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, with a relative uncertainty of 10⁻⁴. 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, so that the mass of the Earth–Moon system is close to 6.0457×10²⁴ kg.

Most of the mass is accounted for by iron and oxygen (c. 32% each), magnesium and silicon (c. 15% each), calcium, aluminium and nickel (c. 1.5% each). Precise measurement of the Earth mass is difficult, as it is equivalent to measuring the gravitational constant, which is the fundamental physical constant known with least accuracy, due to the relative weakness of the gravitational force. The mass of the Earth was first measured with any accuracy (within about 20% of the correct value) 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, using 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, with only a tiny bit of uncertainty.

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, which is a very small fraction. 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, and its density changes depending on where you look. The Earth's core, which is right in the middle, makes up only 15% of the Earth's space but holds more than 30% of its 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 makes up a big part of the core, and silicon and oxygen, which are 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 studying things like gravity and density. Early attempts in the 1700s gave answers that were not quite right, but by 1798, an experiment by Henry Cavendish got very close to the true value. Since then, scientists have kept improving their measurements.

We now know the mass of Earth with great precision, thanks to careful experiments and modern tools. The uncertainty in these measurements comes mainly from how hard it is to measure a key number called the gravitational constant exactly.

Variation

Main article: Atmospheric escape

Earth's mass changes 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 like the Chicxulub impact 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.