Earth mass

An Earth mass (denoted as M_🜨, M_♁ or M_E, where 🜨 and ♁ are the astronomical symbols for Earth) is a unit of mass equal to the mass of the planet Earth. The current best estimate for the mass of Earth is M_🜨 = 5.9722×10²⁴ kg. It is equivalent to an average density of 5515 kg/m³. Using the nearest metric prefix, the Earth mass is approximately six ronnagrams, or 6.0 Rg.

The Earth mass is a standard unit of mass in astronomy that is used to indicate the masses of other planets, including rocky terrestrial planets and exoplanets. One Solar mass is close to 333000 Earth masses. The Earth mass excludes the mass of the Moon. The mass of the Moon is about 1.2% of that of the Earth.

Most of the mass is accounted for by iron and oxygen, magnesium and silicon, calcium, aluminium and nickel. Precise measurement of the Earth mass is difficult. The mass of the Earth was first measured with any accuracy in the Schiehallion experiment in the 1770s, and within 1% of the modern value in the Cavendish experiment of 1798.

Unit of mass in astronomy

Further information: Solar mass, Standard gravitational parameter, and Gaussian gravitational constant

The Earth mass is a way scientists measure big amounts of matter. They use the total amount of matter that makes up our planet Earth. The best guess for Earth’s mass is about 5.9722 × 10²⁴ kilograms. This number is very close to the true value.

Scientists sometimes compare Earth’s mass to the mass of the Sun. Earth’s mass is about 1/332,946 of the Sun’s mass. They also know how Earth’s mass compares to the Moon’s mass — Earth is about 81.3 times more massive than the Moon. These comparisons help astronomers study planets and other objects in space more easily.

Composition

Further information: Internal structure of Earth and Abundance of elements in Earth's crust

The Earth is made of many different materials. Its density changes depending on where you look. The Earth's core, right in the middle, is small but holds a lot of the Earth's mass. Most of this core is made of iron and nickel. Around the core is the mantle, which takes up most of the Earth's space and mass. The outer layer, called the crust, is very thin and makes up less than 1% of the Earth's mass.

Important elements in the Earth include iron, which is a big part of the core, and silicon and oxygen, found in the mantle and crust. There are also smaller amounts of other elements like magnesium, aluminum, and calcium, as well as tiny amounts of carbon, water, and gases in the atmosphere.

History of measurement

Main article: Gravitational constant § History of measurement

Scientists measure the mass of Earth by looking at gravity and how thick Earth is. Long ago, in the 1700s, their answers were not perfect. But in 1798, an experiment by Henry Cavendish came very close to the right value. Since then, scientists have kept making their measurements better.

Today, we know the mass of Earth very well, thanks to careful work and new tools. The biggest challenge is measuring a special number called the gravitational constant exactly.

Variation

Main article: Atmospheric escape

Earth's mass changes only a little over time. It loses some weight from gases floating away into space and gains a bit from tiny pieces of space dust and meteorites falling onto it. Each year, Earth loses about 100,000 tons of gas but gains around 45,000 tons of space material. These changes are very small compared to Earth's whole mass, so they don’t affect our overall estimate much.

Most of the loss is from hydrogen and helium gas escaping into space. The gain comes mostly from cosmic dust, meteors, and other space material. Sometimes, really big space events can add a lot more mass all at once. Other tiny changes happen because of natural processes inside Earth and because of spacecraft leaving our planet, but these are also very small.