When we celebrated World Space Week (4-10th October), much of the focus was on, well, space! And when you think about fields like astrobiology, you probably think of exoplanets, exploring the solar system or maybe aliens in sci-fi movies. But a lot of space research, especially that of astrobiology or planetary science, is actually conducted in the field here on Earth!
Earth is often called the Goldilocks planet, with an environment just right for life. But the Earth isn’t one homogeneous continent of temperate habitat: Earth’s mean temperature may be a comfortable 14 °C, but the energy received from the sun warms the earth unevenly across bands of latitude, giving us icy poles and a warm equator. Complicating this further is the varied topography and other geographical features characteristic of our tectonically active world, all of which have an impact on local and global climate. It’s this variety that has allowed the Earth’s life to diversify into many millions of species1, some of which have adapted to the specific niches of Earth’s most extreme environments, surviving conditions more similar to space than the average Earth habitat.
These Extreme Earth Environments are known as Analogue Sites, as they are analogous (similar) to environments in space. Most life on Earth, like most animals and plants, struggle to survive here, but some organisms- extremophiles- have adaptations which allow them to survive extreme temperature, droughts, pH or a mixture of all and more. Quite often, the ecosystems which build up here don’t rely on photosynthesisers like plants. Instead, the producers (the first step in the food chain) use chemosynthetic processes to harness the energy of specific compounds in their environment.
The idea goes that if life can exist in places on earth which are so similar to the environments we see in space, then why couldn’t life exist in those environments in space? Whether this is accurate or not remains to be seen (no concrete evidence for extra-terrestrial life has been found just yet!), but it is still an interesting area of science which can help guide us as we plan missions to the solar system and beyond.
First, let’s head to one of the most remote, uninhabitable places on Earth… Antarctica!
If you read my article from earlier this year2, you’ll probably know just how fascinating of a place I think Antarctica is. Once covered in a rainforest; today the cold temperatures and dry conditions (all the water is locked up in ice!) aren’t exactly conducive to life, especially plant life, meaning that typical terrestrial food chains with plants as the dominant primary producers tend not to form. But it’s not entirely lifeless: colonies of penguins, seals and birds line the coast, and the surrounding and underlying seas have far more diverse ecosystems than was originally expected.
Much of the life on Antarctica is reliant on the life beneath the ice sheet and in the wider Southern Ocean, where phytoplankton and algae convert the sun’s energy into biomass for other species to feed on. These complex food webs extend far from the open ocean, beneath the ice shelf, with one community of sessile (stationary) animals like sponges found earlier this year beneath 900m of ice, 260km from the ocean.3
These vulnerable ecosystems are really important to be studied to assess and monitor the impact of climate change- scientists do this by using long range autonomous or remotely controlled submersibles, such as Boaty McBoatface developed at NOC in Southampton!4 This sort of technology could eventually be repurposed for use in future missions to Icy Moons such as Europa, to investigate what actually is below its icy crust!
Elsewhere in the ocean, hydrothermal vents are also being explored in this way for similar reasons. But where the ecosystems under the Antarctic Ice Sheet may resemble potential communities living close to the surface under Europa’s ice sheet, hydrothermal vents may reveal how communities could spring up without any need for light to reach through the ice. Hydrothermal Vents are where geothermally heated water is released back into the ocean at high temperatures from cracks in the crust. The water is rich in minerals such as silica (in lower temperature ‘white smokers’) and iron sulphides (in higher temperature ‘black smokers’). At the vents, the water is scaldingly hot- up to 400 degrees!5
But the heat dissipates into the surrounding ocean, heating the near freezing waters of the open ocean to something more tropical! So far below the surface, the lack of light means photosynthesis is impossible. Instead, the producers use the heat and metal ions provided by hydrothermal vents for energy through chemosynthesis. The vents are usually found where the crust is warmer near volcanically active areas, and although Europa may not have plate tectonics, the gravitational interaction between the Galilean Moons keeps its mantle warm, so similar processes could form these hydrothermal vents where the subsurface ocean and mantle interact.
These oceanic communities are fairly good analogues for the icy moons of the solar system like Europa or Enceladus, but they are still connected to the rest of the (decidedly non icy) ocean, so are very much affected by the global system. In fact, the cold, extra salty oceans of the poles (the salt concentration increases as ice forms from pure water, leaving salt behind), is what powers some of the world’s largest and strongest ocean currents.
But you don’t have to go far to find an even better analogue: beneath the ice sheet of mainland Antarctica lies a buried continent, fit with its own mountains, valleys, and even lakes. It has been hypothesised that a huge lake and river system of salty water could exist in certain places beneath the ice, such as Lake Vostok, 900m below the ice and 1500km from the ocean. The huge subglacial lake is currently being explored, and the researchers are having to implement techniques to prevent contamination of a potentially pristine ecosystem vulnerable to change. This ethical dilemma of interfering with an isolated ecosystem is one which is important to consider when planning space missions.6
Antarctica isn’t only an analogue site for icy moons, but also Mars. The McMurdo Dry Valleys are the largest area of Antarctica not covered by ice. Protected by mountains, they are some of the most extreme deserts on earth, having not seen any rain for about 2 million years, and any snow that falls quickly evaporates due to the incredibly dry wind. Some areas receive enough humidity for lichens to grow, but the driest regions most similar to Mars have an almost entirely sterile permafrost. The few microbes which do live here, live within rocks, which have more moisture than the air around them!7
As Mars is probably the most similar place in the solar system to the earth, it’s unsurprising that there are many environments on Earth with similarities to Mars, past or present!8 The Atacama Desert in Chile and Yilgarn Craton in Australia are very dry and have a similar geology to Mars, and the Rio Tinto in Spain is extremely acidic and has a high iron concentration, may be a good analogue for ancient mars rivers or underground rivers that may exist today.
Returning to orbit around the gas giants, in the case Saturn, we find Titan. At first glance, it’s easy to see that it’s very different from most other moons- bigger than Mercury and with a thick, hazy atmosphere, it more closely resembles early Earth or modern Venus than the moons close to it. Looking with infra-red reveals a world with remarkable similarities to Earth- it has clouds, mountains, rivers, lakes… but not a drop of liquid water can be found on its surface! This far from the sun, water is frozen solid- in fact the ‘rocky’ surface seen in the photos returned by the Huygens lander is actually water ice coated with a hydrocarbon snow. The pebbles are also made of ice, eroded from the mountainside fluvially (ie. by rivers).
But all the water is locked away as ice, so instead of having a hydrological cycle, Titan has a hydrocarbon cycle! This means its lakes are made out of liquid methane and ethane, so any life there would probably also be carbon based, but without water it would look very different from life on earth. However there are some environments which can be considered analogue sites to Titan, such as Pitch Lake, a huge natural deposit of asphalt in Trinidad, as well as big oil fields, especially those under permafrost like in Siberia or Canada. Specially adapted microbes can be found here, including archaea, bacteria and fungi which survive using hydrocarbons as their energy source.
Analogue Sites aren’t just used for purely scientific research, but also the testing of new technology which could be used on future missions! Lava Tubes and Caves are very interesting to study in terms of what life is found in them and how deep, but they are also formed by physical processes that aren’t limited to earth- we see lava tube like structures on both Mars and the moon, and these have been suggested as potential locations for future artificial habitats of crewed missions, as the rock above would act as a natural barrier against radiation.
For now, interplanetary (and certainly interstellar) missions have to be uncrewed, or explored solely by passive exploration with telescopes. One key technique when looking for signs of life- but also just to learn about the chemistry of a site more generally- is spectroscopy. 9 Spectroscopy involves splitting the light from a source to show its whole spectrum (like when a prism creates a rainbow) and then looking for missing wavelengths or brighter wavelengths (depending on the type of spectroscopy). This ‘barcode’ pattern can be analysed to reveal the chemical composition of a source.
Absorption Spectroscopy is often used in planetary science to analyse atmospheres, and works by looking at light that has passed through the atmosphere, and seeing which wavelengths were absorbed. This can be matched to specific molecules as each atom or molecule will absorb a different set of specific wavelengths. To learn about the geology of planetary surfaces, reflection and emission spectroscopy can also be used.10 The catch is that biosignatures often have non-biological sources as well as biological ones, so the technology has to be able to differentiate between the 2 origins. Again, scientists are using earth analogue sites to test this…
The Borup Fiord Pass in the Canadian Arctic is a glacial system which has deposits of elemental sulphur on its surface, which is produced as a result of chemotrophic bacteria which use sulphates in their surroundings for energy by turning them into sulphides, which is carried to the surface by pipes/springs, where it is re-oxidised to produce elemental sulphur, which has a characteristic yellow colour. When the sulphur is in the bacteria, it has a unique biogenic structure. Scientists have already been able to identify and map the compounds from space using satellites, so investigating whether or not satellites can pick up the biogenic structure would be useful for technology applied to Europa, which also has sulphur deposits at its surface, origin unknown!11
Circling back to Antarctica, it’s not just the life existing in spite of the environmental challenges that makes it such a valuable analogue site- it’s inaccessibility and hostility to human life also plays a part: researchers who go there have to live in a similar way to astronauts! Technology means that they are better connected virtually, but they are still physically very isolated, having to live with a changing day night cycle which messes with their circadian rhythms, and an environment which can only be survived by wearing appropriate equipment… so the researchers are practically taking part in analogue space missions of their own!
There are so many more analogue environments across the globe, that’s without mentioning the lab-based artificial analogues which can be precisely controlled for experiments, or the use of Low Earth Orbit! Additionally, astrobiologists are also interested in how life on earth began, so stay tuned for my next post which will be about analogues of the early earth!
I’d originally done some brief research into analogue sites for my webinar with Girls in Aerospace last month, but had wanted to go into more detail.. so here it is! Hope you enjoyed, as always let me know if you have any questions, and stay curious!
Sources sources sources!
- https://noc.ac.uk/technology/technology-development/autonomous-vehicles https://www.bas.ac.uk/polar-operations/sites-and-facilities/facility/rrs-sir-david-attenborough/science-facilities/marine-robotics/
- https://www.researchgate.net/publication/317889242_Earth_as_a_Tool_for_Astrobiology_-_A_European_Perspective https://core.ac.uk/download/pdf/160504075.pdf