KATRINA SHERMAN, VIA EMAIL IS THE MOON SHRINKING?
Email your questions to questicns@sciencefocus.com or submit on X (formerly Twitter) at @sciencefocus
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 diameterand 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 lunarover 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.
