Science & Technology

Why discovering organic molecules in the Solar System could indicate the existence of extraterrestrial life

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The past month has seen several fairly major developments with regards to the discovery of organic molecules at different locations in the solar system. First NASA announced the discovery of variations in methane concentrations on Mars, followed by the discovery of organic molecules in Martian mudstone. More recently it was revealed that the Cassini probe had observed even more massive organic molecules on Saturn’s moon Enceladus. But if organic can have abiotic origins, what is it about these recent discoveries that point to the possibility of life elsewhere in the Solar-system? 

The news of the observations listed above and in more detail below has generally been met with one of two reactions. Some are excited about the revelations and believe that they indicate that we may finally be on the trail of evidence that life has or does exist elsewhere in our solar system with implications for the possibility of life elsewhere in the universe. Sceptics correctly point out, however, that the phrase ‘organic molecules’ simply refers to molecules comprised of rings or chains of carbon with other common elements such as oxygen, hydrogen and nitrogen. As such, these molecules can’t be too closely associated with life. Organic molecules, they point out, can be created by mechanisms that don’t involve life and it’s wrong to see ‘organic molecule’ and immediately assume biology. They also point out that methane has been discovered on Mars before and that this discovery didn’t immediately lead to speculation of life.

So who is right here?

The answer is a little from column A, a little from column B.

Consider complexity

Whilst the recent discoveries resemble similar finds in the past and there remains a possibility that these molecules arose through processes other than life, what makes these discoveries so significant is the circumstances that surround them, the situations that we have found these carbon-rich molecules in and the patterns in those observations.

When it comes to the discovery of life elsewhere in the solar system, the devil is in the details, or perhaps more accurately in the complexity.

Lee Cronin, a chemist at the University of Glasgow indicates why astrobiologists look for complex organic molecules as potential signs of extraterrestrial life by pointing out that the more complex the more likely these molecules are a sign of biological processes. “Biology has one signature: the ability to produce complex things that could not arise in the natural environment,” Cronin suggests the use of a system to count the number of unique steps such as chemical side groups or ring structures in the formation of a compound to indicate molecules which are biological in origin. He suggests that any structure requiring 15 or so unique steps would indicate a strong chance of biological origin.

Life on Mars?

The idea of life on Mars has been explored in various forms of media, not least lurid science fiction

Without a shadow of a doubt the most interest in extraterrestrial life concerns our neighbour Mars. This is perhaps to be expected as the idea of life having existed on Mars has been popularised by the media and fiction writers for decades. To the scientific community though, Mars represents perhaps the most logical first step in the search for life in the solar system for several reasons. In terms of proximity, it’s our easiest option for exploration and sample collection. In addition to this, there are some similarities between Mars and Earth that would favour the type of life that has thrived on Earth or would have at some point in our neighbour’s history.

the fact that Mars has a similar axial tilt to Earth means that it has similar changes in seasons

These similarities include a similar axial tilt leading to seasonal variability, similar atmospheric conditions, similar land surface area, the presence of polar ice caps and crucially the presence of surface water. Evidence also suggests that Mars may once have had a magnetosphere as Earth does now. This is crucial in the development of early life as it would have protected the surface from solar and cosmic radiation allowing early molecules to flourish and grow in complexity without the risk of being broken down or destroyed by harmful radiation.

Since landing on the surface of Mars in 2012, the Curiosity Rover has detected traces of atmospheric methane, albeit in far scarer supply in the atmosphere of Mars than it is found in  Earth’s atmosphere, 0.4 parts per billion (ppb) in comparison to 1800 ppb. On Earth, the majority of atmospheric methane is generated by microbial life but before ascribing the same cause to Mar’s methane one must consider that methane can be generated in other ways. Ways that don’t involve biology. Hydrothermal reactions with olivine-rich rocks underground can generate it, as can reactions that are driven by ultraviolet (UV) light striking the carbon-containing meteoroids and dust that constantly rain down on the planet from space.

Methane’s atmospheric methane as recorded by the Curiosity rover (NASA)

What is interesting about Mars’ atmospheric methane isn’t its presence alone though. Rather, it’s the fact that Mars seems to have an excess of methane and also displays seasonal variation in its atmospheric methane concentration. This variation is approximately three times higher than can currently be explained by known events such as the thinning of the atmosphere caused by the freezing of carbon dioxide at the planet’s southern polar cap. Even though we’ve been aware of methane in Mars’ atmosphere since at least 2004 thanks to Earth-based remote sensing and telescopes, this seasonal variation and some variations in time and location weren’t seen until we were able to make in-situ observations.

One of two pieces of recent research concerning organic molecules on Mars published in Science earlier this month concludes that the observed spikes in methane concentration are consistent with it being released from small localized sources on the surface or subsurface reservoirs. The seasonal variation indicates a process by which methane is held at the surface and released at times of higher temperature. This could indicate methane is stored at or just under the planet’s surface and is released in greater volume when heat causes cracks and fissures in the surface layer to expand.

This would seem to indicate that Mars possesses methane-rich sedimentary rock, much like Earth. The question remains how did this methane become trapped in sedimentary rock in the first place?

The Curiosity rover takes a selfie as it drills for mudstone samples. (NASA/Caltech)

A second study also published in Science this month indicates that the storage and preservation of methane and other organic molecules geologically is key to understanding the possibility of ancient life having once existed on Mars. To analyse this, researchers collected samples of mudstone from various sites on the planet’s surface including a dried out lake bed. These samples were baked in Curiosity’s onboard oven, breaking down to release aromatic, aliphatic and thiophenic vapours. These substances could well be the breakdown products of even larger organic molecules.

Aromatic molecules such as benzene are formed with one or more hexagonal carbon rings

They concluded that carbon-rich molecules such as benzenes are widespread in the Martian rock record and there could well be an abundance of such materials further beneath the planet’s surface protected from solar radiation.

Of course, this doesn’t tell us that biological processes are definitely responsible for depositing organic molecules in the geological record of Mars. But it’s encouraging because it does tell us that at some point some process that we are currently unaware of is responsible for these overabundances in carbon-rich organic molecules. We can couple this with the knowledge that in our own planet’s geological record the presence of an excess of organic molecules is a result of biological processes.

Again, a little from column A, a little from column B.

Tantalising signs of life from Saturn’s moon Enceladus?

Perhaps even more significant than the traces of organic molecules found on Mars are the traces of more massive organic compounds from Saturn’s moon Enceladus. Enceladus, a mere 500 km in diameter with a gravitational force 1% that of Earth’s, has long been an interest of astrobiologists due to its global ocean which lies between a surface ice sheet and a rocky core.

Enceladus’ composition makes it a primary candidate for simple life (NASA/Caltech)

The collection of samples from Enceladus is possible via a fly-by from the Cassini probe due to the fact that ice-grains and gas are ejected from the moon to space in cryo-volcanic plumes at the moon’s southern pole. These plumes are likely the result of hydrothermal activity deep within the moon’s porous core. Cassini made several passes through these gas plumes collecting samples to examine.

 

An artist’s impression of Cassini’s Enceladus flyby

Research published in Nature this month discussed the discovery of molecules of 200 atomic masses and above on Saturn’s moon. To put that into context, molecules 10 times more massive than methane. The finding suggests a thin-film of organic macromolecules on the top of  Enceladus’ water table. The sheer size of these molecules has taken researchers by surprise and when coupled with some of Enceladus’ other characteristics, a salty-liquid water ocean, hydrothermal vents and sub-surface geysers, it makes Enceladus a tantalising prospect for extraterrestrial life. The molecules found aren’t amino acids, but they aren’t far away from that size and complexity.

Unfortunately, as Cassini, the probe which collected these samples has now crashed meaning that the collection of further samples may have to wait for now.

Increasingly scientists are coming around to the idea that Enceladus should now be made our primary focus in the search for life in the solar system overtaking previous best candidate, Jovian moon Europa. Carolyn Porco, a prominent planetary scientist who has worked on several key NASA missions, is a strong supporter of Enceladus becoming a primary focus of research. “We simply know more about Enceladus,” Porco stated at the National Geographic Awards in June. “We just don’t know that much about Europa with certainty,” Porco said. “There is a lot of excitement, but it’s speculation at this point. Of course, I’d choose Enceladus. We know it’s the best, and it stands the greatest chance of making that next big step.”

Again, it must be noted that there are possible abiotic pathways to the creation of these molecules, but what remains striking here is that some of the detected organic compounds bare a strong resemblance to compounds found on Earth as a result of the decomposition of biological life.

A little from column A… you get the point.

Don’t call this evidence of life… yet

You’ll notice that articles discussing these findings generally tend to avoid the word ‘evidence’. That’s simply because that as the sceptics point out, at this stage, we simply can’t conclude that detected organic molecules are a result of biological activity. But what we should also consider that the more data points like those we’ve described above that are collected the more likely we are to find something extraordinary. Something else like the extremely massive compounds or seasonal variations in concentration detailed above. Maybe something more significant. It is these outliers in the normal data that make scientists sit up and pay attention.

Other options for the discovery of life elsewhere in the solar-system also exist. As mentioned above, Europa is believed to be a primary candidate due to the fact it hosts the largest ocean in the solar system that we are aware of. In addition to this, another Jovian moon Ganymede is believed to possess a subsurface ocean similar to Enceladus. Saturn’s moon Titan has a subsurface ocean and surface hydrocarbon liquids which could increase the probability of life.

A cross-section of Europa’s surface shown its subsurface salty water ocean

This is coupled with further research recently published that reveals complex organic molecules, the necessary building blocks for life, are far more abundant in the Universe than previously suspected.

Though clearly caution is of the utmost importance when considering the discovery of complex organic molecules and that discovery’s possible significance with regards to the possibility of biological systems, the future does seem promising. These recent discoveries though, suggest we may well be on the verge of discovering extraterrestrial life in our solar system, whether extant or extinct.

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