How would life start in extreme environments on other planets?

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“Are we alone?” is an age-old question asked by humans about whether or not there is life on other planets. Whether there is evolved, intelligent life, is beyond this blog post. Instead, I’m going to cover the potential for early, microbial life on other planets. For this subject, it is likely that life would start in a similar way to how it started here on Earth, billions of years ago.

What would the life look like?

Early life on Earth is believed to be related to, or have been microbes known as archaea, which resemble bacteria. Many species of archaea are what’s known as extremophile organisms, which is to say they live in extreme environments.

Extreme environments refer to extremely high or low levels of a certain part of the environment. For example, halobacterium (right) live in high salt content water, such as salt lakes or the Dead Sea. Other types of archaea live in highly acidic or alkaline environments (such as hydrothermal vents and soda lakes respectively), extremely hot or cold environments (such as geothermal power plant outflow or deep ice) or low oxygen environments (such as swamps).

Most likely, humans could discover cold thermophiles or acidophiles on other planets, for reasons I will explain near the end of this post.

How would the life start?

Taking notes from Earth’s early history, we can get an idea of how the requirements for life got to these planets.

An early theory as to how water and the chemicals commonly referred to as the building blocks of life is that asteroids containing them arrived on Earth. It is known that many asteroids struck the planet approximately 4 billion years ago, though other theories have arisen on the topic of water and compounds such as amino acids.

Amino acids for instance may have formed from some sort of shock (ie: from lightning) out of volcanic gases, such as hydrogen sulfide. An experiment devised by the late chemist Stanley Miller that was finally analyzed circa 2008 revealed that such a shock would synthesize compounds known as precursor reagents, such as formaldehyde. These compounds could be absorbed by water in the atmosphere and rain down, where synthesis reactions for amino acids would occur. Amino acids themselves are not life, but eventually, could undergo the reactions to become an organism.

Water, on the other hand, could still have arrived via asteroids, based on analysis of the ratio of “heavy” water to “ordinary” water, the former featuring the deuterium isotope of hydrogen instead, which weighs more due to the presence of a neutron. However, new theories indicate that much of water could have also formed from the solar nebula that once surrounded the sun, or the gases which were a key part of the formation of planets. With the latter two theories, the water would have formed at the same time as Earth.

What planets could support life? (Or: Case studies)

According to the University of Puerto Rico, Arecibo’s Habitable Exoplanets Catalogue, there are 55 total discovered exoplanets that could theoretically support life (an exoplanet being a planet outside our solar system). Due to the distance between us and these planets however, we don’t quite know details about these planets. We do however, have three moons in the solar system that could be home to life: Europa, Titan and Enceladus.

Europa is perhaps the best well known; it is covered in ice, and presumably has a massive subsurface ocean. It is one of Jupiter’s 79 moons. Europa is believed to have formed around the same time as most of the solar system, 4.5 billion years ago. Also in these oceans, there may be hydrothermal vents or volcanoes on the sea floor, providing nutrient chemicals, making it one of the most likely bodies to harbour life.

Titan is one of the most Earth-like celestial bodies in the solar system; it has an atmosphere and is the only other body to have a liquid cycle and bodies of liquid such as lakes. It orbits Saturn. The reason I say “liquid” and not “water” is because the surface of Titan is cold enough for liquid methane to be present; this liquid is methane. Gravitational and radio analysis by probes also indicates that there is an underground water ocean. These environments are potentially habitable for extremophile organisms.

Enceladus, a moon of Saturn and an icy one, similar to Europa, is unique in that it sprays water out of its icy shell. Samples of this spray have revealed that Enceladus does contain many chemical requirements for life, and the presence of the spray itself indicates that Enceladus has a subsurface ocean. These chemical requirements likely come from hydrothermal vents as well. Some of Enceladus’ spray makes its way into Saturn’s rings, where small amounts of silica have been found; silica being formed from interactions from hydrothermal activity. All of this evidence means that Enceladus is yet another candidate for life in the solar system.

It is very likely that other potentially habitable planets would have similar characteristics to these three moons in our solar system, but other conditions are still possible.

_{Note: All images sourced from NASA or affiliates are in the public domain, according to their image use policy.}