Astrobiology 103: Looking for Life closer to home

This post was created as part of my crest project in 2020. You can read the original here or watch it as a youtube video!

In my last post, I talked about how cool exoplanets are, and how we can find them and look for life there. But there is a small possibility that we won’t have to travel so far to find life. Although unlikely to be intelligent life, there are some locations that NASA has highlighted as potential habitats. Time to visit Earth’s neighbours…

Earth’s Evil Twin

Before the advent of modern space travel, Venus was enshrouded in thick clouds and an air of mystery. From afar (108.2 million kilometres afar), Venus is a similar size and density to Earth, is made of mostly the same stuff, and has an atmosphere.

So it was considered our twin planet, the most likely place to find aliens… Then we visited it and found it is the atmosphere that makes it so different to earth: 96% Carbon Dioxide, and no oxygen to speak of! Surprisingly, this is actually very similar to the atmosphere of early earth, despite the fact that the surface pressure is 92x that at sea level on earth! This immense pressure combined with the greenhouse effect caused by the CO2, means that Venus’ surface is the hottest place in the Solar System (excluding the sun), at a boiling 462 degrees! This makes the possibility of life incredibly unlikely.

Some scientists have theorised that life could exist in its upper atmosphere, where the temperature and pressure are more pleasant. One downside? This is where the highest concentrations of Sulphuric Acid are found! The hot, acidic conditions make active explorations of venus difficult- all missions that entered Venus’ atmosphere stopped transmitting within 2 hours. 

Image of Venus, showing its beige coloured clouds
Venus today has a thick atmosphere of carbon dioxide and clouds of hydrochloric and sulphuric acid

Although this doesn’t bode well for the possibility of life existing there now, it’s possible that Venus was habitable in the past! This is because early Venus (and early Mars actually) was actually quite similar to Earth. It may even have had oceans! Why? When the sun was younger, it wasn’t as bright, so the habitable zone was closer to the sun, allowing conditions to be more habitable. 

Early Venus, yellow surface and blue oceans
Early Venus may have had oceans and a habitable atmosphere

However, as the sun aged, the extra insolation (energy from the sun hitting the surface) meant the temperature reached a tipping point: oceans began to evaporate (and as water is a greenhouse gas, the speed of evaporation increased exponentially) until the surface was barren of water.

Water acts as a lubricant at plate boundaries, so without it, the plates ‘froze’ in place. Volcanoes occurring at plate boundaries are a vital part of the carbon cycle, which would usually regulate the temperature…. leaving us with the hellish world that is Venus today!

Graph showing Venus, Mars and Early Earth atmospheres as all being similar, with high concentrations of CO2 (over 90%), some nitrogen, and very little oxygen.
All the three ‘habitable zone’ planets started off with similar atmospheres

Even if we managed to design a lander that survived a substantial length of time, we probably wouldn’t be able to find any evidence of past life, had it formed in the distant past, as Venus seems to have been completely resurfaced within the last billion years, and any life would have died long before then.

The Red Planet

Our next closest neighbour, after Venus, is Mars. Despite many failures in the past, Mars is a much more forgiving planet to send missions to, and so has become the focus of many.

Like Venus, it may once have been similar enough to Earth for life to have begun! This time, however, the reason for its demise is more linked to its size, than its place in the solar system. Mars is much smaller than the Earth (about half the size), meaning that it cooled far quicker than the Earth. This cooling meant that the originally liquid outer core (which carried a current and thereby created a magnetic field due to the rotation of the – the dynamo effect) solidified, reducing the magnetic field to patchy sporadic areas. This means that its surface is subject to large amounts of radiation, which also poses a threat to any future crewed missions.

However, it is possible that if life had begun during Mars’ habitable period (when Mars may have had oceans, a habitable atmosphere and a reliable magnetic field), some microbes could still survive, beneath the ground! These could exist just within cracks in surface rock (microbes on earth have been found living up to 5km underground!), or in subsurface lakes or caves!

In 2018, scientists discovered a large, probably saltwater, lake a mile beneath Mars’ south pole! If it is a real lake, then there could be life living within it. It would have to survive super salty conditions, but we do have examples of halophilic extremophiles on earth! More research is required, as it could just be an area of supersaturated rock, which would still be cool, as would be one of the very few sources of liquid water on mars.

Image via EarthSky of the possible lake underneath Mars’ south pole

Mars is probably the most likely planet in our solar system to be home to extraterrestrial life, which is why most probes to Mars are equipped with some sort of astrobiological instrument, often one that looks for chemical biosignatures of present life, or analyses rocks for trace fossils of past life!

For example the Perseverance rover launching in 2020 will carry a drill, to sample underneath the regolith.  However, due to the amount of missions we have sent, and plan to send, we need to be really careful about contamination- we don’t want to find evidence of life 2.0 and it ending up just being the remnants of someone’s sneeze!

The Moon

Realistically, the moon isn’t a prime candidate for life, due to the fact it has little atmosphere, no magnetic field, and it’s only water is trapped in ice or within rock. But like with many places in the solar system, there is a small possibility that life existed long ago.

This is because for a short time after its formation, the moon was volcanically active (providing energy) and may have had some water and a substantial atmosphere. If life had formed then, or was transferred from Earth via Panspermia (the theory that life can be transferred between planets/ moons via asteroids etc) and against the odds some have survived, it would probably be deep under the surface, or within lava tunnels!

Despite the incredibly low likelihood of organic life existing there, it’s possible that artificial panspermia may have occurred as a result of past moon missions! It has already been proven by experiments on the ISS that tardigrades can survive the harsh environment of space, so it is possible that some could have been dropped there during moon missions! As well as this, I really hope that if we check back in a decade, we will find more than microbial life on the moon- perhaps we will have started long term moon missions by then with NASA’s Artemis program! 

What might we discover if we head back to the moon? Image via moon.nasa.gov

So what do you think? Will we find life in any of these places? Do you know of any other bodies in the solar system that might be home to life?