New research reveals that the abundance of so-called highly siderophile, or metal-loving, elements like gold and platinum found in the mantles of Earth, the moon and Mars were delivered by massive impactors during the final phase of planet formation over 4.5 billion years ago. The predicted sizes of the projectiles, which hit within tens of millions of years of the giant impact that produced our moon, are consistent with current planet formation models. They predict that the largest of the late impactors on Earth — at 1,500 to 2,000 miles in diameter — potentially modified Earth’s obliquity by approximately 10 degrees, while those for the Moon, at approximately 150-200 miles, may have delivered water to its mantle.