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Last Word is Â鶹´«Ã½â€™s long-running series in which readers give scientific answers to each other’s questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com
Does Earth’s mass remain constant, or is it slowly gaining or losing any?
Mike Follows
Sutton Coldfield, West Midlands, UK
Between 50,000 and 100,000 tonnes of light gases such as hydrogen and helium escape from Earth’s upper atmosphere each year. This is partly offset by the annual gain of approximately 40,000 to 50,000 tonnes – mostly in the form of meteorites and cosmic dust. The result is a net decline of up to 60,000 tonnes annually, which seems like a lot.
However, over the time since Earth was created about 4.54 billion years ago, this adds up to a total loss of only about 45 parts per billion of its mass. If stated in terms of Earth’s human population of roughly 8 billion, this is equivalent to about 360 people – just enough to fill two Boeing 737-800 aircraft, or roughly equal to the increase in the global population that takes place in just under 3 minutes.
Of course, this is the situation after Earth was struck by Theia, a Mars-sized object, shortly after it was formed. The collision led to the creation of the moon and, though computer simulations of the event differ, probably resulted in a roughly 10 per cent increase in the mass of Earth.
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Earth’s mass goes down by about 65 tonnes a year as a result of sending out spacecraft that never return
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Alex McDowell
London, UK
Overall, Earth is losing mass at around 50,000 tonnes a year – about 8.4 × 10-18 per cent of its total.
Around 100,000 tonnes of our atmosphere escape into space – mainly hydrogen and helium.
A gas at a given temperature contains molecules that travel at a wide range of speeds. Some of the fastest exceed the velocity needed to escape Earth’s gravity, which is 11 kilometres per second. Lighter gases travel faster and thus have a higher percentage of molecules with speeds greater than this velocity.
In the upper atmosphere, ultraviolet light can break up water molecules, producing hydrogen, some of which escapes from our atmosphere. Helium is constantly produced inside Earth through radioactive decay and can also escape into space.
We only lose about 65 tonnes of mass a year as a result of sending spacecraft on escape trajectories. Orbiting satellites will eventually fall back to Earth. Earth gains about 40,000 tonnes a year by accreting meteoric dust and debris from space.
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