Mars is far the most earthly planet in the solar system … but that doesn't say much.
We know that Mars once organized oceans, rivers, lakes and streams. It had a thick carbon dioxide-rich atmosphere. Mars and Earth looked like siblings billions of years ago. Yes, the red planet was probably cooler than the earth, which meant that it probably contained much larger polar ice caps and was much drier, but all conditions were definitely in the area to support life. And at the same time, billions of years ago, we know that microorganisms roam the earth – or, floated or flagate or whatever it is that microorganisms do this.
So was Mars alive in the distant past? Have the remains survived to this day? Or is there at least a fossil stock that could find one day?
Since Mars is the next, what every world of the solar system comes to earth, we have the best chance of finding the next analogues for earth-like life here. And so in our assessment of Mars we have to evaluate the conditions for life as a past or current whereabouts for life using four criteria:
First, we need water. Liquid water. The universal solvent. It cannot be frozen or evaporated. It must be available in abundance in life. And it must have the right solity and the right pH value – too salty, not salty enough, too acidic, too fundamental and life cannot work. Yes, many extremophiles on Earth can live in a wild row of conditions, but there is still a limit for the basic biochemistry.
Second, life needs the right chemical environment. It takes many good things, the elementary building blocks such as carbon, hydrogen, nitrogen and oxygen. And these elements are sufficiently sufficient and in the right places: as in or near the surface near water bodies. And at the same time, life needs low concentrations of bad things, heavy metals such as zinc and nickel. Life needs to work safely to work, but too high concentrations lead to a toxic environment.
Next, life needs energy. Only direct energy. A power source. This can be the sun, it can also be geochemical, like hydrothermal ventilation slots. It also requires oxidizing agents and reducing agents – it needs chemical paths so that it can save, transform and use energy.
After all, physics must be correct. It can't be too hot or too cold. The pressure cannot be too high or too low – life will not have a great time in the vacuum of the room or in the core of a planet. There cannot be too many cosmic rays that are really good at destroying endangered molecules. A substrate is needed, something to anchor yourself. It needs transport so that it can encounter new resources.
Life is … in need. And although we know that the past Mars has certainly held some of these boxes, we do not know whether it has held all these boxes. And even if all conditions have been met in the past, we do not know whether life is guaranteed to happen or the chance of not making it.
In today's environments on Mars, which could possibly support life, there are only a few, and it is about following the water.
Mars still has a lot of water, but it is almost only frozen in the polar ice cap or under the surface. Over the years, however, there has been a lot of evidence that there can be liquid bags. For example, in 2018, scientists discovered what they had thought of a liquid water lake under the southern ice lid, liquid due to the pressure of the upper limit. This discovery was based on Radar mapping, which were recorded with the Mars Express Orbiter. However, further investigations have doubts as to whether liquid water is the best interpretation of the radar signals.
And then there is the favorite, the recurring slope lineae, which are rejuvenated dark strips that appear on some crater walls in the summer months that were discovered in 2015 with the Mars Reconnaissance Orbiter. One possibility is that they are thawed through underground water and understood on the surface, although other options such as moisture from the atmosphere or only sulfates and chlorine salts that close to create grains that roll downhill are pulled out.
The last place where we could find water on Mars – and therefore where life could find a home – is deep underground. In 2024, scientists who worked with the Insight of NASA came to the conclusion that strange seismometer readers in the crust of 10 to 20 kilometers agreed with liquid water. This would not be a huge underground ocean (that is at the corner), but hydrated minerals: small pieces of water that are hidden in crevices and cracks and were kept warm from the remaining heat remaining down there.
We know that life on Earth under similar conditions is an existence, so that the old Mars life hiked down there when the planet had cooled and dried, similar to the life of Venusian life.
The endurance Rover of the NASA is currently crawling around Jezero Crater on the edge of an old lake and is looking for signs of past life, while Rosalind Franklin Rover is to reach the surface of the European Space Agency in 2028.
