This question already has an answer here:
I know atomic mass can be (over) simplified as the average mass of all isotopes of a given element weighted by the ratio of their natural occurrence. I also know that, as far as we know, refining relative atomic mass is a matter of finding new significant figures, not finding new isotopes. Are we reasonably sure these ratios won't change as we explore the universe?
As an over simplified example, do we have a reasonable certainty that 70% of the Calcium on Mars won't turn out to be Ca-41?