How Did the Moon Really Form

Planetary scientists have long believed that our moon formed following a collision between Earth and another planet, but studies of Earth and moon rocks suggest otherwise. A new analysis of the composition of moon rocks brought back by Apollo astronauts may help finally resolve the mystery.Here’s the current thinking about how the moon formed. Early in its history, Earth was struck a glancing blow by a Mars-sized planet. That planet was destroyed by the impact, but much of its debris—and some of Earth’s—formed into a disk around Earth that eventually coalesced into the moon. Much evidence supports this scenario. The moon would have ended up hot, boiling off light elements and water, leaving the arid rocky moon we see today; the moon has a small core, consistent with being made from parts of the colliding planet and outer parts of Earth; the Earth-moon system rotates fast, consistent with a glancing blow.But one bit of evidence just doesn’t fit: the composition of moon rocks. Researchers have found that rocks from different parts of the solar system (brought to Earth as meteorites) have subtle differences in their composition. Oxygen, for example, comes in different varieties, called isotopes. Oxygen-16 (O-16) is the most common type, followed by oxygen-17 (O-17)—which has one extra neutron in its nucleus—and oxygen-18, with two extra neutrons. Meteorites from different parts of the solar system have different proportions of these isotopes. So a rock from Mars would have a markedly different ratio of O-17 compared with O-16 than, say, a piece of an asteroid or a rock from Earth. These ratios are so reliable that researchers use them to identify where meteorites come from. Email Sign up for our daily newsletter Get more great content like this delivered right to you! Country Click to view the privacy policy. Required fields are indicated by an asterisk (*) Country * Afghanistan Aland Islands Albania Algeria Andorra Angola Anguilla Antarctica Antigua and Barbuda Argentina Armenia Aruba Australia Austria Azerbaijan Bahamas Bahrain Bangladesh Barbados Belarus Belgium Belize Benin Bermuda Bhutan Bolivia, Plurinational State of Bonaire, Sint Eustatius and Saba Bosnia and Herzegovina Botswana Bouvet Island Brazil British Indian Ocean Territory Brunei Darussalam Bulgaria Burkina Faso Burundi Cambodia Cameroon Canada Cape Verde Cayman Islands Central African Republic Chad Chile China Christmas Island Cocos (Keeling) Islands Colombia Comoros Congo Congo, the Democratic Republic of the Cook Islands Costa Rica Cote d’Ivoire Croatia Cuba Curaçao Cyprus Czech Republic Denmark Djibouti Dominica Dominican Republic Ecuador Egypt El Salvador Equatorial Guinea Eritrea Estonia Ethiopia Falkland Islands (Malvinas) Faroe Islands Fiji Finland France French Guiana French Polynesia French Southern Territories Gabon Gambia Georgia Germany Ghana Gibraltar Greece Greenland Grenada Guadeloupe Guatemala Guernsey Guinea Guinea-Bissau Guyana Haiti Heard Island and McDonald Islands Holy See (Vatican City State) Honduras Hungary Iceland India Indonesia Iran, Islamic Republic of Iraq Ireland Isle of Man Israel Italy Jamaica Japan Jersey Jordan Kazakhstan Kenya Kiribati Korea, Democratic People’s Republic of Korea, Republic of Kuwait Kyrgyzstan Lao People’s Democratic Republic Latvia Lebanon Lesotho Liberia Libyan Arab Jamahiriya Liechtenstein Lithuania Luxembourg Macao Macedonia, the former Yugoslav Republic of Madagascar Malawi Malaysia Maldives Mali Malta Martinique Mauritania Mauritius Mayotte Mexico Moldova, Republic of Monaco Mongolia Montenegro Montserrat Morocco Mozambique Myanmar Namibia Nauru Nepal Netherlands New Caledonia New Zealand Nicaragua Niger Nigeria Niue Norfolk Island Norway Oman Pakistan Palestine Panama Papua New Guinea Paraguay Peru Philippines Pitcairn Poland Portugal Qatar Reunion Romania Russian Federation Rwanda Saint Barthélemy Saint Helena, Ascension and Tristan da Cunha Saint Kitts and Nevis Saint Lucia Saint Martin (French part) Saint Pierre and Miquelon Saint Vincent and the Grenadines Samoa San Marino Sao Tome and Principe Saudi Arabia Senegal Serbia Seychelles Sierra Leone Singapore Sint Maarten (Dutch part) Slovakia Slovenia Solomon Islands Somalia South Africa South Georgia and the South Sandwich Islands South Sudan Spain Sri Lanka Sudan Suriname Svalbard and Jan Mayen Swaziland Sweden Switzerland Syrian Arab Republic Taiwan Tajikistan Tanzania, United Republic of Thailand Timor-Leste Togo Tokelau Tonga Trinidad and Tobago Tunisia Turkey Turkmenistan Turks and Caicos Islands Tuvalu Uganda Ukraine United Arab Emirates United Kingdom United States Uruguay Uzbekistan Vanuatu Venezuela, Bolivarian Republic of Vietnam Virgin Islands, British Wallis and Futuna Western Sahara Yemen Zambia Zimbabwe Here’s the puzzle: The giant impact hypothesis predicts that the moon should be made of about 70% to 90% material from the impactor, so its isotope ratios should be different from Earth’s. But ever since researchers got hold of Apollo moon rocks for analysis, they have failed to find any significant difference in isotope ratios on Earth and the moon. Studies of the isotopes of oxygen, titanium, calcium, silicon, and tungsten have all drawn a blank.This discrepancy has troubled planetary scientists so much that in recent years they have put forward a number of alternative scenarios to explain the moon’s origins. One hypothesis suggests that there could have been much greater mixing between Earth and the debris disk as it coalesced after the impact, or if Earth was hit head-on by a similarly sized impactor, their remains could have mixed completely. Another possibility is that a fast-spinning Earth could have been hit by a much smaller impactor, which would have provided little material for the moon. Yet it has been hard to show how you could get from one of those events to the Earth-moon system we have today.Researchers would prefer to stick with the original, plain vanilla impact scenario because it explains so many things so well. New results, published online today in Science, will give them some hope. Lunar rocks have a measurably higher ratio of O-17 over O-16 compared with those from Earth. The new study began because a team of researchers led by Daniel Herwartz of the University of Cologne in Germany had recently upgraded its mass spectrometer—a form of supersensitive atomic scale—and decided to test the device out on the Earth-moon isotope problem. “Our analysis is now an order of magnitude better than other laboratories,” says team member Andreas Pack of the University of Göttingen in Germany.They started out analyzing moon rocks that arrived on Earth as meteorites but found that the weathering these rocks experienced on Earth was skewing the results. So they got hold of some rock samples from NASA that had been brought back by Apollo missions 11, 12, and 16. They extracted oxygen from all the samples and then passed it through the spectrometer to find out the proportions of each isotope. Their conclusion was that the lunar samples had an O-17 to O-16 ratio that was 12 parts per million higher than rocks derived from Earth’s mantle. This difference “supports the view that the Moon formed by a giant collision of the proto-Earth with [an impactor],” the team writes. “It is a relief that a [disparity in ratios] has been found, since the total absence of difference between Earth and moon would be hard to explain,” comments planetary scientist David Stevenson of the California Institute of Technology in Pasadena, in an e-mail.   The team acknowledges other possible explanations for the difference, including that Earth was bombarded by material with a lower oxygen isotope ratio at some time after the impact. “Now that a difference has been found, many will work to confirm or deny it and do battle over what it means,” Stevenson says.The team says the results suggest that the moon is a roughly 50-50 mix of Earth and impactor material. Moreover, the high oxygen isotope ratio suggests that the impactor was principally made of a rare material called enstatite chondrite. The vast majority of meteorites that land on Earth are chondrites, but only about 2% of those are enstatite chondrites. “The possible significance of enstatite chondrites is interesting, but at present we are stuck with speculating about the bodies that went into making Earth, since they are no longer around,” Stevenson says.

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