The University of Colorado’s physics department has achieved yet another landmark discovery in geophysical studies: the timescale in which the moon initially separated from Earth and receded to its position as we know it in the sky today.
The study was a collaboration between University of Colorado physics Professor Shijie Zhong and former Boulder researchers Chuan Qin and Roger Phillips. The research was funded by NASA and the National Science Foundation, according to a news release.
“I was trying to understand how the moon’s fossil bulge came to be formed,” Zhong said. “We know the observation well, for more than 200 years.”
Most studies in the past pointed out observations and not the facts, Zhong said. He and his team wanted to create a dynamic, physical model to really understand the moon’s ancient characteristics, he said.
“It’s basically applying the laws of physics to describe how the moon moves away from the Earth, how the forcing changed,” he said. “This is really due to the rotational force, how that varies with time, and how the moon itself actually responded to the changing forces.”
The team discovered that the moon’s recession from Earth was not a rapid process. Instead, the moon slowly pulled away over a span of 400 million years between 3.8 billion to 4.5 billion years ago, the study found.
“For the first time, we can reproduce the observation,” Zhong said.
As the moon’s detachment from Earth was underway, the moon began to physically morph due to the strength of the external forces acting upon it, either gravitational or tidal, the study found. It stretched slightly at its poles, which created a permanent bulge in its crust — the feature known as the moon’s fossil bulge, according to the release.
Yet the bulge is nearly 20 times too large for its one-revolution-per-month rotational rate, Zhong said. This is compared to Earth’s standard model, which calculates the size of the equator relative to Earth’s rotational rate, he said.
“Even in the old days, we didn’t know. We knew it was way too large compared to the theoretical prediction,” he said.
Many of the challenges Zhong and his team encountered during their experimentation were mathematical, but his partners were able to break down most of those barriers, he said. Qin is a graduate of the University of Colorado who is now engaged in postdoctoral research at Harvard University, and Phillips studied at the Southwest Research Institute in Boulder, he said.
The research spanned over three years. The study, named “Formation of the Lunar Fossil Bulges and Its Implication for the Early Earth and Moon,” was published on Friday by Geophysical Research Letters, a journal of the American Geophysical Union, according to the release.
Despite the study’s breakthroughs, the exact geophysical conditions needed to produce the moon’s recession 4 billion years ago are still unclear.
“That environment could be very different from today,” Zhong said.
The tidal dissipation on Earth must have been extremely small, if existent, at the time the moon began to recede from Earth to produce the fossil bulge, and the sun could have radiated nearly 30 percent less heat to Earth 4 billion years ago, he said.
“We’re still working on the problem. Hopefully, we’ll continue to work on this to figure it out,” Zhong said.
The moon currently retreats from Earth at a rate of 4 centimeters a year, as observed by lunar-laser operations from the Apollo missions, Zhong said. Yet the focus of the study was on the conditions of the past, and less so the implications for the future, he said.
“We know some things are missing, the moon receded much slower in the past,” Zhong said. “The lunar fossil bulge might actually record something here, how quickly the moon migrated out in early times.”