radiometric dating, planetary accretion models, and many proxies through - out our planet’s history—yet we don’t know when and how exactly did Earth’s plate tectonics start, what was the composition of early Earth’s atmosphere, whether life could have appeared before the Late Heavy Bombardment. The further into the past we go, the more uncertain our picture gets. But to compare Earth’s and Venus’ evolution, we perhaps must go to the very beginning. Both planets formed approximately 4.6 billion years ago in the circumstellar disk around the young Sun. Both are very similar in size and composition. The most marked difference anyone will point out first are their different orbits. Earth orbits roughly 150 million km far from the Sun, or 1 au (astronomical unit). Venus circles the Sun just a little over 0.7 au away from our star. “Cir - cles” is a metaphor not far from reality in this case. Venus has the lowest orbital eccentricity of all planets in our system. Is its proximity to the Sun the sole cause of its hellishness today? Being closer to our star, Venus has always received much more insolation than Earth. But would it alone suffice to turn Earth’s sister into Hell—and if so, how quickly? Climate models can help out. The first came in late 1980s, courtesy of professor James Kast - ing of Penn State—the same Kasting who later developed the concept of habitable zones, the in - ner edge of which is defined by the runaway greenhouse effect. Water vapor is a powerful greenhouse gas. Imagine you have water oceans, but your planet re - ceives a lot of sunlight, enough to lead to substantial evaporation of the oceans. More water va - por in the atmosphere gets you even hotter, even though clouds can increase the planet’s albedo and reflect a lot of sunlight. The evaporation continues, and you enter a positive feedback loop, until all of your oceans turn to vapor and you have a scorching steam atmosphere. That, in short, is runaway greenhouse effect. But how quickly did it happen on Venus? The estimates vary wildly. Kasting’s model led to an es - timate of six hundred million years, if we discount the cooling influence of clouds. Other models postpone ocean evaporation to over two billion years of existence—roughly half the age of the So - lar System. Optimistic, isn’t it? If Venus kept its oceans for two billion years, life could have devel - oped there, couldn’t it? These models usually take into account the cooling effect of clouds, raising the planet’s albe - do, and Venus’ slow rotation (a day on Venus takes longer than its year), which would allow a temperate climate even under higher insolation values, up to some extent—it’s largely an effect of cooling through the planet’s nightside and concentration of cloud cover on the dayside. But there are also models suggesting that although Venus may have had lots of water in its past (we can conjecture that from the hydrogen/deuterium ratio in Venus’ atmosphere, pointing at substantial water loss), it was never in a liquid state. So far, we cannot completely exclude the possibility that Venus started out with a thick vapor atmosphere and a magma ocean that had quickly outgassed more greenhouse gases. Solar irradiation broke down water vapor into hy - drogen and oxygen, the hydrogen escaped, oxygen was sequestered, and Venus soon gained its thick carbon dioxide atmosphere. So how can we tell which of these “retellings of Venusian past” rings closest to the truth? 62 JULIE NOVAKOVA JANUARY /F EBRUARY 2018 Simulated view of Venus based on the Magellan radar images. https://photojournal.jpl.nasa.gov/catalog/PIA00104 Most terrestrial objects have their history written all over them. We can see the cratering, ridges, cracks, fossil outflow channels . What about Venus? Starting in 1990, the Magellan probe imaged Venus through radar observations and found that it sports very few craters, too few even when accounting for its thick atmosphere. There were also vast plains, likely of volcanic origin, and more volcanic features than you can count. What happened there? It seems that roughly a half a billion years ago, the planet underwent a massive volcanic event leading to global resurfacing. Very little, if anything, of earlier would have survived that. But if something had . it might provide evidence of rock-water interaction, past temperatures, and more.