The Moon Has Disappeared. Now What?

Pixie

It’s the Mid-Autumn Festival, and the whole family has gathered to play with lanterns and gaze at the beautiful full moon. Suddenly, the Moon disappears. What happens to our family and the Earth?

The Moon is a 3,475 kilometer-wide ball of rock. How could it disappear? Let’s take a look at a few different ideas, and what the effects of each would be.

One fundamental law of physics is that matter cannot be created or destroyed, so the moon could not just cease to exist. However, matter can be converted into energy, as described by Einstein’s famous equation E=mc². Suppose the Moon was suddenly converted into energy, in the form of radiation: radio waves, microwaves, light, x-rays and cosmic rays. That would be 6.57 x 10³⁹ Joules, which is about the same as the Sun’s total energy output for thirty five and a half years. Everything on the side of the Earth facing the Moon would be instantly vaporised, and a shockwave of hot gasses would cook everything on the far side. The Earth would be sterilised.

Second, suppose something just blew up the moon. Star Wars showed the Death Star, a space station with a powerful weapon, annihilate an entire planet. If the Death Star was real and targeted our moon, what would we see? As the Death Star approached, astronomers would notice and there would be emergency news bulletins talking frenetically about the alien spacecraft and first contact with extraterrestrial intelligence. Depending on the firing range and line of approach, the space station would probably be visible in the night sky. Then there would be a bright flash, followed by the explosion, with a cloud of twinkling debris.

In the film, the debris spreads to three times the diameter of the planet in a second, so, if the planet was Earth-sized, the debris was travelling very fast. The debris would cross the distance from the Earth to the Moon in eleven seconds. Less than 0.01 per cent of the debris would hit the Earth, but that would still be five thousand trillion tonnes of rock. That is eleven times the estimated mass of the Chicxulub Impactor, the meteorite that is thought to have killed the dinosaurs.

Unfortunately, it is far worse than that because the estimated speed is 12 per cent of the speed of light, far faster than the Chicxulub Impactor was travelling. Taking into account the speed, the debris hitting the Earth would have the energy of 56 million Chicxulub Impactors. This is only 1 per cent of the energy hitting the Earth in our first scenario, but still, everything on the side of the Earth facing the Moon would be obliterated, seas would boil, and a devastating shock-wave would raze the opposite side.

Anything that survived the scalding steam and winds that would make a typhoon look like a calm day would face an ice-age as dust in the atmosphere would reflect sunlight. The cold years that followed would be punctuated by occasional Chicxulub Impactor-sized meteorite that took a different orbit. Life might survive. Life is pretty resilient, so some bacteria can grow in hot springs and other extreme conditions, but birds and mammals would be extinct. Humanity would be no more.

Alternatively, suppose the Moon shrank and became a black hole. This is a much friendlier scenario. A black hole with the mass of our Moon would be about the size of a sand grain. The Moon would no longer be reflecting sunlight, and our Mid-Autumn Festival family would be surprised by its sudden absence, but not much else would change. The black hole moon would still have the same mass and be moving in the same orbit, so its gravitational effects would be the same. We would still have tides of the same size in a 28 day cycle.

However, there would be effects on Earth’s life from the missing moonlight. It might actually be a benefit to night-migrating birds, because they are thought to use stars for navigation and bright moonlight confuses them. Some insects seem to navigate by moonlight, so they would suffer. Turtles synchronise their breeding with the tides, using the spring high tide at full moon to get high up the beach to lay their eggs. The eggs hatch on another spring high tide, so the young turtles have the shortest distance to crawl to the sea.

What would happen without moonlight? It seems unlikely that turtles could directly feel the tiny decrease in gravity at a full moon, so it is probably moonlight that keeps their breeding cycle synchronised with the tide. Without moonlight, the cycle would gradually drift and the breeding success of turtles would suffer. Many other activities of different species are synchronised to the moon’s cycle, including coral breeding and Arctic zooplankton migrations. The results in the months and years following the Moon becoming a black hole would be complex. Some species would decline, or even become extinct, others would increase, from direct effects or because they benefit from another species’ decline.

Try to think about other ways the Moon might disappear, and what the consequences would be. One thing is certain: Mid-Autumn Festival would never be the same again.