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The idea of intelligent alien life has been an exciting topic in the world of science fiction for a very long time. In reality, if there are any forms of extraterrestrial life within our solar system, they will most likely be microbial.

It has been hypothesised that life could have existed on both Mars and Venus, when the environmental conditions were similar to the early Earth. However, the climates of both Venus and Mars rapidly changed, with the latter getting colder and the atmosphere thinning, and the former getting hotter as the atmosphere got thicker. Until recently, it was believed that both Mars and Venus no longer had any liquid water. Sounds pretty inhospitable, right?

Surprisingly, the recent discovery of hydrated perchlorate salts suggests that briny liquid water is actually still present on Mars, flowing just below the surface only when the temperature reaches above -23oC. It has been hypothesised that potential Martian life may be halophilic (thriving in high salinity), living inside salt crystals, as they are in the image below.

Pinkish-red sodium chloride (NaCl) crystals.
Image 1: These pinkish-red crystals of sodium chloride (NaCl) are coloured by millions of halobacteria. The bacteria survive inside the salt crust, in California’s Owens Valley.

 

As for our ‘sister planet’, Venus: the build-up of CO2 and runaway greenhouse effect in the Venusian atmosphere resulted in the planet’s current surface temperature of 467oC and atmospheric pressure 90-100 times that of Earth, with highly concentrated sulphuric acid clouds (with pH 0 or below). Although this sounds hellish, there is a section of Venusian cloud, 50-65km above the surface, where the temperature ranges between 30-70oC, with pressure similar to Earth’s surface, where water vapour may be present. ‘Dark streaks’, containing microbe-sized, non-spherical particles have also been observed (see Image 2). Since 20% of small particles in Earth’s upper atmosphere are living bacterial cells, there has been speculation that these ‘dark streaks’ could be an indication of life in Venus’ clouds.

The planet Venus as captured by NASA's Pioneer Venus Orbiter in 1979
Image 2: Image captured by NASA’s Pioneer Venus Orbiter (1979).

In contrast, Enceladus, an icy moon orbiting Saturn has a global ocean beneath the ice crust, and active plumes. Due to tidal heating from Saturn’s gravitational field, Enceladus’ oceans have the potential to nurture multi-cellular organisms.

However, although NASA’s mantra – “follow the water” – suggests that life cannot exist without liquid water, other biosignatures (e.g. ozone, hydrogen, methane) are also important in determining a planet’s habitability. Biosignatures are substances that are unlikely to be produced abiotically (derived from sources that are not biologically active), and can be markers of life.

In 2004, methane was discovered in the Martian atmosphere. Most of the methane on Earth is produced by methanogens, which are extremophiles – organisms that thrive in extreme conditions – that metabolise hydrogen and carbon dioxide to produce methane. Methanogens do not require aerobic conditions (meaning they do not require air or oxygen), light, or organic nutrients, which means they could potentially survive just below the topsoil on Mars.

On Venus, the presence of both sulphur dioxide (SO2) and hydrogen sulphide (H2S) is unusual. These two substances have a high affinity for reacting together. One explanation for the presence of both substances is the presence of microbes that can metabolise sulphur into sulphuric acid. Another possibility could be that Venusian microbes are similar to the sulphur-oxidising bacteria that existed in Earth’s ancient oxygen-poor oceans, or the hydrogen sulphide-oxidising thermophiles (heat-loving microbes) present in hot springs.

Additionally, Enceladus’ plumes have been confirmed as the product of hydrothermal activity, due to the 1-2% presence of molecular hydrogen (H2). On Earth, hydrothermal vents producing hydrogen gas are home to methanogenic microorganisms, and are believed to be the origin of life!

Computer model of Enceladus’ plumes. Showing a cross section of the moon's crust.
Image 3: Model of Enceladus’ plumes, the proposed connection to hydrothermal vents, and liquid water ocean beneath the thick icy crust. From theconversation.com.

However, radiation is the most significant challenge facing any potential extraterrestrial life forms.

The levels of UV radiation on Venus are extremely high, due to its’ proximity to the Sun. However, the thick atmosphere reflects a lot of radiation away, and the sulphur compounds from the clouds may act as ‘sunblock’ for potential microbial life. Despite this, life on Venus is still quite unlikely, and would be extremely difficult to detect.

Due to the lack of atmosphere, radiation exposure on the surface of Mars is 30µSv per hour (or 200 msV over 180 days) during solar minimum, compared to 20mSv per year on Earth. Although water and methane are present, life on Mars is extremely unlikely.

Ultimately, the most likely place for life in the solar system is below the icy surface of Enceladus. This is because the thick icy crust blocks most harmful UV and gamma radiation, removing the most difficult obstacle from the equation of microbial survival. Compounded by the overwhelming presence of water and potential hydrothermal vents, Enceladus seems to be the most ideal home for extraterrestrial life in our solar system.

Closer to home, our GreenSat team is currently working towards creating a satellite habitat for nitrogen-fixing bacteria. Although the Earth’s surface and some of the atmosphere is mostly habitable, once you reach space, it is a different story entirely. Our aim is to eventually develop the technologies to grow food in Earth’s orbit, or on another planet (provided that we do not colonise/wipe out any existing life forms on that planet, of course). More info can be found here.