Tag: supercritical carbon dioxide

A mystery on Venus

Scientists have reported that they have found abnormal amounts of a toxic compound called phosphine in Venus’s atmosphere, at 55-80 km altitude. This story is currently all over my Twitter feed because one way to explain this unexpected abundance is that microbes could be producing this gas – as we know them to do on Earth – in oxygen-starved conditions. Nonetheless, we shouldn’t lose sight of the fact that the real proposition here is that there is too much phosphine, not that there is a potential sign of life.

While some scientists have been issuing words of caution along similar lines, others have cut to the other end, writing that making sense of this discovery doesn’t require “alien microbes” at all because chemistry offers possibilities that are much more likely to be the case – and verging on the argument that this possibly can’t be aliens. Between them is the option to keep an open mind, so difficult these days – between an Avi Loeb-esque conception of the universe in which the role of creativity is overemphasised to dream up plausible (but improbable) theories and a hyper-conservative reality that refuses to admit new possibilities because we haven’t plumbed the depths of what we already know to be true enough.

Nonetheless, this is where it is best to stand today – considering we simply don’t know enough about the Venusian atmosphere to refute one argument or support the other. At the same time, I would like to make a finer point. In November 2014, I had published a post explaining the contents of a scientific paper published around then, describing how an exotic form of carbon dioxide could host life. As I wrote:

At about 305 kelvin and 73-times Earth’s atmospheric pressure, carbon dioxide becomes supercritical, a form of matter that exhibits the physical properties of both liquids and gases. … As the study’s authors found, some enzymes were more stable in supercritical carbon dioxide because it contains no water. The anhydrous property also enables a “molecular memory” in the enzymes, when they ‘remember’ their acidity from previous reactions to guide the future construction of organic molecules more easily. The easiest way – no matter that it’s still difficult – to check if life could exist in supercritical carbon dioxide naturally is to … investigate shallow depths below the surface of Venus. Carbon dioxide is abundant on Venus and the planet has the hottest surface in the Solar System. Its subsurface pressures could then harbour supercritical carbon dioxide.

When we do muster as much caution as we can when reporting on recently published papers presenting evidence of new mysteries, we evoke the possibility of ‘unknown unknowns’ – things that we don’t know we don’t know, as perfectly illustrated in the case of carbon monoxide on Titan. At the same time, are we aware that ‘unknown unknowns’ also make way for the possibility of alien life-forms with biological foundations we may never conceive of until we encounter a real, live example? I am not saying that there is life on Venus or elsewhere. I am saying that the knowledge-based defences we employ to protect ourselves from hype and reckless speculation in this case could just as easily work against our favour, and close us off to new possibilities. And since such caution is often considered a virtue, it is quite important that we don’t indulge it.

There is a wonderful paragraph in a paper from 2004 that I’m reminded of from time to time, when considering the possibility of aliens for a science article or a game of Dungeons & Dragons:

The universe of chemical possibilities is huge. For example, the number of different proteins 100 amino acids long, built from combinations of the natural 20 amino acids, is larger than the number of atoms in the cosmos. Life on Earth certainly did not have time to sample all possible sequences to find the best. What exists in modern Terran life must therefore reflect some contingencies, chance events in history that led to one choice over another, whether or not the choice was optimal.

How Venus could harbor life: supercritical carbon dioxide

The dark spot of Venus crossed our parent star in 2012. Pictured above during the occultation, the Sun was imaged in three colors of ultraviolet light by the Earth-orbiting Solar Dynamics Observatory.
The dark spot of Venus crossed our parent star in 2012. Pictured above during the occultation, the Sun was imaged in three colors of ultraviolet light by the Earth-orbiting Solar Dynamics Observatory. Image: NASA/SDO & the AIA, EVE, and HMI teams

A new study published in the online journal Life says a hotter, pressurized form of carbon dioxide could harbor life in a similar way water does on Earth. This is an interesting find, theoretical though it is, because it might obviate the need for water to be present for life to exist on other planets. In fact, of the more than 2,700 exoplanet candidates, more than 2,000 are massive enough to have such carbon dioxide present on their surface.

At about 305 kelvin and 73-times Earth’s atmospheric pressure, carbon dioxide becomes supercritical, a form of matter that exhibits the physical properties of both liquids and gases. Its properties are very different from what they usually are in its common state – in the same way highly pressurized water is acidic but normal water isn’t. Supercritical carbon dioxide is often used as a sterilization agent because it can deactivate microorganisms quickly at low temperatures.

As the study’s authors found, some enzymes were more stable in supercritical carbon dioxide because it contains no water. The anhydrous property also enables a “molecular memory” in the enzymes, when they ‘remember’ their acidity from previous reactions to guide the future construction of organic molecules more easily. Moreover, as stated in the paper,

… the surface tension in carbon dioxide is much lower than that of water, whereas the diffusivity of solutes in scCO2 is markedly higher [because of lower viscosity]. Thus, scCO2 can much easier penetrate [cell membranes] than subcritical fluids can.

The easiest way – no matter that it’s still difficult – to check if life could exist in supercritical carbon dioxide naturally is to check the oceans at about a kilometer’s depth, where pressures are sufficient to entertain pockets of supercritical fluids. As the authors write in their paper, supercritical carbon dioxide is less dense than water, so they could be trapped under rocky formations which in turn could be probed for signs of life.

A similarly accessible place to investigate would be at shallow depths below the surface of Venus. Carbon dioxide is abundant on Venus and the planet has the hottest surface in the Solar System. Its subsurface pressures could then harbor supercritical carbon dioxide. Dirk Schulze-Makuch, a coauthor of the paper and an astrobiologist at Washington State University, notes,

An interesting twist is that Venus was located in the habitable zone of our Solar System in its early history. [Him and his coworkers] suggested the presence of an early biosphere on the surface of this planet, before a run-away greenhouse effect made all life near the Venusian surface all but impossible.

The probability that Venus could once have harbored life is as strange as it is fascinating. In fact, if further studies indicate that supercritical carbon dioxide can play the role of a viable bio-organic solvent,  the implications will stretch far out into anywhere that a super-Earth or gas-giant is found. Because its reactions with complex organic molecules such as amines will not be the same as water’s, the life-forms supercritical carbon dioxide could harbor will be different – perhaps more primitive and/or short-lived. We don’t know yet.

This study continues a persistent trend among astrobiologists since the 1980s to imagine, and then rationalize, if and how life could take root in environments considered extreme on Earth. After the NASA Kepler space telescope launched in 2009 and, in only four years of observation, yielded almost 4,100 exoplanet candidates (more than a thousand confirmed as of now), astrobiologists began to acquire a better picture of the natural laboratories their hypotheses had at their disposal, as well as which hypotheses seemed more viable.

In August this year, Schulze-Makuch himself had another paper, in Science, that discussed how a lake of asphalt in Trinidad harbored life despite a very low water content (13.5%), and what this said about the possibilities of life on Saturn’s moon Titan, which exhibits a similar chemistry on its surface. The Science paper had cited another study from 2004. Titled ‘Is there a common chemical model for life in the universe?‘, it contained a pertinent paragraph about why the search for alien life is important as well as likely endless:

The universe of chemical possibilities is huge. For example, the number of different proteins 100 amino acids long, built from combinations of the natural 20 amino acids, is larger than the number of atoms in the cosmos. Life on Earth certainly did not have time to sample all possible sequences to find the best. What exists in modern Terran [i.e. Earth-bound] life must therefore reflect some contingencies, chance events in history that led to one choice over another, whether or not the choice was optimal.