Question: What are the chances there are aliens on Mars?

Andrew McDowall answered on 22 Nov 2021:

It seems very unlikely we’ll discover any form of advanced life on Mars, but the prospects of something more basic are rather better.
The suface environment on Mars is very harsh – cold, dry, oxidising and with a high radiation flux. The subsurface environment and the historical surface environment look to be rather better so it’s possible life might have evolved and retreated as the surface environment decayed. Certainly on Earth we find life in the most extraordinary environments, capable of tollerating and thriving in conditions that you would have expected to be instantly lethal, environments kind of similar to those on Mars, so it is possible, but on Earth there has been a long period of time for organisms to develop the complex mechanisms needed and then adapt them to the extreme environments. It’s a question as to whether the environment on Mars was benign enough, for long enough, for these mechanisms to evolve and adapt before the planet lost most of its atmosphere and became a radiation-blasted desert.
At the moment I’m pessimistic regarding the prospect of finding any life of any sort on Mars (not accidentally carried recently from Earth) in the reasonable future. If there is any it’s likely to be buried deep and I’m not sure there was time before Mars froze. But our understanding of the Martian environment and history is advancing so rapidly (and also our understanding of the conditions life can endure on Earth) that anything we think we know is likely to be rewritten in weeks or months and the great thing about science is you get to change your mind.