Does the mass of the earth change over time?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

11 December 2022

Peter Mbugua Kiiru asks this: “What was the original weight of the earth! Is it measurable? Does it change over time due to construction, manufacturing or crop farming e.g. fruits? Does the population have a weight impact due to increase considering that at one time we only had Adam & Eve?”

Before attempting to answer these questions, let me first clarify that there is a difference between ‘weight’ and ‘mass’. Weight is the force of gravity acting on something and, therefore, it depends on the gravitational intensity at the place where the measurement is being taken. For example, the weight of an object on earth is greater than its weight on the moon.

Mass is the quantity of matter in the object. This does not depend on gravity. The mass of an object on earth is exactly the same as it is on the moon. I presume that Peter is asking about the mass of the Earth and not the weight.

The first question is somewhat confusing: what is the meaning of “original” mass? Is it at the time when the earth formed? My view is that this planet is incomplete work in progress; it is still undergoing formation. It hasn’t finished forming yet.

The current mass, however, of the earth is known; it is about 5,972 quintillion tonnes; that is, the number 5,972 followed by 18 zeros. Peter’s second question is whether this mass is measurable. The answer is yes. Anyone who has studied secondary school physics can measure it! It does require complicated equipment: a simple pendulum, a ruler and a stop clock is all that you need…plus knowledge of Newton’s law of universal gravitation.

Does the mass of the earth change due to construction, manufacturing, farming etc.? The answer is no. All these human activities only change the distribution of the mass – moving it from one place on the planet to another. Changes in population also don’t change the mass of the earth since people live on food taken from the very same planet.

The earth gains mass when it encounters objects from outer space. From the ground, we see them as shooting stars. It is estimated that these add 40,000 tonnes every year. But the planet also loses mass when light gasses (hydrogen and helium) evaporate from the atmosphere into outer space. The rate of loss is about 90,000 tonnes per year. Hence the net change is a loss of about 50,000 tonnes annually. Is this something to worry about?