Moon is 40 million years older than previously thought

Newswand: By analyzing tiny lunar crystals gathered by Apollo 17 astronauts in 1972, researchers recalculated the age of the Earth’s Moon. Although previous assessments estimated the Moon as 4.425 billion years old, the new study discovered it is actually 4.46 billion years old — 40 million years older than previously thought.

A lunar zircon grain under a microscope.
Credit: Jennika Greer

Led by researchers at the Field Museum and the University of Glasgow, the study was made possible by Northwestern University’s atom-probe tomography facility, which “nailed down” the age of the oldest crystal in the sample. By revealing the age of these telltale zircon crystals — found hidden within dust collected from the Moon — researchers were able to piece together the timeline of the Moon’s formation.

The study was published on Oct. 23 in the journal Geochemical Perspectives Letters.

“This study is a testament to immense technological progress we have made since 1972 when the last manned Moon mission returned to Earth,” said Northwestern’s Dieter Isheim, who co-authored the study. “These samples were brought to Earth half-a-century ago, but only today do we have the necessary tools to perform microanalysis at the requisite level, including atom-probe tomography.”

The atom-by-atom analysis enabled researchers to count how many atoms in the zircon crystals have undergone radioactive decay. When an atom undergoes decay, it sheds protons and neutrons to transform into different elements. Uranium, for example, decays into lead. Because scientists have established how long it takes for this process to unfold, they can assess the age of a sample by looking at the proportion of uranium and lead atoms.

“Radiometric dating works a little bit like an hourglass,” said the Field Museum’s Philipp Heck, the study’s senior author. “In an hourglass, sand flows from one glass bulb to another, with the passage of time indicated by the accumulation of sand in the lower bulb. Radiometric dating works similarly by counting the number of parent atoms and the number of daughter atoms they have transformed to. The passage of time can then be calculated because the transformation rate is known.”

More than 4 billion years ago, when the solar system was still young and the Earth was still growing, a giant Mars-sized object crashed into the Earth. A colossal hunk broke off Earth to form the Moon, and the energy of the impact melted the rock that eventually became the Moon’s surface.

“When the surface was molten like that, zircon crystals couldn’t form and survive,” Heck said. “So, any crystals on the Moon’s surface must have formed after this lunar magma ocean cooled. Otherwise, they would have been melted and their chemical signatures would be erased.”

Because the crystals must have formed after the magma ocean cooled, determining the age of the zircon crystals would reveal the minimum possible age of the Moon. But, to pinpoint the maximum possible age of the Moon, researchers turned to Northwestern’s atom-probe tomography instruments.

In atom-probe tomography, we start by sharpening a piece of the lunar sample into a very sharp tip, using a focused ion beam microscope, almost like a very fancy pencil sharpener, the researchers said. Then, they use UV lasers to evaporate atoms from the surface of that tip. The atoms travel through a mass spectrometer, and how fast they move tells how heavy they are, which in turn tells what they’re made of.

After determining the materials in the sample and performing radiometric dating, the researchers concluded that the oldest crystals are about 4.46 billion years old. That means the Moon must be at least that old.

It’s important to know when the Moon formed, a researcher said, because “The Moon is an important partner in our planetary system. It stabilizes the Earth’s rotational axis. It’s the reason there are 24 hours in a day. It’s the reason we have tides. Without the Moon, life on Earth would look different. It’s a part of natural system that we want to better understand, and our study provides a tiny puzzle piece in that whole picture.”

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Lucy to meet asteroid Dinkinesh on November 1

Newswand: NASA’s Lucy spacecraft is about 25 million kilometers away from asteroid Dinkinesh and it may reach the asteroid in about 14 days.

Since NASA’s Lucy spacecraft first imaged the asteroid Dinkinesh on Sept. 3, 2023, Lucy has traveled over 33 million miles (54 million km) and is now 4.7 million miles (7.6 million km) away from the small asteroid. However, as Dinkinesh continues on its orbit around the Sun, Lucy still has another almost 16 million miles (25 million km) to travel to its meet-up with the asteroid on Nov. 1.

Over the last month, the spacecraft team has seen the target asteroid generally brightening as Lucy approaches it and has also seen a subtle brightness variation consistent with the previously observed 52.7-hour rotation period.

Since Lucy first observed the asteroid on Sept. 3, the team has used images collected by the spacecraft’s high-resolution camera, L’LORRI, to refine their knowledge of the relative positions of the spacecraft and asteroid, optically navigating Lucy towards the encounter.

Using this information, on Sept. 29 the spacecraft carried out a small trajectory correction maneuver, changing the spacecraft’s speed by just 6 cm/s (around 0.1 mph). This nudge is predicted to send the spacecraft on a path that will pass within 265 miles (425 km) of the asteroid. In late October the team will have another opportunity to adjust the trajectory if necessary.            

On Oct. 6, the spacecraft passed behind the Sun as viewed from Earth, beginning a planned communications blackout. The spacecraft has continued to image the asteroid and will return these images to Earth once communications resume after the end of the solar conjunction period in mid-October.

Lucy’s principal investigator, Hal Levison, is based out of the Boulder, Colorado, branch of Southwest Research Institute, headquartered in San Antonio, Texas. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, provides overall mission management, systems engineering, and safety and mission assurance. Lockheed Martin Space in Littleton, Colorado, built the spacecraft. Lucy is the 13th mission in NASA’s Discovery Program. NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Discovery Program for the Science Mission Directorate at NASA Headquarters in Washington.

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Hunt for ninth planet

Newswand: Theoretical physicists are reporting that the some observations inspiring the hunt for a ninth planet might instead be evidence within the solar system of a modified law of gravity originally developed to understand the rotation of galaxies.

Researchers Harsh Mathur,  professor of physics at Case Western Reserve University, and Katherine Brown, an associate professor of physics at Hamilton College, made the assertion after studying the effect, that the Milky Way galaxy would have an object in the outer solar system—if the laws of gravity were governed by a theory known as Modified Newtonian Dynamics ( MOND).

MOND proposes that Isaac Newton’s famous law of gravity is valid up to a point. That is, when the gravitational acceleration predicted by Newton’s law becomes small enough, MOND allows for a different gravitational behavior to take over.

 “MOND is really good at explaining galactic-scale observations,” Mathur said, “but I hadn’t expected that it would have noticeable effects on the outer solar system.”

A ‘striking’ alignment

Mathur and Brown had studied MOND’s effect on galactic dynamics before. But they became interested in MOND’s more local effects after astronomers announced in 2016 that a handful of objects in the outer solar system showed orbital anomalies that could be explained by a ninth planet.

Orbital peculiarities have led to historic discoveries before: Neptune was discovered through its gravitational tug on the orbits of nearby object, the minute precession of Mercury provided early evidence in support of Einstein’s theory of general relativity, and astronomers have recently used orbital dynamics to infer the presence of a super massive black hole at the center of our Galaxy.

Brown realized MOND’s predictions might be at odds with the observations that had motivated the search for a ninth planet. “We wanted to see if the data that support the Planet Nine hypothesis would effectively rule out MOND,” she said.

Instead, Mathur and Brown found MOND predicts precisely clustering that astronomers have observed. Over millions of years, they argue, the orbits of some objects in the outer solar system would be dragged into alignment with the galaxy’s own gravitational field.

When they plotted the orbits of the objects from the Planet Nine dataset against the galaxy’s own gravitational field, “the alignment was striking,” Mathur said.

The authors caution that the current dataset is small and that that any number of other possibilities might prove to be correct; other astronomers have argued the orbital peculiarities are the result of observational bias.

“Regardless of the outcome,” Brown said, “this work highlights the potential for the outer solar system to serve as a laboratory for testing gravity and studying fundamental problems of physics.”

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