Tuesday, January 31, 2012

Is the Earth getting lighter?

The recent crash landing of Russian spacecraft Phobos-Grunt has focused attention on the increasing amount of space junk orbiting the planet. So does this mean the Earth has been getting lighter? The BBC's Radio 4 programme More or Less turned to a group of Cambridge University academics for the answer.

There are factors that are causing Earth to both gain and lose mass over time, according to Dr Chris Smith, a medical microbiologist and broadcaster who tries to improve the public understanding of science.

Using some back-of-the-envelope-style calculations, Dr Smith, with help from physicist and Cambridge University colleague Dave Ansell, drew up a balance sheet of what's coming in, and what's going out. All figures are estimated.

By far the biggest contributor to the world's mass is the 40,000 tonnes of dust that is falling from space to Earth, says Dr Smith.

"[The dust] is basically the vestiges of the solar system that spawned us, either asteroids that broke up or things that never formed into a planet, and it's drifting around.

"The Earth is acting like a giant vacuum cleaner powered by gravity in space, pulling in particles of dust," says Dr Smith.

Another much less significant reason the planet is gaining mass is because of global warming.

"Nasa has calculated that the Earth is gaining about 160 tonnes a year because the temperature of the Earth is going up. If we are adding energy to the system, the mass must go up," says Dr Smith, referring to Einstein's equation that energy equals mass times the speed of light.

This means that in total between 40,000 and 41,000 tonnes is being added to the mass of the planet each year.

But overall, Dr Smith has calculated that the Earth - including the sea and the atmosphere - is losing mass. He points to a handful of reasons.

For instance, the Earth's core is like a giant nuclear reactor that is gradually losing energy over time, and that loss in energy translates into a loss of mass.

But this is a tiny amount - he estimates no more than 16 tonnes a year.

And what about launching rockets and satellites into space, like Phobos-Grunt? Dr Smith discounts this as most of it will fall back down to Earth again.

But there is something else that is making the planet lose mass. Gases such as hydrogen are so light, they are escaping from the atmosphere.

"Physicists have shown that the Earth is losing about three kilograms of hydrogen gas every second. It's about 95,000 tonnes of hydrogen that the planet is losing every year.

"The other very light gas this is happening to is helium and there is much less of that around, so it's about 1,600 tonnes a year of helium that we lose."

So taking into account the gains and the losses, Dr Smith reckons the Earth is getting about 50,000 tonnes lighter a year, which is just less than half the weight of the Costa Concordia, the Italian cruise liner, that ran aground recently.