The Moon is shrinking, by about 50m (164ft) in radius over the last several hundred million years. Scientists discovered this by analysing images of thrust faults (known as scarps) on the lunar surface.
The images were taken by Apollo astronauts and, more recently, NASA’s Lunar Reconnaissance Orbiter. Researchers found that the epicentres of some shallow moonquakes, detected by seismometers left on the Moon during the Apollo era, seem to be associated with these thrust faults.
They also found that the Moon has an inner core that’s about 500km (310 miles) in diameter and partially molten, but much less dense than Earth’s core.
The Moon is tectonically active because its interior is still cooling and contracting. The Moon’s crust is very brittle, so as the interior shrinks, the crust breaks apart, resulting in scarps as portions of crust are pushed over others.
At least some lunar scarps are cracks and wrinkles formed by that slow contraction. All the evidence suggests this process is still going on today, although the stresses on the Moon due to Earth’s gravity (tidal forces) have a greater effect on the occurrence of moonquakes.
Are there any consequences for Earth (or humans) from the shrinking Moon? Not really.
This rate of shrinking is almost imperceptible, being equivalent to a change in the Moon’s radius by about one quintillionth (10-18) of a per cent per year.
So, the Moon’s apparent size in the sky won’t change noticeably due to lunar shrinking over Earth’s expected lifetime. Also, the Moon isn’t shrinking due to loss of mass, so the gravitational force between Earth and the Moon will remain the same.
This doesn’t mean that the evolution of the Moon won’t affect Earth in the future, though. Due to tidal forces between Earth and the Moon, the size of the Moon’s orbit is increasing by about 3.8cm (1.5in) per year.
As it recedes from us, the Moon’s orbital period increases and Earth’s rotation slows down. Every century, that process adds about 2.3 milliseconds to the length of a day on Earth.
But, again, this effect is almost imperceptible. The change in the Moon’s apparent size due to its recession from us is vanishingly small and is completely dwarfed by the monthly change in size due to its elliptical orbit around Earth.
A receding Moon will eventually have severe consequences for Earth, however. Ocean currents would be disrupted, meaning many aquatic species would probably cease to exist.
And Earth itself would become unstable and begin to wobble, disrupting the planet’s seasons and causing devastating variations in climate. It’s not something you need to worry about right now, however. This won’t happen until many billions of years from now.
That said, the fact that the Moon is tectonically active will have consequences for human exploration. Shallow moonquakes can cause strong seismic shaking, so the location of thrust faults may determine the selection of sites for long-term lunar bases.
This article is an answer to the question (asked by Katrina Sherman, via email) 'Is the Moon shrinking?'
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