Query: But the new research shows that the amount of oxygen in those shells doesn’t actually remain constant over time.

Relevant passages:
Oxygen <=SEP=> Oxygen is a chemical element with symbol O and atomic number 8 . It is a member of the chalcogen group on the periodic table and is a highly reactive nonmetal and oxidizing agent that readily forms oxides with most elements as well as other compounds . By mass , oxygen is the third-most abundant element in the universe , after hydrogen and helium . At standard temperature and pressure , two atoms of the element bind to form dioxygen , a colorless and odorless diatomic gas with the formula . This is an important part of the atmosphere and diatomic oxygen gas constitutes 20.8 % of the Earth 's atmosphere . Additionally , as oxides the element makes up almost half of the Earth 's crust .   Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen , such as proteins , nucleic acids , carbohydrates , and fats , as do the major constituent inorganic compounds of animal shells , teeth , and bone . Most of the mass of living organisms is oxygen as a component of water , the major constituent of lifeforms . Conversely , oxygen is continuously replenished by photosynthesis , which uses the energy of sunlight to produce oxygen from water and carbon dioxide . Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms . Another form ( allotrope ) of oxygen , ozone , strongly absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation . But ozone is a pollutant near the surface where it is a by-product of smog . At low earth orbit altitudes , sufficient atomic oxygen is present to cause corrosion of spacecraft .   Oxygen was discovered independently by Carl Wilhelm Scheele , in Uppsala , in 1773 or earlier , and Joseph Priestley in Wiltshire , in 1774 , but Priestley is often given priority because his work was published first . The name oxygen was coined in 1777 by Antoine Lavoisier , whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion . Its name derives from the Greek roots ὀξύς oxys , `` acid '' , literally `` sharp '' , referring to the sour taste of acids and - γενής - genes , `` producer '' , literally `` begetter '' , because at the time of naming , it was mistakenly thought that all acids required oxygen in their composition .   Common use of oxygen includes residential heating , internal combustion engines , production of steel , plastics and textiles , brazing , welding and cutting of steels and other metals , rocket propellant , oxygen therapy , and life support systems in aircraft , submarines , spaceflight and diving .

Radiocarbon dating <=SEP=> Radiocarbon dating ( also referred to as carbon dating or carbon-14 dating ) is a method for determining the age of an object containing organic material by using the properties of radiocarbon , a radioactive isotope of carbon .   The method was developed by Willard Libby in the late 1940s and soon became a standard tool for archaeologists . Libby received the Nobel Prize in Chemistry for his work in 1960 . The radiocarbon dating method is based on the fact that radiocarbon is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen . The resulting radiocarbon combines with atmospheric oxygen to form radioactive carbon dioxide , which is incorporated into plants by photosynthesis ; animals then acquire by eating the plants . When the animal or plant dies , it stops exchanging carbon with its environment , and from that point onwards the amount of it contains begins to decrease as the undergoes radioactive decay . Measuring the amount of in a sample from a dead plant or animal such as a piece of wood or a fragment of bone provides information that can be used to calculate when the animal or plant died . The older a sample is , the less there is to be detected , and because the half-life of ( the period of time after which half of a given sample will have decayed ) is about 5,730 years , the oldest dates that can be reliably measured by this process date to around 50,000 years ago , although special preparation methods occasionally permit accurate analysis of older samples .   The idea behind radiocarbon dating is straightforward , but years of work were required to develop the technique to the point where accurate dates could be obtained . Research has been ongoing since the 1960s to determine what the proportion of in the atmosphere has been over the past fifty thousand years . The resulting data , in the form of a calibration curve , is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample 's calendar age . Other corrections must be made to account for the proportion of in different types of organisms ( fractionation ) , and the varying levels of throughout the biosphere ( reservoir effects ) . Additional complications come from the burning of fossil fuels such as coal and oil , and from the above-ground nuclear tests done in the 1950s and 1960s . Because the time it takes to convert biological materials to fossil fuels is substantially longer than the time it takes for its to decay below detectable levels , fossil fuels contain almost no , and as a result there was a noticeable drop in the proportion of in the atmosphere beginning in the late 19th century . Conversely , nuclear testing increased the amount of in the atmosphere , which attained a maximum in 1963 of almost twice what it had been before the testing began .   Measurement of radiocarbon was originally done by beta-counting devices , which counted the amount of beta radiation emitted by decaying atoms in a sample . More recently , accelerator mass spectrometry has become the method of choice ; it counts all the atoms in the sample and not just the few that happen to decay during the measurements ; it can therefore be used with much smaller samples ( as small as individual plant seeds ) , and gives results much more quickly . The development of radiocarbon dating has had a profound impact on archaeology . In addition to permitting more accurate dating within archaeological sites than previous methods , it allows comparison of dates of events across great distances . Histories of archaeology often refer to its impact as the `` radiocarbon revolution '' . Radiocarbon dating has allowed key transitions in prehistory to be dated , such as the end of the last ice age , and the beginning of the Neolithic and Bronze Age in different regions .

Europa (moon) <=SEP=> Europa -LSB- jʊˈroʊpə -RSB- ( Jupiter II ) , is the smallest of the four Galilean moons orbiting Jupiter , and the sixth-closest to the planet . It is also the sixth-largest moon in the Solar System . Europa was discovered in 1610 by Galileo Galilei and was named after Europa , the legendary mother of King Minos of Crete and lover of Zeus ( the Greek equivalent of the Roman god Jupiter ) .   Slightly smaller than Earth 's Moon , Europa is primarily made of silicate rock and has a water-ice crust and probably an iron -- nickel core . It has a tenuous atmosphere composed primarily of oxygen . Its surface is striated by cracks and streaks , whereas craters are relatively rare . In addition to Earth-bound telescope observations , Europa has been examined by a succession of space probe flybys , the first occurring in the early 1970s .   Europa has the smoothest surface of any known solid object in the Solar System . The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it , which could conceivably harbor extraterrestrial life . The predominant model suggests that heat from tidal flexing causes the ocean to remain liquid and drives ice movement similar to plate tectonics , absorbing chemicals from the surface into the ocean below . Sea salt from a subsurface ocean may be coating some geological features on Europa , suggesting that the ocean is interacting with the seafloor . This may be important in determining if Europa could be habitable . In addition , the Hubble Space Telescope detected water vapor plumes similar to those observed on Saturn 's moon Enceladus , which are thought to be caused by erupting cryogeysers .   The Galileo mission , launched in 1989 , provides the bulk of current data on Europa . No spacecraft has yet landed on Europa , although there have been several proposed exploration missions . The European Space Agency 's Jupiter Icy Moon Explorer ( JUICE ) is a mission to Ganymede that is due to launch in 2022 , and will include two flybys of Europa . NASA 's planned Europa Clipper will be launched in the mid-2020s .