Final answer:
The term 'Ir' stands for iridium, a rare and corrosion-resistant element with atomic number 77. It is much more common in meteorites than in Earth's crust, suggesting an extraterrestrial origin for much of the iridium on Earth.
Step-by-step explanation:
The term 'Ir' refers to iridium, which is a chemical element with the symbol Ir and atomic number 77. Iridium is noteworthy for its high density and for having the highest corrosion resistance of any element. Additionally, iridium is rare in the Earth's crust and is much more prevalent in meteorites. The high concentrations of iridium in meteorites compared to terrestrial rocks support the hypothesis that some of the Earth's iridium has extraterrestrial origins.