There's nothing special about Argon, per se. Think of the Martian atmosphere as a pot that has recently boiled water an unknown number of times. read more
When looking at Argon, or really any other element in the Martian atmosphere, we know what the naturally occurring ration of isotopes is. The Argon 36 and Argon 38 you mentioned are atomic weights, 38 being two neutrons heavier. read more
Mars is a planet that shows climate change on a large scale. Although its atmosphere used to be thick enough for water to run on the surface, today that water is either scarce or non-existent. The atmosphere today is also too thin to easily support life as we know it, although life may have existed in the ancient past. read more