Making the Moon: Two New Models

Simulation of a Moon-forming impact [Harvard University].

"A common origin for the Moon and Earth is required by their identical isotopic composition. However, simulations of the current giant impact hypothesis for Moon formation find that most lunar material originated from the impactor, which should have had a different isotopic signature. Previous Moon-formation studies assumed that the angular momentum after the impact was similar to the present day; however, Earth-mass planets are expected to have higher spin rates at the end of accretion. Here, we show that typical last giant impacts onto a fast-spinning proto-Earth can produce a Moon-forming disk derived primarily from Earth's mantle. Furthermore, we find that a faster-spinning early Earth-Moon system can lose angular momentum and reach the present state through an orbital resonance between the Sun and Moon."

- Matija Ćuk & Sarah T. Stewart-Mukhopadhyay, "Making the Moon from a Fast-Spinning Earth: A Giant Impact Followed by Resonant Spinning," Science DOI: 10.1126/science.1225542 (Online October 17, 2012)

Scientists have long believed the Moon formed as a result of a collision between the early Earth and a smaller planet, but computer models of the giant impact have always predicted the wrong composition for the Moon. Now researchers at Harvard University and the SETI Institute are proposing a new spin on the giant impact model to match the observed composition of the Moon. Understanding how the Moon formed is important for astrobiologists who are studying how the Earth became habitable for life as we know it.

The previous giant impact models have held that the small planet, Theia, hit the Earth, sending a cloud of debris from Theia into orbit that formed the Moon. But the chemistry of the Moon matches the Earth. Now Sarah T. Stewart-Mukhopadhyay, a professor in Harvard's Department of Earth and Planetary Sciences, and her SETI colleague Matija Ćuk propose a new giant impact model that resulted in pieces of the Earth breaking off and forming the Moon.

The researchers present a dynamic model of their theory, motivated by the results of chemical analyzes of isotopes from the Earth and Moon, in a paper published online today in Science. The results were also presented at the 44th meeting of the AAS Division for Planetary Sciences in Reno, NV.

Additionally, Stewart and Ćuk propose that prior to the collision and creation of the Moon, the Earth was spinning much faster than it does now, and had a day that was only two to three hours long.

Many scientists believe that Earth itself emerged from a series of giant impacts. These impacts made the early Earth spin near its stability limit of about 2 hours per revolution. The last giant impact, they believe, formed a Moon that is a twin of the Earth. Stewart and Ćuk posit that when the giant impact occurred between Theia and the fast-spinning Earth, the high speed of the Earth's spin caused the ejection of material from Earth into orbit. The ejected material formed a Moon with chemical composition similar to Earth. After the impact, the rapidly rotating Earth was slowed down by the gravitational interaction between the Sun and the Moon.

Previous giant impact models could match the size of the Moon and the present angular momentum of the Earth and Moon but did not explain the similar chemistry of the Earth and Moon. But the new theory, with the discovery of a mechanism to slow the spin of the Earth after the impact, explains how a giant impact with a fast-spinning Earth could result in a Moon with a similar chemical composition

Almost a "double planet," the Earth-Moon system imaged by the ESA Mars Express in Mars orbit [ESA].

As part of their dynamic model, Ćuk and Stewart found that a resonance between Earth's orbit around the Sun and the Moon's orbit around Earth can pass angular momentum to the Sun. Furthermore, Ćuk and Stewart showed that if the Earth was fast-spinning before the impact then a giant impact would eject enough Earth material into orbit to make the Moon.

Today, tides between the Earth and Moon both slow Earth's rotation and push the Moon's orbit further away. But the total angular momentum of the system is conserved. The finding is significant because without a fast-spinning Earth preceding impact, "a giant impact could not make the Moon originate from the Earth's mantle with today's angular momentum," says Stewart.

The origin of the Moon had been called into question by isotope analyzes of material from both Earth and the Moon. The isotope signatures of celestial bodies differ greatly and often are used to 'fingerprint' different planets and meteorite groups. The data show that the Earth and Moon are 'isotopic twins,' a contradiction to the Moon origin story from the original giant impact model. If the original model were correct, then the Moon should have had a different isotopic fingerprint than the Earth.

Nineteenth century scientists speculated about a fast-spinning early Earth. George H. Darwin, son of Charles Darwin, studied the relation between tides and the Moon. In 1879, he suggested that the Moon formed by fission from the Earth, but he did not know how early Earth might have being spinning so quickly. A similar dynamic model for a great impact resulting in the formation of the Moon from Earth material is described in a second paper in the same issue of Science. This alternative dynamic model is presented by Dr. Robin Canup of the Southwest Research Institute (SwRI).

Related:

Forming the Moon with an Earth-Like Composition via a Giant Impact (Canup, SwRI; Science)
Water from the Sun (October 17, 2012)
Hit-and-Run Science (September 30, 2012)
A Sawtooth-like timeline for the first billion years of lunar bombardment (August 28, 2012)
A new 'hit and run' Giant Impact scenario (July 28, 2012)
"Our view of the Moon has turned upside down" (April 26, 2012)
Ti paternity test fingers Earth as Moon's parent (March 28, 2012)
NLSI team sheds light on 'late heavy bombardment' (February 28, 2012)
Spudis: Cataclysmic Conundrum (February 14, 2012)
'Significant change' in bombardment timing (January 6, 2012)
LOLA reveals distinct populations in bombardment record,
Diviner finds "no pristine lunar mantle" even within SPA (September 16, 2010)
'The Grand Lunar Cataclysm and how LRO can help test it' (September 7, 2009)

The Astrobiology Institute, Harvard Crimson, SETI Institute and Southwest Research Institute contributed to this digest.

Zuber shares lunar morphology insights at Harvard

Daniel J. Kramer and Fatima Mirza
The Harvard Crimson

Geophysicist Maria T. Zuber applied her knowledge of Earth science to one of the world’s most universally studied astronomical bodies—the moon—at this year’s annual Neekeyfar Lecture on Math and Science, hosted Tuesday, April 17, 2012 by the (Harvard University) Undergraduate Council’s Student Advisory Board on Science.

“Basically every human who has lived on the rocky earth has observed the moon,” she said. “So it’s not surprising that when humans first stepped on the moon it was viewed as one of the greatest achievements, if not the greatest achievement, ever accomplished by humanity, at least from a technological standpoint.”

Zuber, E.A. Griswold Professor of Geophysics at MIT, focused on her work on planetary phenomena in the moon’s interior and lunar surface modeling at the talk, titled “Journey to the Center of the Moon.”

Zuber, who is also the department head of Earth, Atmospheric, and Planetary Sciences, is the first female department head in the history of MIT and also the first woman to be selected by NASA to head a major planetary spacecraft mission.

Zuber related her experiences with lunar geophysics to the NASA Gravity Recovery And Interior Laboratory (GRAIL) mission, which launched for the moon in September 2011.

In particular, Zuber explained the details of how to go from research projects in the lab to an applied astronomical endeavor.

Zuber explained to the audience, which ranged from curious undergraduates to astrophysics concentrators and professors, that the moon’s surface consists of two types of rock. One type, anorthosite, or white rock, represents the areas of the moon’s surface that were once covered by the lunar magma ocean.

The other type is basalt, or black rock, whose formation is indicative of lava flows from volcanic melting in the moon’s distant past. Zuber mentioned that the discovery of the once-present magma ocean is considered one of the greatest fruits of the Apollo Mission.

Early in the Earth’s history, Zuber said, it was hit by a Mars-sized object that struck at a sharp angle, tearing off a piece of the Earth’s mantle and sending it into the atmosphere.

Over time, the piece of the mantle folded into a disk and then condensed into the modern-day moon.

The moon also has a rich history of “impact events” or being hit by giant rocks, as indicated by the numerous craters scattered across its surface.

The biggest crater, or impact basin, is on the side of the moon not visible from Earth. This impact basin is two-thirds the width of the United States and is 8 km deep.

MIT geophysicist Dr. Maria T. Zuber, principal investigator for the GRAIL mission, shares the stage with Apollo 11 lunar module pilot Buzz Aldrin, Steve Squyres, and James T. Belllingham at the MIT+150 Symposia Earth, Air, Ocean and Space: The Future of Exploration, April 26, 2011 [Flickr].

One still-unanswered question Zuber discussed was why the near side of the moon is topologically different from the far side. One theory is that the differences are caused by internal lunar activity.

“If you’ve sent 109 space crafts that have essentially mapped the surface and brought back samples, and you haven’t found the answer, then you’re probably not looking in the right place,” she said.

This internal activity could include phase changes in matter, causing fluctuations in energy flow and distribution, as well as “moon quakes” that occur more often when the moon is closest to Earth.

In her research, Zuber works to study the moon’s gravitational field by increasing the resolution of imaging on the moon.

“The results are really transformative when you can improve measurements by three orders of magnitude or better,” said Zuber.

However, she feels her most profound contribution to society stems from her ability to engage the minds of the future. As part of her GRAIL mission, she incorporated cameras on her spacecraft specifically devoted to educating students around the country. The students use online software to monitor the location of the spacecraft in orbit and look at images.

“I want to teach kids physics is not only interesting but also fun,” said Zuber.

According to Deana Ste. Marie, Executive Assistant to the Dean of Science and an advisor to SABS, said that this year, students were able to spend time with Zuber in breakout sessions throughout the day, which proved to be “engaging and rewarding.”

Attendee and one of the event’s organizers, Alexander L. Jaffe ’15, who is also a member of SABS, enjoyed hearing about brand new topics from such a renowned expert.

“The subjects covered were some that I had no experience with before,” he said. “To hear about them, especially from a first hand research project, was fascinating.”

Solar radio burst imaging from the lunar surface

Notional field array of 30-meter (short wave) antennae deployed on the lunar surface, beyond the effects of Earth's dynamic ionosphere, which is notoriously refractive of radio waves to HF wavelengths [NLSI].

Robert MacDowall, GSFC

NASA Lunar Science Institute
NLSI Director's Seminar

The lunar surface is often identified as a prime location for acquiring radio observations at frequencies below the terrestrial ionospheric cutoff or for lunar far-side observatories that would be shielded from terrestrial radio interference. We consider a candidate observatory for solar radio burst imaging below 10 MHz. 

The Radio Observatory on the Lunar Surface for Solar studies (ROLSS) consists of 3 arms of thin polyimide film, each 500 meters in length and radiating from a central hub, providing approximately 2 degrees angular resolution at 30-meters wavelength (10 MHz). Each arm includes 16 dipole antennas consisting of metal deposited on the film and transmission lines connecting to receivers at the central hub. These arms could be unrolled using a crewed or robotic rover. 

The data collected by the antennas are processed at the central hub and down-linked to Earth for final radio image synthesis. This antenna system is uniquely suited to the low mass and low volume requirements for delivery to the lunar surface. 

In an online presentation sponsored by the NASA Lunar Science Institute, Feb. 28, Robert MacDowell of NASA Goddard reviewed the scientific goals of ROLSS and their relationships to heliophysics, hardware components ROLSS requires, the current status and work to be completed and the role of a pathfinder mission to provide mission risk reduction at modest cost.

The ROLSS concept study was funded by the NASA Lunar Sortie Science Opportunities (LSSO) program. The LUNAR consortium (Jack Burns, P.I.) is funded by the NASA Lunar Science Institute to investigate concepts for astrophysical observatories on the Moon.

Robert MacDowall has worked at NASA Goddard Space Flight Center (GSFC) since 1979, originally as an employee of Computer Sciences Corporation and other contractors, and later as a civil servant. During his years at Goddard he has worked in the fields of solar and planetary radio astronomy, in analyses of the solar wind and interplanetary magnetic fields, and plasma wave physics. He is currently the Lab Chief for the Planetary Magnetosphere Laboratory. 

MacDowell's "other recent or ongoing responsibilities include" the Ulysses Unified Radio & Plasma Wave Investigation (as Principal Investigator.), Solar Probe Plus Magnetometers (Co-PI), Interstellar Boundary Explorer (Mission Scientist), NASA Lunar Science Institute/LUNAR team (Co-I), WIND/WAVES (Co-I), STEREO/WAVES (Co-I), Cassini/RPWS (Co-I) and Radiation Belt Storm Probes Magnetometers. His interests also include space-based radio aperture synthesis imaging from space by microsatellite clusters or from the surface of the moon.

PETA protests target radiation testing

Dave Wedge
Boston Herald

While about two dozen animal rights supporters gathered outside McLean Hospital in Belmont this morning to protest controversial radiation tests on monkeys, researchers defended the testing as adhering to government guidelines.

Wearing T-shirts and holding signs, People for the Ethical Treatment of Animals members voiced their opposition to a NASA project that will zap about 20 squirrel monkeys with radiation. The primates will be hit with radiation doses equivalent to three years of space travel at a New York facility and then will be shipped to McLean to live under the watch of Harvard Medical School researchers.

PETA officials say the testing is cruel because it could lead to cancer, premature aging and cognitive damage in the monkeys. Six PETA protesters donned monkey masks and sat in cages during the peaceful protest.

Follow this story HERE.