If an object hadn鈥檛 crashed into Earth to form the moon, as is hypothesised, Earth would be bigger. Would it still sustain life?
Ron Dippold
San Diego, California, US
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You reference the giant impact hypothesis, the most favoured explanation for the moon鈥檚 existence and composition.
It suggests that, about 100 million years after our solar system started to form, proto-Earth got smacked by Theia, a Mars-sized planet. This knocked enough debris into orbit to form the moon. The early solar system was so crowded and chaotic, it was like the seven-lane 鈥淢agic Roundabout鈥� in Swindon, UK, if all the drivers were drunk. It is thought that early Earth had dozens of small planet impacts and Theia was the last major one, though it continued to be bombarded by tons of comets and meteorites.
But the key word here is 鈥淢ars-sized鈥�. Mars and the hypothetical Theia are a tenth of Earth鈥檚 mass. Our moon is only a hundredth of Earth鈥檚 mass. Theia is thought to have hit our world pretty square on, so the bulk of it would have sunk into Earth, which would have actually gained up to about 10 per cent mass from this impact.
Without the moon, we wouldn鈥檛 have our tides. Some think that these helped start life by repeatedly refreshing (at high tide) and concentrating (at low tide) life鈥檚 starter components in tidal zones. So maybe we wouldn鈥檛 have life in the first place without the moon. There are lots of other ideas about how life began, though.
If life would have still started on Earth without the moon, a mere 10 per cent increase or decrease in mass would be entirely survivable 鈥� though dieters might either be distraught or ecstatic.
Mike Follows
Sutton Coldfield, West Midlands, UK
There are several conditions that make life possible on Earth and most or all were present before the hypothetical impact that created the moon. It is possible life would have flourished even without it.
Liquid water is required for the biochemical reactions essential to life as we know it. Luckily, our planet orbits at a distance from the sun that is inside the habitable (or 鈥�Goldilocks鈥�) zone, where Earth鈥檚 global mean surface temperature allows liquid water to collect on the surface. The planet鈥檚 magnetic field protects our atmosphere by deflecting solar wind; the depletion of Mars鈥檚 atmosphere can be traced back to the collapse of its magnetic field. Earth is massive and dense enough to produce a sufficiently strong gravitational field to prevent the escape of our atmosphere, but has this always been the case?
According to the giant impact hypothesis, a planet called Theia collided with proto-Earth and, out of the wreckage, we ended up with our moon orbiting a more massive Earth. The mass of Theia has been proposed at between 10 and 45 per cent of the current mass of Earth. This implies that, before the collision, Earth had a mass of between 56 and 91 per cent of what it is today. Assuming Earth鈥檚 density hasn鈥檛 changed, the gravitational field strength on the planet鈥檚 surface would have been as low as 82 per cent of its current value. Earth鈥檚 escape velocity may have been as low as 9.2 kilometres per second (compared with our current 11km/s).
The exosphere is where our atmosphere blurs with outer space. The bottom of the exosphere is called the exobase and its height varies between 500 and 1000 km above Earth鈥檚 surface. In the atmosphere, gas particles constantly collide with each other. If they are moving away from Earth鈥檚 surface with the required escape velocity when they reach the exobase, they will never again collide with another terrestrial gas particle and will be lost forever from Earth鈥檚 atmosphere. This is a bit like evaporation from the surface of a liquid.
Due to heating by ultraviolet radiation from the sun, the temperature at the exobase is about 1000 kelvin. At this temperature, hydrogen atoms have an average speed of about 5 km/s (below Earth鈥檚 escape velocity of 10.8 km/s at the exobase). But there are enough fast atoms that 3 kilograms of hydrogen escapes the atmosphere every second, which is insignificant. A significant loss of hydrogen risks drying out the planet.
This is known as Jean鈥檚 escape. A less massive Earth would mean a smaller escape velocity and potentially more Jean鈥檚 escape. However, according to the faint young sun paradox, when Earth was formed, the luminosity of the sun would have been about 30 per cent less than it is today, so the gas particles in the atmosphere would have been travelling slower. Regardless, even a thinner atmosphere would probably have been sufficient to sustain life.
The luminosity of the sun is increasing by 10 per cent every billion years. As Earth warms, the stratosphere will get wetter, increasing the diffusion of water vapour upwards towards the exobase. Geoscientist James Kasting predicted Earth will have lost most of its oceans in 2 billion years. The bigger the mass we gained from the Theia impact, if it occurred, the longer this nightmare scenario is postponed.
The existence of the moon is the wild card. The tides produced by the moon lead to the alternate wetting and drying of the intertidal zone, which may have been crucial for the origin of life, so perhaps the impact was vital after all.
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