UC Davis professor coins new theory that answers questions about the moon
The textbook story of how the moon was made begins with an object the size of Mars clipping the Earth, shattering the foreign object and creating a cloud of rocky material, from which the moon condensed. Despite some prominent concerns, the “Giant Impact Theory” has remained the prevalent theory on the formation of the moon due to lack of alternative hypotheses.
One problem with this theory is that it dictates that the moon should be made out of the material from the object that hit the Earth. However, the moon’s isotopic chemical fingerprint, a diagnostically unique ratio of isotopes of certain chemical elements, is very similar to the Earth’s. Another problem with the theory is that the moon’s orbit is offset 5-degrees from the ecliptic plane, or the plane at which the rest of the solar system orbits.
“If the moon formed by a giant impact, it would have grown from the debris that were initially around the earth’s equator,” said Sarah Stewart, a professor in the Department of Earth and Planetary Sciences. “If you calculate what happens to the moon after that, it shouldn’t have any inclination.”
Stewart, along with then-postdoctoral fellow and current scientist at the Science for Extraterrestrial Intelligence (SETI) Institute, Matija Ćuk, and two other researchers, published a paper in Nature which detailed a new theory. Published in Oct. 2016, the theory explained both the moon’s 5-degree tilt and the similar isotopic fingerprint to Earth.
The theory starts with a high energy, head-on collision between a larger-than-current-day Earth and a celestial body half the size of Mars. The collision combined the foreign body and the Earth, and blew a part of that combined mass into a cloud that ultimately resulted in the modern-day moon.
This mixing and subsequent spewing of combined material explains the almost identical chemical isotopes of the Earth and the moon. The collision also set the Earth rotating at a rate that made days two to three hours long. The degree of rotation was skewed to a 60 to 80-degree angle — as opposed to the current 23-degree angle — which explains the 5-degree tilt of the moon’s orbit.
“[We proposed] that the moon formed in the equator around the Earth when it was tilted over,” Stewart said. “During the evolution from that initial condition, two dynamical events happen that straighten up the Earth so that our obliquity is lowered, [which] lowers the inclination of the moon, but not all the way down to zero.”
At its core, the new theory still revolves around a “giant impact,” but the details of the collision, as well as the things that follow, are drastically different.
This theory answered a lot of questions; however, it also left scientists an oblong Earth with too much angular momentum, causing the earth to rotate more rapidly. Part of what Stewart and Ćuk were able to show in their paper was how to bring the earth to a 23-degree tilt, while dissipating the some of the angular momentum and maintaining the 5-degree tilt of the moon’s orbit. These feats were achieved with the two dynamical events known as the LaPlace and the Cassini transitions.
Simon Lock is a graduate student in the Department of Earth and Planetary Sciences at Harvard University, as well as a co-author on the paper.
“If the moon starts [at the equatorial plane of the Earth], and the Earth starts on its side, then through something called the LaPlace plane transition, the Earth gets tilted upright, but the moon gets left in a very high inclination orbit,” Lock said. “When there’s a second-state transition which we call the Cassini state transition […] you damp inclination, and you bring [the moon’s orbit] down to the 5 degrees you see today.”
As for why the two transitions take place, Lock said that they are driven by the search for a stable state.
“There are very stable what we call ‘orbital arrangements’ between the rotation axis of the Earth [and] the orbital axis of the moon,” Lock said. “At a point in the moon’s history, you move from one of these stable configurations to the other; one becomes more preferable than the other.”
Because the LaPlace and the Cassini transition states are integral to the paper’s hypothesis, finding evidence that these transitions took place can further validate the updated theory.
“These events dissipate energy in the moon and the Earth, so they’re heating events, and we can look for the record of heating events in the moon,” Stewart said. “There are isotopes that imply that the moon had been hot at a time that seemed to be after its date of origin, and it could be that our dynamical transitions are responsible […] One of the tests for our hypothesis is to look for, in more detail, a fingerprint — that’s a thermal pulse — that occurred during our tidal evolution.”
Dawn Sumner, the chair of the Department of Earth and Planetary Sciences at UC Davis, attests to the significance of narrowing down a theory on the formation of the moon on our understanding of the basic dynamics of the Universe.
“Stewart’s research has lots of important scientific implications for the origin of the moon and how it has affected Earth.” Sumner said in an e-mail interview. “It also explains some of the characteristics of the Earth-moon-sun system that people observe on a regular basis. Stewart’s model explains aspects of our daily life — particularly the length of a day — for the first time.”
This research was supported by NASA.
Written by: Meral Basit – email@example.com