The accepted value for Earth’s total mass is approximately 5.9722 x 10^24 kilograms, a figure so immense it is almost impossible to imagine. This number represents the amount of matter that makes up the entire planet, from its atmosphere to its deepest core. Determining this mass speaks to the power of physics, as the planet cannot be placed on a physical scale. However, the phrasing “how much does the world weigh” is technically imprecise, requiring a clarification of the distinction between mass and weight.
Mass vs. Weight: Clarifying the Terminology
Mass is an intrinsic property of an object that measures the amount of matter it contains and its resistance to changes in motion, known as inertia. This property remains constant regardless of where the object is located in the universe. Mass is measured in kilograms (kg), which is the standard unit used for the Earth’s total bulk.
Weight, conversely, is a measure of the force of gravity acting on an object’s mass. It depends on both the object’s mass and the strength of the gravitational field it is in. Because weight is a force, it is measured in Newtons (N). Since the Earth is not resting on a larger object, it technically has no “weight” in the traditional sense, which is why scientists focus on calculating its mass.
The Gravitational Constant: How Earth’s Mass is Calculated
Since it is impossible to place the Earth on a scale, its mass must be calculated indirectly using the laws of gravitation. This calculation relies on Isaac Newton’s Law of Universal Gravitation, which describes the attractive force between any two objects with mass. The formula includes a proportionality factor known as the universal gravitational constant, or “Big G.”
The value of G was first determined accurately by Henry Cavendish in 1798 using a torsion balance apparatus. His experiment measured the tiny gravitational force between known masses in a laboratory setting. By measuring the attraction between lead spheres, Cavendish was able to calculate G.
Once G was known, scientists could determine Earth’s mass (M) by rearranging the gravitational equation. The calculation uses the known acceleration due to gravity (g), which is about 9.8 m/s^2 at the surface, and the Earth’s radius (R). The relationship g = GM/R^2 allows the total mass of the planet to be inferred from the gravitational pull experienced on the surface. The accuracy of the Earth’s mass value depends directly on the accuracy of the gravitational constant, which is difficult to measure precisely.
Compositional Layers: What Accounts for the Mass
The enormous mass of the Earth is accounted for by the planet’s composition and its deep internal structure, which features a dramatic increase in density toward the center. The Earth is divided into three main compositional layers: the crust, the mantle, and the core. The outermost layer, the crust, is the thinnest and least dense, making up less than one percent of the Earth’s total mass.
The mantle lies beneath the crust and is a dense layer of silicate rocks rich in iron and magnesium. It accounts for approximately 67 percent of the planet’s mass and 83 percent of its volume.
The core, composed primarily of a dense iron-nickel alloy, contributes about 31 percent of the Earth’s total mass. This central region is split into a liquid outer core and a solid inner core, where the density reaches its maximum. Although the core makes up only about 15 percent of the Earth’s volume, its extreme density, reaching up to 13,000 kg/m^3 in the inner core, validates the high total mass calculated from gravitational measurements.
Is Earth’s Mass Constant?
Although the Earth’s mass is immense, it is not perfectly constant and is subject to continuous, tiny change over time. The planet experiences both a gain and a loss of material, creating a dynamic mass balance.
Mass is gained primarily from the constant accretion of cosmic debris, including micrometeorites and cosmic dust, which fall onto the planet. Scientists estimate this infalling material adds tens of thousands of tonnes to the Earth annually.
A simultaneous process of mass loss occurs due to atmospheric escape, where very light gases, mainly hydrogen and helium, leak into space. Earth’s gravity is not strong enough to permanently hold these lightest elements, resulting in a loss greater than the mass gained from space dust. The net effect is an estimated mass reduction of approximately 50,000 tonnes per year. This change is negligible compared to the total mass of the Earth, meaning the planet’s mass is effectively constant for all practical purposes.