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A set of base units in the atomic system as in one proposal are the electron rest mass, the magnitude of the electronic charge, the Planck constant, and the permittivity. [6] [9] In the atomic units system, each of these takes the value 1; the corresponding values in the International System of Units [10]: 132 are given in the table.
A given atom has an atomic mass approximately equal (within 1%) to its mass number times the atomic mass unit (for example the mass of a nitrogen-14 is roughly 14 Da), but this number will not be exactly an integer except (by definition) in the case of carbon-12. [67] The heaviest stable atom is lead-208, [59] with a mass of 207.976 6521 Da. [68]
A picture of Earth's troposphere, with different cloud types at low and high altitudes casting shadows. Sunlight, filtered into a reddish hue by passing through much of the troposphere at sunset, is reflected off the ocean. The above-lying stratosphere can be seen at the horizon as a band of its characteristic glow of blue scattered sunlight.
The geomagnetic activity levels of the Earth are measured by the fluctuation of the Earth's magnetic field in SI units called teslas (or in non-SI gauss, especially in older literature). The Earth's magnetic field is measured around the planet by many observatories. The data retrieved is processed and turned into measurement indices.
Most of Earth's surface is ocean water: 70.8% or 361 million km 2 (139 million sq mi). [97] This vast pool of salty water is often called the world ocean, [98] [99] and makes Earth with its dynamic hydrosphere a water world [100] [101] or ocean world. [102] [103] Indeed, in Earth's early history the ocean may have covered Earth completely. [104]
The Earth's crust is one "reservoir" for measurements of abundance. A reservoir is any large body to be studied as unit, like the ocean, atmosphere, mantle or crust. Different reservoirs may have different relative amounts of each element due to different chemical or mechanical processes involved in the creation of the reservoir. [1]: 18
The composition of Earth's atmosphere is determined by the by-products of the life that it sustains. Dry air (mixture of gases) from Earth's atmosphere contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and traces of hydrogen, helium, and other "noble" gases (by volume), but generally a variable amount of water vapor is ...
Near the Earth's surface, it is a pollutant formed as a by-product of automobile exhaust. [43] At low earth orbit altitudes, sufficient atomic oxygen is present to cause corrosion of spacecraft. [44] The metastable molecule tetraoxygen (O 4) was discovered in 2001, [45] [46] and was assumed to exist in one of the six phases of solid oxygen.