Astrobiology, Physics

Astrobiology 103: Looking for life closer to home

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, and plans to send missions to! 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. But it is this atmosphere that makes it so different to earth. It is 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, at sea level, the atmospheric pressure is 92 times that on earth! This immense pressure, and the greenhouse effect caused by the CO2, means that Venus’ surface is the hottest place in the Solar System (excluding the sun). 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, and Sulphuric Acid clouds exist! This is a big problem, and has caused all missions that entered Venus’ atmosphere to stop 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 acid

Although this doesn’t bode well for life’s current possibilities of existing there, it’s possible that Venus was habitable earlier in its history! 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. This meant that the conditions would have been habitable, and thanks to volcanoes being a good energy source, life could possibly have existed!

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 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. This lack of water caused any existing plate tectonics to ‘freeze’ as water acts as a lubricant at plate boundaries, so the Carbon Dioxide cycling that generally regulates temperature also stopped. And so we are left 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, due to the fact that Venus seems to have been completely resurfaced within the last billion years, as it doesn’t have as many scars from crater impacts as we would expect, when compared to the Moon, Mars, and even Earth!

The Red Planet

Our next closest neighbour, after Venus, is Mars. It is a much more forgiving planet to send missions to (although many have failed), 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 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 earth- 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 manned 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 oceans 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 few sources of liquid water off-earth! 

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 and past life! 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

Unfortunately, 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. 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 their 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

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? Let me know in the comments below!


If you can’t trust an atom… trust in science!

☆it’s like magic, but it’s true whether you believe it or not!☆

See you next time!



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