Faint young Sun paradox

Faint young Sun paradox

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Encyclopedia
The faint young Sun paradox or problem describes the apparent contradiction between observations of liquid water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 early in the Earth's history
History of Earth
The history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation 4.578 billion years ago to the present day. Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's...

 and the astrophysical expectation that the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

's output would be only 70% as intense during that epoch
Epoch (astronomy)
In astronomy, an epoch is a moment in time used as a reference point for some time-varying astronomical quantity, such as celestial coordinates, or elliptical orbital elements of a celestial body, where these are subject to perturbations and vary with time...

 as it is during the modern epoch. The issue was raised by astronomers Carl Sagan
Carl Sagan
Carl Edward Sagan was an American astronomer, astrophysicist, cosmologist, author, science popularizer and science communicator in astronomy and natural sciences. He published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books...

 and George Mullen in 1972. Explanations of this paradox have taken into account greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

s, astrophysical influences, or a combination of the two.

Early solar output


Early in the Earth's history
History of Earth
The history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation 4.578 billion years ago to the present day. Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's...

, the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

's output would have been only 70% as intense during that epoch as it is during the modern epoch. In the then current environmental conditions, this solar output would have been insufficient to maintain a liquid ocean. Astronomers Carl Sagan
Carl Sagan
Carl Edward Sagan was an American astronomer, astrophysicist, cosmologist, author, science popularizer and science communicator in astronomy and natural sciences. He published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books...

 and George Mullen pointed out in 1972 that this is contrary to the geological and paleontological evidence.

According to the Standard Solar Model
Standard Solar Model
The Standard Solar Model refers to a mathematical treatment of the Sun as a spherical ball of gas...

, stars similar to the Sun should gradually brighten over their main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

 lifetime. However, with the predicted solar luminosity  ago and with greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

 concentrations the same as are current for the modern Earth, any liquid water exposed to the surface would freeze. However, the geological record shows a continually relatively warm surface in the full early temperature record
Temperature record
The temperature record shows the fluctuations of the temperature of the atmosphere and the oceans through various spans of time. The most detailed information exists since 1850, when methodical thermometer-based records began. There are numerous estimates of temperatures since the end of the...

 of the Earth, with the exception of a cold phase, the Huronian glaciation, about 2.4 to 2.1 billion years ago. Water-related sediments have been found that date to as early as 3.8 billion years ago. Hints of early life forms have been dated from as early as 3.5 billion years, and the basic carbon isotopy is very much in line with what is found today. A regular alternation between ice ages and warm periods is only to be found occurring in the period since one billion years ago.

Greenhouse hypothesis


When it first formed, Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 may have contained more greenhouse gases. Carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 concentrations may have been higher, with estimated partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....

 as large as 1000 kPa (10 bar), because there was no bacterial photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 to reduce
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 the gas to carbon and oxygen. Methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

, a very active greenhouse gas which reacts with oxygen to produce carbon dioxide and water vapor, may have been more prevalent as well, with a mixing ratio of 10−4 parts per million by volume.

Based on a study of geological sulfur isotopes, in 2009 a group of scientists including Yuichiro Ueno from the University of Tokyo
University of Tokyo
, abbreviated as , is a major research university located in Tokyo, Japan. The University has 10 faculties with a total of around 30,000 students, 2,100 of whom are foreign. Its five campuses are in Hongō, Komaba, Kashiwa, Shirokane and Nakano. It is considered to be the most prestigious university...

 proposed that carbonyl sulfide
Carbonyl sulfide
Carbonyl sulfide is the chemical compound with the formula OCS. Commonly written as COS, it is a colourless flammable gas with an unpleasant odor. It is a linear molecule consisting of a carbonyl group double bonded to a sulfur atom...

 (OCS) was present in the Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...

 atmosphere. Carbonyl sulfide is an efficient greenhouse gas and the scientists estimate that the additional greenhouse effect would have been sufficient to prevent the Earth from freezing over.

Following the initial accretion of the continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...

s after about 1 billion years, geo-botanist Heinrich Walter and others believe that a non-biological version of the carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...

 provided a negative temperature feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...

. The carbon dioxide in the atmosphere dissolved in liquid water and combined with metal ions derived from silicate weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...

 to produce carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

s. During ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

 periods, this part of the cycle would shut down. Volcanic
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

 carbon emissions would then restart a warming cycle due to the greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

.

According to the Snowball Earth
Snowball Earth
The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...

 hypothesis, there may have been a number of periods when the Earth's oceans froze over completely. The most recent such period may have been about 630 million years ago. Afterwards, the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...

 of new multicellular life forms started.

Alternatives


A minority view, propounded by the Israeli-American physicist Nir Shaviv
Nir Shaviv
Nir Joseph Shaviv is an Israeli/American physics professor, carrying out research in the fields of astrophysics and climate science. He is currently an associate professor at the Racah Institute of Physics of the Hebrew University of Jerusalem....

, uses climatological influences of solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

, combined with a hypothesis of Danish physicist Henrik Svensmark
Henrik Svensmark
Henrik Svensmark is a physicist at the Danish National Space Center in Copenhagen who studies the effects of cosmic rays on cloud formation. His work presents hypotheses about solar activity as an indirect cause of global warming; his research has suggested a possible link through the interaction...

 for a cooling effect of cosmic rays, to explain the paradox. According to Shaviv, the early Sun had emitted a stronger solar wind that produced a protective effect against cosmic rays. In that early age, a moderate greenhouse effect comparable to today's would have been sufficient to explain an ice-free Earth. Evidence for a more active early Sun has been found in meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...

s.

The temperature minimum around 2.4 billion years goes along with a cosmic ray flux modulation by a variable star formation rate in the Milky Way Galaxy. The reduced solar impact later results into a stronger impact of cosmic ray flux (CRF), which is hypothesized to lead to a relationship with climatological variations.

An alternative model of solar evolution may explain the faint young sun paradox. In this model, the early Sun underwent an extended period of higher solar wind output. This caused a mass loss from the Sun on the order of 5−10% over its lifetime, resulting in a more consistent level of solar luminosity (as the early Sun had more mass, resulting in more energy output than was predicted). In order to explain the warm conditions in the Archean era, this mass loss must have occurred over an interval of about one billion years. However, records of ion implantation from meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...

s and lunar samples show that the elevated rate of solar wind flux only lasted for a period of 0.1 billion years. Observations of the young Sun-like star π1 Ursae Majoris
Pi1 Ursae Majoris
Pi¹ Ursae Majoris is a yellow G-type main sequence dwarf with a mean apparent magnitude of +5.63. It is approximately 46.8 light years from Earth, and is a relatively young star with an age of about 200 million years. It is classified as a BY Draconis type variable star and its brightness varies by...

 matches this rate of decline in the stellar wind output, suggesting that a higher mass loss rate can not by itself resolve the paradox.

Examination of Archaean sediments appears inconsistent with the hypothesis of high greenhouse concentrations. Instead, the moderate temperature range may be explained by a lower surface albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

brought about by less continental area and the "lack of biologically induced cloud condensation nuclei". This would have led to increased absorption of solar energy, thereby compensating for the lower solar output.

Further reading