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Venus today seems like an embodiment of Hell. Its surface temperatures and pressures wouldquickly kill even the most resilient Earth microbes. It’s scorched dry and inhospitable. The thicksulfurous 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 wroteabout Venus: “A great part of the surface of Venus is no doubt covered with swamps . . . Theconstantly uniform climatic conditions which exist everywhere result in an entire absence ofadaptation 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 theever-present clouds could have easily been water vapor.

The popular image of Venus consisted of hot humid rainforests, swamps, and oceans teemingwith 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 slowlyshifting 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 conditionsthan 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 summarizedthe current state of knowledge and some conjectures about the planet (which included “oilseas” hidden beneath a smog layer, or a “soda ocean,” besides the more likely possibility of adesert), 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 molecularoxygen. . . . Before such a scheme can be seriously considered, much more information must beacquired about the composition and meteorology of the Cytherean [Venusian] atmosphere, andextensive laboratory biological investigations must be performed. Nevertheless, some tentative

Science Fact

Hell Is OtherPlanets

Julie Novakova

How our image of Venus went from paradise to hell—and possibly back

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 thereis no liquid water anywhere on Venus, the organism must be able to utilize water vapor (fromthe 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 microf lora awaiting the next ecologicalsuccession, Venus will have become a much less forbidding environment than it appears to beat 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 fitthat 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 bythe Venera, Pioneer Venus, and Vega spacecraft specified the conditions, and also delivered amortal blow to Sagan’s terraformation vision, which depended upon a thinner atmosphere. Noalgae could possibly convert tha t much carbon dioxide, even if they managed to survive the

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Mariner 2, artistic depiction.https://commons.m.wikimedia.org/wiki/Mariner_2#/media/File%3AMariner_2.jpg

acidic, low-water conditions of Venus’ cloud deck; and even supposing the scheme worked, theresult would be nothing like habitable: a thick layer of fine graphite on the surface, and thickpure-oxygen atmosphere. One of the f irst, and in its simplicity quite ingenious, terraformingproposals was therefore off the table.

In 1968, Brian Aldiss edited a science fiction anthology Farewell, Fantastic Venus. It honoredthe old view of Venus, while the world was coming to terms with the new one. The dreamVenus of our past was irrevocably gone. But perhaps . . . we should look into Venus’ own pastand see what we find there.

* * *Through the looking glass into the

land of modelsReconstructing 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 tectonicsstart, what was the composition of early Earth’s atmosphere, whether life could have appearedbefore the Late Heavy Bombardment. The further into the past we go, the more uncertain ourpicture gets.

But to compare Earth’s and Venus’ evolution, we perhaps must go to the very beginning. Bothplanets formed approximately 4.6 billion years ago in the circumstellar disk around the youngSun. Both are very similar in size and composition. The most marked difference anyone willpoint out first are their different orbits. Earth orbits roughly 150 million km far from the Sun, or1 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 ofall 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 turnEarth’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 albedoand 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 modelspostpone 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 atemperate climate even under higher insolation values, up to some extent—it’s largely an effectof 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 itspast (we can conjecture that from the hydrogen/deuterium ratio in Venus’ atmosphere, pointingat substantial water loss), it was never in a liquid state. So far, we cannot completely exclude thepossibility that Venus started out with a thick vapor atmosphere and a magma ocean that hadquickly 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 itsthick carbon dioxide atmosphere.

So how can we tell which of these “retellings of Venusian past” rings closest to the truth?

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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 thatit sports very few craters, too few even when accounting for its thick atmosphere. There werealso vast plains, likely of volcanic origin, and more volcanic features than you can count. Whathappened there?

It seems that roughly a half a billion years ago, the planet underwent a massive volcanic eventleading to global resurfacing. Very little, if anything, of earlier would have survived that. But ifsomething had . . . it might provide evidence of rock-water interaction, past temperatures, andmore. We could also study the isotopic composition of Venus’ atmosphere, which can furtherconstrain things such as outgassing of rock, water loss, water delivery by impacts, and so on.

Sending a mission capable of doing that to Earth’s “sister planet” would be advisable. Butsome are proposing other missions goals as well—among them search for life.

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Simulated view of Venus based on the Magellan radar images. https://photojournal.jpl.nasa.gov/catalog/PIA00104

* * *Venus as an abode of life?

It might sound preposterous: life on Venus?! But the notion is not nearly as crazy as it mayseem. First, let us consider the more optimistic of Venus’ climate models. If they prove correct,then Venus may have had liquid water oceans for about half of its history, more than long enoughfor life to originate and develop. But could anything have survived the ocean evaporation?

On Earth, you can find life practically anywhere you look—be it on the surface, below it in thedepths of the ocean or rock . . . or above it. It would be exaggerating to say that Earth’s tropos-phere—the lower part of the atmosphere, essentially where weather happens—is teeming withlife. But it is present there. We don’t know of any microorganisms specialized for life in the tinywater droplets or on dust grains in the air, but we know that many can survive such an environ-ment for a prolonged period of time, and it’s probably a common way of transport of microor-ganism across the planet. They can’t control how long they’ll stay there or where they end up,but there is enough water and basic nutrients for them to survive. Some can even be lifted bystrong currents and electric fields in storms into the stratosphere.

But could life adapt to living solely in the air? On Venus, the current conditions seem quite fa-vorable. Its cloud deck has temperature and pressure similar to Earth’s surface, and unlike inEarth’s atmosphere, the circulation is very stable. With the currents present there, even a largedust grain could spend months up there before sinking to the hell below. Dust grains presentthere could potentially provide basic nutrients and shielding against UV irradiation. The wateravailability isn’t good and acidity is high, but overall, simple acidophilic algae known from manyEarth environments should be able to survive there.

There have been f indings raising our hopes in this matter, among them especially severalchemical disequilibria in Venus’ atmosphere. If two chemical species that react together are pre-sent, generally an equilibrium will establish—unless something is producing or depleting one ofthe reactants. On the other hand, these disequilibria can be just as easily explained by UV-drivenchemistry on the grains. There is also the question of the long-term stability of the cloud envi-ronment. The clouds need gases such as sulphur dioxide to form, and it is produced by volcanicactivity. Has Venus been constantly volcanically active, or did it have quiescent periods? If so, didthe clouds persist? And could life adapt to changes in the atmospheric pressure and composition?On Earth, life adapted to catastrophes such as the great oxygenation event, which overturned theprevious atmospheric composition; but changes in temperature and pressure had been compar-atively small.

Alas, it seems unlikely that Venus today harbors life—but to test the possibility, we should gothere. As to long-extinct life . . . To those who enjoy big, hard-to-achieve dreams, I suggest lookingfor Venusian meteorites. Finding a rock from some epoch of the planet’s history could tell usmore about its atmosphere and temperature during that time, and if we’re being very optimistic,it could even yield fossils of hypothetical Venusian life. That’s why the Moon presents a great as-trobiological location: it must be laden with meteorites from various epochs on Earth, providinggreat grounds for fossil hunters. The meteorites would remain pristine in the Moon’s vacuum.Temperature changes, radiation and particle sputtering might erode the tiny ones in time, butthere should still be a lot of work for space paleontologists. Could there also be some meteoritesfrom Venus?

We know of at least 120 Martian meteorites on Earth, and that’s only the recent ones thathaven’t been destroyed by the active geology of our planet and that the scientists have found. ButMars is smaller and has a lower escape velocity. Moreover, it’s outward in the Solar System, andany ejected debris would more likely spiral inward—down the gravitational well on a slow jour-ney into the Sun. Or Earth, if it crashes here. Venus is inward, larger than Mars, and currently hasan atmosphere so dense that only a huge impact might be able to first hit the surface withoutburning up in the atmosphere, and then eject debris with sufficient velocity. Therefore we canhardly expect finding any recent Venusian meteorites anywhere. But what of its past? If it had athinner atmosphere two or three billion years ago, could there be something? The odds arestrongly against us, but it’s not impossible.

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* * *The quest for answers

How long, if ever, did Venus harbor liquid water? Did it ever possess plate tectonics, so farknown only from Earth, or perhaps an intrinsic magnetic field like Earth or Mercury? How andwhen did it gain its extremely slow rotation? How wild were its “mood swings” throughout his-tory? If we can say anything with certainty about Venus, it’s that we need to go there to get the an-swers to our questions. We need to scour the clouds with atmospheric probes to try to find thepossible, if highly improbable extant life. We need to measure the levels of various elements’ iso-topes in Venusian atmosphere to constrain the history of outgassing, atmospheric loss, and more.We need to land on the surface and analyze the oldest rocks we can find to learn whether theyhave been altered by chemical interaction with water, or whether they hold traces of an ancientmagnetic field (Venus currently has none, but that’s not saying it has always been so), if past tem-peratures on Venus allowed magnetization to persist . . .

There are detailed concepts on how to do it. But none of them is currently slated to actually goto Venus. ESA’s Venus Express provided us with a good picture of the circulation in Venus’ atmo-sphere, exciting hints for ongoing volcanism, and more, but the mission ended in early 2015 andwasn’t equipped to answer many of the remaining questions. JAXA’s Akatsuki enjoyed miracu-lous success in its second attempt to enter Venus’ orbit and is currently observing the planet, butit too wasn’t built for measuring isotopic composition or search for potential biosignatures. NASAhad two Venus missions in its latest Discovery-class mission selection, DAVINCI and VERITAS—both focused on the second planet’s atmosphere and potentially able to provide some of the an-swers we’re waiting for. But neither was selected for implementation.

Missions focused on surface geochemistry would be much more demanding. But many suchconcepts exist, such as VISE, Venus Mobile Explorer, VITL . . . Apart from those, JPL scientist andwell-known SF writer Geoffrey Landis is the author of an intriguing proposal of a Venus landsail-ing rover. The rover is conceived to be low-energy, sturdy, as heat-resistant as possible and yet ca-pable of achieving impressive science results. But it remains on paper as of now, and likely will fora prolonged time or forever. Mission selection process is always difficult and must rely on whatthe scientists want, what the engineers can do, what are the launch options, how much financeis at disposal, whether the mission exceeds the cost cap or has its resources trimmed, and more.Venus has the bad luck of not being among the top priorities of any space agency as of this time.In Solar System exploration, NASA focuses mostly on Mars, and the “Ocean Worlds” are becom-ing the next great priority. In addition, an ice giant orbiter might become the next big mission(Flagship-class). ESA also has Mars plans with its ExoMars 2020, it plans to go to Mercury in co-operation with JAXA, and Jupiter’s icy moons have a green light for the JUICE mission. Venus iscurrently not on the menu, despite being much easier to reach than Mars. Does Venus need bet-ter PR in order to spark the interest of established as well as new space agencies, or even privatecompanies?

Luckily, Earth’s sister planet has a lot of outspoken advocates. Their arguments don’t centerjust on the planet itself. Venus’ fate is increasingly relevant for exoplanet research. If we want tosearch for “Earth 2.0,” we should be able to discern it from “early Venus 2.0.” Why so? WhenVenus’ oceans evaporated and it was rapidly losing its hydrogen, a lot of oxygen could have stayedin its atmosphere for a prolonged period of time. Let us imagine we find a planet near the inneredge of the “habitable zone,” with the right size and mass, and with oxygen spectral signature inits atmosphere. Is it a reason for celebration, for we have found alien life?

Not quite. Without other biosignatures or good temperature measurements, it’s possible thatwe’ve found a second Venus. Moreover, learning more about Venus’ history enables us to form abetter picture of Earth’s future. As the Sun’s luminosity increases, Earth is in for a drastic if slowand distant-future change. Will its oceans evaporate soon enough and leave behind a potentiallyhabitable if quite hot desert planet? Or will it suffer the same fate as Venus and turn into a pres-surized hellscape? We must hope that some Venus missions will be selected in the foreseeable fu-ture. Only that will enable us to learn more.

For now, Venus remains shrouded in her veil of mystery.

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