Venus, Titan and the Great Filter

Diverse living worlds may be bad news for would-be galaxy colonists

Colin Robinson
Nov 16, 2020 · 9 min read
The Dragonfly probe is scheduled to reach Titan in 2036 and look for evidence of life there. (NASA graphic)

There are good reasons to think that Titan and Venus may have life, or something very like it. If so, it implies an awe-inspiring diversity of living worlds in the Milky Way Galaxy. For those who dream of humans colonising the Galaxy, this would be bad news, because of what it says about the Great Filter…

The recent report about Venusian phosphine is the latest in a series of surprises from that planet, and from another world with clouds, Saturn’s moon Titan.

The phosphine report by Jane S. Greaves and others was both serious and exciting, though we don’t yet whether in the long run it will be confirmed or proven wrong. A paper criticising the mathematical tools used has been made available online. On the other hand, there is also a paper which re-examines data from the 1978 Pioneer Venus Large Probe, and finds that this data strengthens the case for phosphine in Venus’ clouds. Unlike Greaves’ paper, neither of these two responses has yet completed the peer-review process.

By Earth standards, Venus’ surface is extremely hot, and its clouds are highly acidic; while Titan’s surface is extremely cold, and its clouds consist of droplets of liquid methane. Yet these two cloudy worlds have repeatedly given the sort of chemical data that worlds with life would logically produce.

Life chemistry is active chemistry. As such, it’s more likely to be found on a world whose atmosphere is chemically active than on one whose atmosphere is at or near chemical equilibrium. This point was argued in detail back in 1967 by James Lovelock and Dian Hitchcock, and was later developed by Lynn Margulis and David Grinspoon.

The atmosphere of Venus is dominated by oxygen-rich molecules such as carbon dioxide (CO₂), carbon monoxide (CO), and sulfur dioxide (SO₂). If it was at equilibrium, it would not contain hydrogen-rich molecules such as methane (CH₄), ammonia (NH₃), hydrogen sulfide (H₂S), or, for that matter, phosphine (PH₃).

Yet the Soviet Venera 8 mission (1972) sent back data indicating ammonia in the atmosphere of Venus, and the US Pioneer Venus Large Probe (1978) sent back data indicating methane and hydrogen sulfide. Efforts were made to explain away this surprising data, for instance by attributing it to faults in the detection process, rather than accepting it as evidence for a chemically active Venusian atmosphere.

Nonetheless, there are planetary scientists, among them Dirk Schulze-Makuch and David Grinspoon, who for many years have taken signs of chemical activity in Venus’ atmosphere as starting points for theories about possible life there. Grinspoon has argued that Venus is more likely to have life than Mars, since Mars is less chemically active. (See David Grinspoon’s book Lonely Planets, published 2003.)

As for Titan, no-one doubts that it has a chemically active upper atmosphere.

Reactions there, powered by ultraviolet light, break up molecules of methane and nitrogen, whose fragments then recombine into a range of carbon-chain compounds, also called organic compounds. This process also releases molecular hydrogen (H₂). An environment containing carbon-chain compounds and H₂ is an out-of-equilibrium environment.

These upper atmosphere reactions by themselves make Titan a place well worth looking at, at least for clues to the origin of life on Earth.

Plus, there is reason to think that the surface and lower atmosphere of Titan may be chemically active too.

Titan went against researchers’ expectations by not having a global covering of ethane (C₂H₆).

Ethane is a gas on Earth, but in Titan conditions it is a liquid. When it forms in the upper atmosphere, it precipitates to the surface, where it was expected to build up without further chemical change.

Based on the age of the Solar System and the rate of the upper-atmosphere reactions, scientists had calculated that the ethane and related hydrocarbons would have accumulated into a global layer hundreds of metres thick.

But Earth-based radar observations in 1990 found that any such global layer must be shallower than that.

The Cassini-Huygens space probe, which went into orbit round Saturn in 2004, found that Titan does not have even a shallow global layer of liquid hydrocarbons.

Theory had predicted an ocean. Cassini’s cameras found lakes.

There were and are several possible explanations for this surprise. One possibility is that ethane and other complex molecules which reach the surface turn back into methane, which can return to the atmosphere through evaporation. This process would involve an input of hydrogen molecules in reactions such as

C₂H₆ + H₂ → 2CH₄

This is the reverse of a common reaction in the upper atmosphere. It would mean a cycle of reactions involving carbon compounds, comparable to the much-studied carbon cycle in Earth’s biosphere.

Conversion of ethane back into methane is a breaking-up, energy-releasing process, which in Titan surface conditions requires a catalyst or a group of catalysts. It’s the sort of process we call decay or decomposition — something microbes do a lot.

Microbial life on the surface of Titan would have to be chemically different from Earth life. It could not use liquid water as a solvent, because the temperature is far below the freezing point of water. It might use liquid methane instead. But like Earth life, it would require energy to grow and reproduce, and it could get that energy by decomposition of organic molecules.

In June 2010, Darrell Strobel of Johns Hopkins University published a study of Titan’s atmospheric hydrogen, based on data from Cassini. Strobel found that H₂ molecules defuse from the upper atmosphere to the surface, but don’t build up there. Something (or someone) is consuming them.

NASA and the scientific community responded cautiously, making the points that Strobel’s work needed confirmation; and that if molecules really are combining with hydrogen and breaking up, this could be due to an unknown non-living catalyst.

Nonetheless, the Cassini data has increased interest in the possibility of life on Titan, encouraging interest in the general possibility of life which isn’t Earth-like.

NASA has approved an innovative mission called Dragonfly, which will use a set of helicopter-like rovers to hop from place to place on Titan’s surface. And Dragonfly’s announced tasks include searching for “chemical evidence of past or extant life”. Note the word “extant”, which means the opposite of “extinct”.

Dragonfly is currently scheduled to take off from Earth in 2027, and touch down on Titan in 2036.

One advantage Venus has over Titan, as a place for us humans to do research, is that travel time from Earth to Venus is a matter of months, not years. But probe missions take time to design and build, and developing a new generation of Venus probes is a serious technological challenge.

People who want answers will need to be patient.

Until new probe missions reach Venus and Titan, we’re not going to know what they’ll find.

Flourishing populations of microbes? Living things big enough to see, clinging like lichen to Titan boulders, or floating like balloons in Venus’ thick atmosphere? Exotic but non-living chemistry? Or, perhaps, something so different from anything here that it will be difficult for us to categorise it either as living or non-living?

If we do find life on Titan or Venus, even in comparatively simple microbial forms, what will it mean?

As Titan specialist Jonathan Lunine has argued, discovery of other life in the Solar System will be most significant if it is different enough from Earth life to imply an independent origin, rather than transfer of organisms via meteors.

In that case, it will enable us to solve a shortened version of the Drake Equation, to make a rough but realistic calculation of the number of life-bearing worlds in the Milky Way galaxy.

It will also change the way we’ve generally thought about planetary habitability. It will force us to abandon the idea that Earth-style habitability is the only version there is, and the idea that conditions hostile to Earth life are hostile to life in general.

To sum up… If Venus and/or Titan have life, we can expect the Milky Way Galaxy to have an awe-inspiring diversity of worlds that are habitats to their own life, whether or not they could ever be habitable to Earth people.

What will that mean to us humans?

It will seem like wonderful news to many, including scientists looking for living things to study, and those who see diverse life as a good thing in itself.

But to others, it will seem like seriously bad news, because of what it implies about the Great Filter.

This Great Filter is the name given by futurologist Robin Hanson to the unknown factor that has so far stopped beings from other worlds from journeying to Earth and colonising it.

Considering that the age of the Galaxy is about 13.5 billion years, while the age of the Solar System is about 4 billion, any living things in older planetary systems have had plenty of time to evolve intelligence and develop technology.

While some civilisations might choose to stay at home rather than colonise other worlds, any that chose to colonise and succeeded in doing so would spread their offspring through the Galaxy. They wouldn’t need to travel faster than 10 percent of the speed of light to spread through the Galaxy in a couple of million years, which is a very small fraction of the Galaxy’s age.

So why hasn’t it happened?

Using the term “Great Filter” doesn’t answer this question, but is a way of coming to grips with it.

Depending on what the Great Filter is, it may or may not also prevent us humans from journeying to other worlds and colonising them.

It depends whether the Filter is behind us, ahead of us, or both.

For instance, it is possible that the emergence of life from non-life requires an event of such low probability that it has happened only once in the Galaxy. The low probability event may occurred here on Earth, or a nearby planet from which microbes spread to Earth via meteor transfer. This would be an example of a Great Filter that’s behind us, one which wouldn’t affect what we can do in the future.

But chemically different life on Titan or Venus would mean that emergence of life from non-life is too frequent to be the Great Filter.

Another possible Great Filter is that evolution of human-level intelligence might be vastly less probable than emergence of life.

Although, as Lynn Margulis famously said: “To go from a bacterium to people is less of a step than to go from a mixture of amino acids to a bacterium.”

Evolution is not a unilinear process, but neither is it pure chance.

Natural selection enables organisms to be competitive in different ways — for instance, staying small and reproducing fast has worked just fine for bacteria. Algae and plants neither have brains nor need them. But brain emergence (cephalisation) has also been a workable evolutionary strategy for multi-celled organisms that can digest large chunks of food, and can move about in search of it.

If evolution has been happening for billions of years, on multitudes of worlds in the Galaxy, it seems reasonable to expect an even greater range of outcomes than on Earth; including, now and then, living things at least as smart and adaptable as ourselves.

Then there is the possibility of a Great Filter that is ahead of us. That’s what worries people like Robin Hanson and Nick Bostrom.

For instance, there may be something about high technology that leads to eventual extinction of every species that begins to use it.

In his 1998 essay about the Great Filter, Hanson recognises nuclear war and collapse of Earth’s eco-system as possibly underrated threats to our own species.

Alternatively, there may be a Great Filter ahead of us that does not destroy a technology-savvy species on its home world, but prevents any attempts at interstellar colonisation from succeeding. Or at least, makes interstellar colonisation sufficiently slow and patchy to explain why we don’t see interstellar colonists in our Solar System right now.

The sticking point may be the process of travelling from one system to another; or it may consist of obstacles interstellar travellers encounter if and when they reach another system and try to settle there (this has been called the Aurora Effect, after a fictional planet that couldn’t be colonised).

It is easy to imagine future humans spreading to different planets and moons in the way past humans spread to different islands and continents.

But wherever past humans settled, there was air they could breath, water they could drink, animals and plants they could eat, and microbes that could break down their faeces. When we eventually find a planet in another system that is a bit more Earth-like than Venus, how likely is it to have all these assets?

It is easy to speak of terraforming, easy to point to the way cyanobacteria and chloroplasts raised the oxygen content of Earth’s own atmosphere… easy to forget that it took them over two billion years to get the oxygen around its present level. That’s over a tenth of the age of the Galaxy.

Robin Hanson thinks space-travelling colonists would overpower “any less developed life in the way”. Is this necessarily true, if the “less developed life in the way” is an entire biosphere, radically different to the biosphere in which the space-travellers evolved?

The Great Filter may be diversity itself — diversity of worlds, diversity of life chemistries, diversity of planetary eco-systems. A diversity which could make other worlds great places for scientists to visit, but unfeasible places for colonists to settle.


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