## Mar 16 The Mass of the Earth is Changing

It is an interesting question to think about: using back of the envelope calculations can you estimate whether the mass of the Earth is going up or down?

Anything which involves turning matter into other matter should keep the mass of the Earth the same. When you were born the atoms that made you up came from food which was already within the atmosphere and when you die your atoms will remain on the planet. However there are some things that do change the mass. Let's start by looking at the processes that the Earth uses to get heavier.

There is a lot of stuff floating out in space. As a rule of thumb there are about four atoms of stuff in every cubic metre even in the most sparse parts of the vacuum, but near planets there is a lot more in the form of dust. In total we gain about 40,000 tonnes of mass a year from space debris which is almost entirely dust. Bigger meteorites make up very little of this.

There is a small other contributor to the mass gain which comes from the fact that the Earth is heating up. Energy and mass are essentially the same thing in different guises, so if the whole system is gaining energy then its mass must also increase. However this only amounts to about 160 tonnes a year.

Let's try to run the numbers on the mass which is lost each year instead. It is tempting to think that all of the satellites and rockets that we send out would make a difference. However, almost all of them eventually come back into the atmosphere. Satellites are retired including the big ones like the ISS. The total mass of rockets that do permanently leave the system is negligible compared with the other numbers we are running in this article.

The big one is the amount of hydrogen and helium that we lose each year. As these light gases rise to the top of the atmosphere they get ripped off an escape into the abyss. This loses about 90,000 tonnes a year. There's plenty of hydrogen left because it forms bonds with pretty much everything, but Helium loss is a real problem. Notice this loss outstrips the gains from space dust by quite a way. However it isn't quite the only loser that makes a difference.

The core of the Earth acts like a mini star where the intense pressure and temperature cause some nuclear reactions which keep it hot. This leads to a net loss of about 16 tonnes of mass a year.

So overall we are losing about 50,000 tonnes of mass a year. However compared with the mass of the Earth this only works out at 0.000000000000001% so none of these effects lead to much change.