Deuterium (or hydrogen-2, symbol 2H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one neutron, whereas the far more common protium has no neutrons in the nucleus. Deuterium has a natural abundance in Earth's oceans of about one atom in 6420 of hydrogen. Thus deuterium accounts for approximately 0.0156% (0.0312% by mass) of all the naturally occurring hydrogen in the oceans, while protium accounts for more than 99.98%. The abundance of deuterium changes slightly from one kind of natural water to another (see Vienna Standard Mean Ocean Water).
The name deuterium is derived from the Greek deuteros, meaning "second", to denote the two particles composing the nucleus. Deuterium was discovered and named in 1931 by Harold Urey. When the neutron was discovered in 1932, this made the nuclear structure of deuterium obvious, and Urey won the Nobel Prize in 1934 "for his discovery of heavy hydrogen". Soon after deuterium's discovery, Urey and others produced samples of "heavy water" in which the deuterium content had been highly concentrated.
Deuterium is destroyed in the interiors of the stars faster than it is produced. Other natural processes are thought to produce only an insignificant amount of deuterium. Nearly all deuterium found in nature was produced in the Big Bang 13.8 billion years ago, as the basic or primordial ratio of hydrogen-1 to deuterium (about 26 atoms of deuterium per million hydrogen atoms) has its origin from that time. This is the ratio found in the gas giant planets, such as Jupiter. The analysis of deuterium–protium ratios in comets found results very similar to the mean ratio in Earth's oceans (156 atoms of deuterium per million hydrogen atoms). This reinforces theories that much of Earth's ocean water is of cometary origin. The deuterium–protium ratio of the comet 67P/Churyumov-Gerasimenko, as measured by the Rosetta space probe, is about three times that of Earth water. This figure is the highest yet measured in a comet.Deuterium–protium ratios thus continue to be an active topic of research in both astronomy and climatology.