I am not a planetary scientist, but I have read a bit on the subject. My understanding is that it’s almost entirely due to the lack of magnetic field. The solar wind just blasts the atmosphere away over time.
If we ever reach a high enough state of technology to terraform Mars, we will have to figure out some way to protect the atmosphere. I found this video to be a really interesting take on the subject: youtu.be/HpcTJW4ur54?si=P-HpbrHPjJFfAaEI
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