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asteroid impact
Has the Earth grown or shrunk since it formed?
Submitted by Saavik Ford on Mon, 7/13/2009, 9:43 PM
Not much. The Earth is inconceivably massive: it is around 6x1024 kg which would weigh about 12x1024 pounds. That's 12 billion quadrillion pounds (yes that's a real number!). The Earth gains and loses very small amounts of mass each day. The main gains are from dust that falls to Earth as micrometeorites. In an average year we gain maybe a hundred million kilograms, which sounds large, but as a percentage of the Earth's total mass, this is tiny. It is so tiny that if the Earth accumulated this much every year since it formed, we would only gain about one one hundred thousandth of a percent of the Earth's mass over its entire history so far.
We also lose mass every year, mainly from our atmosphere. A complete inventory is a little hard to come by, but again, the losses are tiny (most of our atmosphere is still here and breathable, after all!).
Of course, when the Earth was very young, we gained a lot of mass in a very short time period - the Earth formed by accreting "planetary embryos" - essentially super-asteroids. The last accretion event large enough to affect, for example the bulk composition or orbit of the Earth was the Moon forming impact. The Solar System began forming about 4.57 billion years ago, and the Moon-forming impact happened between 4.52 and 4.50 billion years ago. So for the vast majority of the history of the Earth, it has been the same mass as it is today.
