Life in the Solar System

Article - Astrobiology

Life as we know it in the oceans of the solar system.

It is predicted that at least six moons of the outer solar system harbor liquid water oceans below an icy crust. The Jupiter moons Europa, Callisto and Ganymede, the Saturn moons Titan and Enceladus and last but not least Triton, satellite of Neptune. Let us have look at Europa. How could life exist in a body of water under a layer of ice so thick that photosynthesis is not an option? Since the discovery of the hydrothermal vent “Lost City” on Earth in the 1970s we know that life can exist in the cold darkness deep below the water surface. In this particular location 70-100C warm, mineral rich water flows out of the seafloor, building pipe-like structures of carbonate rock. The warm water is not the result of volcanism but instead comes from a reaction called serpentinzation which takes place when highly reduced minerals from the Earth’s crust come in contact with water. This reaction generates heat and a multitude of anorganic compounds, the most relevant for our purposes being Hydrogen. Bacteria and archaea known as methanogens can use this hydrogen as their primary source of energy. In combination with carbon dioxide dissolved in the water they grow, reproduce and generate methane, hence their name. Using hydrogen and carbon dioxide to make methane is maybe the oldest metabolism of life on Earth. And because it requires so few, basic anorganic compounds, it is the best candidate metabolism for microbial life in other bodies of liquid water in the solar system and maybe beyond.

Can microbial life jump from planet to planet?

The panspermia hypothesis suggests that microbial life as we know it has spread between planets and moons of the solar system and that the most likely mechanism responsible is the exchange of rocky meteorites. Orbital modeling has shown that rocks ejected from Mars, either as the result of meteorite impacts themselves or through volcanism will, with a small but non-zero probability, rain down on Earth as meteorites after an average transit journey somewhere between 15 and 25 Million years. The opposite transfer direction from Earth to Mars is also plausible. There is clear evidence for a Mars-to-Earth transfer process since several Earth meteorites have been unambigously identified as being of martian origin. In addition, experiments have shown that a variety of Earth microbes and their spores could survive the acceleration/deceleration, the heating/freezing and the radiation they would be subjected to in such a transfer, simply by being buried in microscale cavities several centimerets below the rocky surface of an asteroid/meteorite. On Earth there is a large variety of bacteria called endoliths living in such habitats permanently [“Impact induced microbial endolithic habitats” by C.Cockell, 2002]. The same meteoritic transfer mechanism would also enable comets of extrasolar origin to become carriers of microscopic life, potentially depositing it on planets and moons of the inner and outer solar system. The TANPOPO experiment running on the International Space Station is set up to detect this type of life arriving on micrometeorites to the outer Earth orbit. The experiment is ongoing. It has also been proposed that the appearance and exceptionally fast radiance of animal phyla on Earth during the Cambrian 540-490 Million years ago could have been the result of such a meteoritic seed event.

But still, we don’t know much about more complex life, do we?

Let’s go back to Europa. Using hydrogen and carbon dioxide as bacterial fuel is ok, but not great. No complex life on Earth relies on this metabolic pathway and for a good reason. It just does not provide a lot of energy. Complex life on Earth only came about after oxygen had started to accumulate in the atmosphere around 2.5 billion years ago. On Earth this oxygen was produced by photosynthetic cyanobacteria, but the ice covered ocean worlds of the solar system are all wrapped in darkness and molecular oxygen or other oxidants must come from somwhere else in order to serve as basis for the more energetic metabolisms required to sustain complex life. In the case of Europa the answer is radiation. Specifically radiation emanating from Jupiter. It is so strong that a human standing on the surface of Europa would receive a lethal dosis in a very short amount of time. But the good thing about it is that when radiation hits the ice layer on the surface, very reactive hydrogen peroxide and even Ozone are created from water molecules and some of these diffuse into the liquid water below the ice. There, assuming the water itself is not completely sterile, they could react with whatever minerals are present, producing oxidants that would now be accessible to potentially complex life present in the cold depths below. Microbial life therefore has a good chance to find enough fuel for its metabolisms in the water that might be convecting downward from the warmer surface layers. But complex life? On earth, the more complex an organism is the more oxygenated compounds it needs to consume. For Europa the question then is how fast the cycling of water from bottom to the surface and back is. If this cycle is fast, then conditions for the emergence of complex life are good. [“Alien Oceans - The Search for Life in the Depths of Space, Kevin Hand, 2020”] On Earth polychaete worms are one example for an animal species that can tolerate very low oxygen levels and survives through a symbiotic relationship with methanogenic bacteria living close to hydrothermal vents. With high oxygen levels fish- and squid-like organisms would be possible on Europa. However, if there is no contact between the liquid subsurface ocean and the underlying mineral base, the water would essentially be sterile, unless some unknown process could consistently deliver minerals to it from above. Even with the water being oxygenated due to the influence of radiation on the top ice sheet,this would mean a lack of essential building blocks for life as we know it and make it very improbable.

Odd life as we do not know it (yet): Titan

The surface of Saturn’s moon Titan might be habitable, even at at -179 degree Celsius, but probably not by carbon based life as we know it. Any life that evolved on or was carried to Titan’s surface would need to have a biochemistry that uses liquid ethane or methane instead of water. While in principle this is not impossible, there is currently a lack of evidence to support the existence of such life. In contrast, it is believed that below Titan’s frozen surface lies a liquid, highly alkaline ocean consisting of a mixture of water and ammonia. Similar to Jupiter’s satellite Europa, this ocean could support earth-like water based biochemistries of life. While the behaviour of proteins under these conditions would be altered, known extremophile bacteria found in the vincinity of Earth’s deep sea vents are again good candidates for microbial life there. [“Life In the Universe - Expectations and Constraints” by Dirk Schulze-Makuch, 2019]. While NASA’s planned “Dragonfly” mission to Titan is proposing the use of a flying drone for extensive surface exploration, drilling deep into the icy surface to sample the water below is currently not part of this mission’s profile.

Of the known extremophile microbes on Earth, which ones are more likely to survive and thrive under Martian conditions?

A recent article on species of Cyanobacteria surviving in high perchlorate salt concentrations like those found on the surface of Mars answers this question. “Survival of Cyanobacteria Under Perchlorate Stress” . But besides photosynthetic cyanobacteria there is a multitude of other extremophile bacteria and even eukaryotes that could endure the conditions found in solar system icy-moon or planetary subsurface water bodies, even on Mars. These bacteria and archaea would use a variety of chemotrophic metabolisms. An overview of the boundary conditions within which they could survive and replicate can be found in D.Schulze-Makuch’s book “Life In The Universe”. Using estimates for pressure, temperature and approximate chemical composition then allows us to choose terrestrial microbes most likely to survive in a given environment. Specifically on Mars, where the perchlorate concentration in the soil is the most limiting factor to life as we know it, entire earth microbial communities could survive, as this study is suggesting “Survival and growth of soil microbial communities under influence of sodium perchlorates”. Here soil samples from an arid mountain environment in Morocco and from the Antarctic desert were used to prepare perchlorate enriched microbial culture media. Unexpectedly, cells multiplied in these conditions in both samples. While bacteria showed higher sensitivity to perchlorates, growing in density by a factor of two only, the archaea grew by a factor of nine. Notably as well, Bacillus subtilis, one of the most widspread soil microbes on Earth seems to have benefited from the presence of perchlorate salts. Another candidate microbe that would probably grow on Mars is Acidithiobacillus ferrooxidans since it survives under simulated conditions similar to those of shallow subsurface environments of Mars (-20 C temperature, 0.006 bar pressure and 0.18% of oxygen).

Does it make sense to look for fossils?

Mars may have been habitable 3.5 billion years ago. The various robotic rovers have revealed that chemically rich lakes, and possibly water oceans covered Mars. If life existed in those environments, then some chemical or structural “fossils” might have been preserved within rocks. However, even considering the existence of visually strange rock formations that are very likely the result of erosion processes, we will only be able to answer the question of macrofossils after we have either set foot on the red planet or returned samples to Earth for a detailed analysis. Rover images are not enough.

And what about Venus? Will we find life there?

Anomalies in the atmosphere of Venus have been detected by the soviets since the 70’s and a long debated one is UV absorption in the top clouds of the atmosphere. There is currently no plausible explanation for this strong, localized but time varying absorbtion pattern of UV light. The substances that have been proposed as possible absorbers either retain light with a different spectral profile or they are not present in sufficient quantity. The privately funded Venus Life Finder mission, scheduled to launch in early 2023, might be able to provide answers since it plans to send a probe on a no-return journey down into the atmosphere. Venus Life Finder Mission The atmosphere of Venus consists of thick clouds of sulfuric acid at high altitudes (>100 km) and of lower layers in which the sulfur dioxide concentration can vary spatially and temporally by up to two orders of magnitude. Measurements and atmospherical modeling have established that the concentration of sulfur dioxide and water in the middle atmospheric layers below the top cloud layer are lower than they should be, according to known photochemical processes. This “depletion anomaly” has remained unexplained to date. “Hydroxide salts in the clouds of Venus” Two possible hypothesis are being discussed. The first explains the sulfur dioxide depletion with the insertion of hydroxy-salts into the middle atmospheric layers by either volcanism or through unknown other sources. These salts would react with the sulfuric acid, decrease the pH and explain the observed depletion. The second hypothesis proposes an unknown biosphere which constantly sacrifices a certain proportion of its own biomass to neutralize the sulfuric acid, lowering the pH to a level that can be tolerated by the remaining organisms. This second hypothesis in combination with the recent discovery of a phosphine anomaly, a gas for which abiogenic sources are improbable, has motivated the renewed interest in Venus from an astrobiological perspective. Only Venus cloud layers between 48 and 66 km altitude have tolerable conditions for known extremophile microbes from Earth, since the temperature at the surface is 735 Kelvin. Recently, authors are proposing a microbial lifecycle in an aerial biosphere extending between 33 and 66 km altitude. Here, microbes and their dessication tolerant spores never touch the surface of Venus and remain airborne at all times. Liquid droplets play a central role since they provide the habitat for these permanently airborne acidophilic microbes. “The Venusian Lower Atmosphere haze as a Depot for Desiccated Microbes” Candidate Earth microbes have been investigated by Dirk Schulze-Makuch and others. Quote: “The archaea Picrophilus oshimae and P. torridus can grow at a pH of -0.06 (Schleper et al. 1995). P. oshimae is a polyextremophile tolerating a pH of 0 and temperatures up to 65 C (Fütterer et al. 2004). There are fewer studies of high alkalinities (pH > 10) than of extreme acidities (pH < 1.0), probably because high alkalinity environments (e.g., soda lakes) are rarer in nature.”

We have given a brief overview of the possibilities for finding present and past life on the planets and moons of the solar system. We hope this insight has been as fascinating to you as it has been to us. Upcoming missions exploring Venus, Mars and the icy moons will continue to feed our curiosity.

This article is the result of a collaboration between myself, @rickywilhelmson and the multinational team behind @astroverbot. The original idea for it came from them and they reached out to me with a set of questions. I researched the answers and wrote the article published here. Thanks and greetings go to @astroverbot (Julia), @stanadmakobo and Anton A. Komarov. The following two websites show what else they do:

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