Argon has a radiogenic isotope 40Ar, which is produced from the radioactive decay of 40K. In contrast, 36Ar is primordial and was incorporated into the atmosphere during the formation of Mars. Observations indicate that Mars is enriched in 40Ar relative to 36Ar, which cannot be attributed to mass-selective loss processes.[45] A possible explanation for the enrichment is that a significant amount of primordial atmosphere, including 36Ar, was lost by impact erosion in the early history of Mars, while 40Ar was emitted to the atmosphere after the impact.[45][2]