Origin of the Moon and of Earth's water

6 problems

• Could the Earth-Moon system have shed a large angular momentum excess?

  Contested

  Some impact scenarios that mix Earth and impactor well leave the system spinning about twice too fast. They work only if a gravitational resonance with the Sun later removed the extra spin.

• Why are Earth and Moon isotopically nearly identical?

  Open

  In the standard giant-impact simulations, most of the Moon comes from the impactor, which should have had a different chemical fingerprint. Yet Moon and Earth rocks have the same isotope ratios to a few parts per million.

• When did the Moon-forming giant impact happen?

  Contested

  Different lunar rocks and minerals give formation ages from about 4.51 to about 4.35 billion years ago. Whether the Moon formed early and was later remelted, or formed late, is disputed.

• Was the Moon-forming impactor isotopically Earth-like?

  Open

  The isotope puzzle disappears if the impactor formed in the same neighborhood of the solar system as Earth. Whether that is plausible depends on where the planets' building blocks came from.

• How much water is stored in Earth's mantle and core?

  Open

  The oceans hold only part of Earth's water; more is locked in minerals deep inside, and hydrogen may also be dissolved in the iron core. Estimates of the hidden amount vary from less than one ocean to many.

• Where did Earth's water come from?

  Contested

  Earth's oceans match the hydrogen chemistry of some water-bearing meteorites, but water could also have come from rocks like those that built Earth or from the gas around the young Sun. The share from each source and its timing are unknown.