Atmospheric super-rotation is a phenomenon where a planet's atmosphere rotates faster than the planet itself. This behavior is observed in the atmospheres of Venus, Titan, Jupiter, and Saturn. Venus exhibits the most extreme super-rotation, with its atmosphere circling the planet in four Earth days, much faster than the planet's own rotation of 243 Earth days. The phenomenon of atmospheric super-rotation can influence a planet's climate and atmospheric dynamics.
In understanding super-rotation, the role of atmospheric waves and instabilities is crucial. These dynamics, including Rossby waves and Kelvin waves, are integral in transferring momentum and energy within atmospheres, contributing to the maintenance of super-rotation. For instance, on Venus, the interaction of thermal tides with planetary-scale Rossby waves is thought to contribute significantly to its rapid super-rotational winds. Similarly, in Earth's atmosphere, Kelvin waves generate eastward along the equator, playing a vital role in phenomena like the El Niño-Southern Oscillation, demonstrating the broader implications of these dynamics in atmospheric science.
The atmosphere of Venus is a prominent case of extreme super-rotation; the Venusian atmosphere circles the planet in just four Earth days, much faster than Venus' sidereal day of 243 Earth days. [1] The initial observations of Venus' super rotation were Earth-based. Modern GCM models and observations are often enhanced by looking at past ancient climates. In a model where Venus is assumed to have an atmospheric mass similar to Earth, SS-AS circulation could have dominated over super-rotation in an ancient thinner atmosphere. [2]
Super-rotation present in the stratosphere of Titan has been inferred by Voyager IRIS, Cassini CIRIS, stellar occultation and temperature observations, and Doppler shifts of the Huygens probe’s radio signal. [3] Latitudinal pressure gradients established from measurements taken by Voyager IRIS were sufficient to produce super-rotation of the atmosphere. [4] Stratospheric zonal winds on Titan were observed on the order of 100-200 m s−1, [5] faster than the highest zonal winds on Earth at ~60-70 m s−1. Questions on the effect of obliquity in super-rotation on Titan is often compared to Venus, as they share similar centrifugal accelerations to achieve dynamic balance. Any seasonal variations effected by obliquity between Titan and Venus is much different, as the small obliquity of Venus at 2.7° negates any strong seasonal effects. Titans obliquity at 26.7° is high enough to cause seasonal variations within the stratospheric spin. [4] Attempts to model super-rotation on the gas giants, including Titan, has been abundant. The first observations of Titan in the 1980's revealed little information about circulation within the atmosphere due to the low contrast photochemical haze covering the moon. The first general circulation model (GCMs) in the 1990s provided insight into the stratospheric properties that should be expected on Titan with further observation, and predicted super-rotation with winds up to 200 m/s. [6] Super-rotation was supported by the first 3D Titan GCM created by the Laboratoire de Météorologie Dynamique (LMD), in which they used an atmosphere similar to the observations of Voyager and recently Cassini.
The most recent GCM that is able to simulate super-rotation in the stratosphere successfully is TitanWRF. Modeled after the PlanetWRF, which was designed to be a global weather, research, and forecasting (WRF) model, TitanWRF added planetary physics and generalized parameters to produce a successful super-rotation model. Work done with TitanWRF v2 was able to simulate gradients in latitudinal temperature, zonal wind jets and super-rotation in the stratosphere. [3] Comparing TitanWRF v2 simulations with constant solar forcing (seasonal cycle removed) models, [7] showed that in the latter, a rapid buildup in rotation, attaining > 100m/s, happened in a few Titan years. The parameters in these older forcing models differ greatly in the mechanisms involved in generating the initial super-rotation compared to the more realistic TitanWRF models. After initial spin up, similarities evolve between the different models when a steady state is produced, [3] but differ again in the final states of the model. The initial mechanism producing spin up to super-rotation is still an on going question, as correlations between models differ greatly within this regime.
The visible cloud tops of Jupiter and Saturn provides further evidence on its deep atmospheric circulation demonstrating the presence of atmospheric super-rotation. [8] Jupiter's auroras, in particular, highlight the planet's rapid atmospheric movements through their ethereal glow and varying cloud depths.
On Earth, there is a phenomenon that its thermosphere has a slight net super-rotation, exceeding the surface rotational velocity. The size of this phenomenon varies widely across different models. [9] [10] [11] Some models suggest that global warming is likely to cause an increase in super-rotation in the future, including possible change in surface winds patterns. [12] [13] In simplified GCM models, equatorial super-rotation emerges without obliquity and the addition of tropical heating anomalies. [5] At present, a counter balance between the easterly Coriolis torque and the westerly torque maintains subrotation in the upper tropical troposphere. This leads to the prospect that with warmer and tropical wave sources in past ancient climates, Earths atmosphere might have super-rotated. [14]
Super-rotation in planetary atmospheres extends to the study of exoplanets, particularly, hot Jupiters. These distant worlds, orbiting close to their stars, often exhibit extreme atmospheric conditions, including super-rotation, which influences their thermal structures and potential habitability. Observations from telescopes like the Hubble Space Telescope have unveiled super-rotational wind speeds of thousands of kilometers per hour on some hot Jupiters. Moreover, the phenomenon shows how hot Jupiters are tidally locked, where one side continuously faces the star. This suggests a mechanism for heat distribution in planets, a factor in understanding their climatic conditions and patterns. [15] [16]
A planet is a large, rounded astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets by the most restrictive definition of the term: the terrestrial planets Mercury, Venus, Earth, and Mars, and the giant planets Jupiter, Saturn, Uranus, and Neptune. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk. Planets grow in this disk by the gradual accumulation of material driven by gravity, a process called accretion.
Venus is the second planet from the Sun. It is a terrestrial planet and is the closest in mass and size to its orbital neighbour Earth. Venus has by far the densest atmosphere of the terrestrial planets, composed mostly of carbon dioxide with a thick, global sulfuric acid cloud cover. At the surface it has a mean temperature of 737 K and a pressure 92 times that of Earth's at sea level. These extreme conditions compress carbon dioxide into a supercritical state at Venus's surface.
Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about nine times that of Earth. It has an eighth the average density of Earth, but is over 95 times more massive. Even though Saturn is almost as big as Jupiter, Saturn has less than a third its mass. Saturn orbits the Sun at a distance of 9.59 AU (1,434 million km), with an orbital period of 29.45 years.
Titan is the largest moon of Saturn and the second-largest in the Solar System. It is the only moon known to have an atmosphere denser than the Earth's and is the only known object in space—other than Earth—on which there is clear evidence that stable bodies of liquid exist. Titan is one of seven gravitationally rounded moons of Saturn and the second-most distant among them. Frequently described as a planet-like moon, Titan is 50% larger in diameter than Earth's Moon and 80% more massive. It is the second-largest moon in the Solar System after Jupiter's Ganymede and is larger than Mercury; yet Titan is only 40% as massive as Mercury, because Mercury is mainly iron and rock while much of Titan is ice, which is less dense.
An atmosphere is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosphere is the outer region of a star, which includes the layers above the opaque photosphere; stars of low temperature might have outer atmospheres containing compound molecules.
Rossby waves, also known as planetary waves, are a type of inertial wave naturally occurring in rotating fluids. They were first identified by Sweden-born American meteorologist Carl-Gustaf Arvid Rossby in the Earth's atmosphere in 1939. They are observed in the atmospheres and oceans of Earth and other planets, owing to the rotation of Earth or of the planet involved. Atmospheric Rossby waves on Earth are giant meanders in high-altitude winds that have a major influence on weather. These waves are associated with pressure systems and the jet stream. Oceanic Rossby waves move along the thermocline: the boundary between the warm upper layer and the cold deeper part of the ocean.
Atmospheric escape is the loss of planetary atmospheric gases to outer space. A number of different mechanisms can be responsible for atmospheric escape; these processes can be divided into thermal escape, non-thermal escape, and impact erosion. The relative importance of each loss process depends on the planet's escape velocity, its atmosphere composition, and its distance from its star. Escape occurs when molecular kinetic energy overcomes gravitational energy; in other words, a molecule can escape when it is moving faster than the escape velocity of its planet. Categorizing the rate of atmospheric escape in exoplanets is necessary to determining whether an atmosphere persists, and so the exoplanet's habitability and likelihood of life.
A runaway greenhouse effect will occur when a planet's atmosphere contains greenhouse gas in an amount sufficient to block thermal radiation from leaving the planet, preventing the planet from cooling and from having liquid water on its surface. A runaway version of the greenhouse effect can be defined by a limit on a planet's outgoing longwave radiation which is asymptotically reached due to higher surface temperatures evaporating water into the atmosphere, increasing its optical depth. This positive feedback means the planet cannot cool down through longwave radiation and continues to heat up until it can radiate outside of the absorption bands of the water vapour.
The atmosphere of Mars is the layer of gases surrounding Mars. It is primarily composed of carbon dioxide (95%), molecular nitrogen (2.85%), and argon (2%). It also contains trace levels of water vapor, oxygen, carbon monoxide, hydrogen, and noble gases. The atmosphere of Mars is much thinner and colder than Earth's having a max density 20g/m3 with a temperature generally below zero down to -60 Celsius. The average surface pressure is about 610 pascals (0.088 psi) which is 0.6% of the Earth's value.
The atmosphere of Venus is the very dense layer of gases surrounding the planet Venus. Venus's atmosphere is composed of 96.5% carbon dioxide and 3.5% nitrogen, with other chemical compounds present only in trace amounts. It is much denser and hotter than that of Earth; the temperature at the surface is 740 K, and the pressure is 93 bar (1,350 psi), roughly the pressure found 900 m (3,000 ft) under water on Earth. The atmosphere of Venus supports decks of opaque clouds of sulfuric acid that cover the entire planet, preventing optical Earth-based and orbital observation of the surface. Information about surface topography has been obtained exclusively by radar imaging.
The Hadley cell, also known as the Hadley circulation, is a global-scale tropical atmospheric circulation that features air rising near the equator, flowing poleward near the tropopause at a height of 12–15 km (7.5–9.3 mi) above the Earth's surface, cooling and descending in the subtropics at around 25 degrees latitude, and then returning equatorward near the surface. It is a thermally direct circulation within the troposphere that emerges due to differences in insolation and heating between the tropics and the subtropics. On a yearly average, the circulation is characterized by a circulation cell on each side of the equator. The Southern Hemisphere Hadley cell is slightly stronger on average than its northern counterpart, extending slightly beyond the equator into the Northern Hemisphere. During the summer and winter months, the Hadley circulation is dominated by a single, cross-equatorial cell with air rising in the summer hemisphere and sinking in the winter hemisphere. Analogous circulations may occur in extraterrestrial atmospheres, such as on Venus and Mars.
An extraterrestrial vortex is a vortex that occurs on planets and natural satellites other than Earth that have sufficient atmospheres. Most observed extraterrestrial vortices have been seen in large cyclones, or anticyclones. However, occasional dust storms have been known to produce vortices on Mars and Titan. Various spacecraft missions have recorded evidence of past and present extraterrestrial vortices. The largest extraterrestrial vortices are found on the gas giants, Jupiter and Saturn; and the ice giants, Uranus and Neptune.
An ocean world, ocean planet or water world is a type of planet that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the surface, as subsurface oceans, or on the surface, potentially submerging all dry land. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid or thalassogen, such as lava, ammonia or hydrocarbons. The study of extraterrestrial oceans is referred to as planetary oceanography.
The study of extraterrestrial atmospheres is an active field of research, both as an aspect of astronomy and to gain insight into Earth's atmosphere. In addition to Earth, many of the other astronomical objects in the Solar System have atmospheres. These include all the giant planets, as well as Mars, Venus and Titan. Several moons and other bodies also have atmospheres, as do comets and the Sun. There is evidence that extrasolar planets can have an atmosphere. Comparisons of these atmospheres to one another and to Earth's atmosphere broaden our basic understanding of atmospheric processes such as the greenhouse effect, aerosol and cloud physics, and atmospheric chemistry and dynamics.
The atmosphere of Titan is the dense layer of gases surrounding Titan, the largest moon of Saturn. Titan is the only natural satellite of a planet in the Solar System with an atmosphere that is denser than the atmosphere of Earth and is one of two moons with an atmosphere significant enough to drive weather. Titan's lower atmosphere is primarily composed of nitrogen (94.2%), methane (5.65%), and hydrogen (0.099%). There are trace amounts of other hydrocarbons, such as ethane, diacetylene, methylacetylene, acetylene, propane, PAHs and of other gases, such as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, acetonitrile, argon and helium. The isotopic study of nitrogen isotopes ratio also suggests acetonitrile may be present in quantities exceeding hydrogen cyanide and cyanoacetylene. The surface pressure is about 50% higher than on Earth at 1.5 bars which is near the triple point of methane and allows there to be gaseous methane in the atmosphere and liquid methane on the surface. The orange color as seen from space is produced by other more complex chemicals in small quantities, possibly tholins, tar-like organic precipitates.
The habitability of natural satellites is the potential of moons to provide habitats for life, though it is not an indicator that they harbor it. Natural satellites are expected to outnumber planets by a large margin and the study of their habitability is therefore important to astrobiology and the search for extraterrestrial life. There are, nevertheless, significant environmental variables specific to moons.
The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly solar proportions; other chemical compounds are present only in small amounts and include methane, ammonia, hydrogen sulfide, and water. Although water is thought to reside deep in the atmosphere, its directly-measured concentration is very low. The nitrogen, sulfur, and noble gas abundances in Jupiter's atmosphere exceed solar values by a factor of about three.
The length of the day (LOD), which has increased over the long term of Earth's history due to tidal effects, is also subject to fluctuations on a shorter scale of time. Exact measurements of time by atomic clocks and satellite laser ranging have revealed that the LOD is subject to a number of different changes. These subtle variations have periods that range from a few weeks to a few years. They are attributed to interactions between the dynamic atmosphere and Earth itself. The International Earth Rotation and Reference Systems Service monitors the changes.
Planetary oceanography, also called astro-oceanography or exo-oceanography, is the study of oceans on planets and moons other than Earth. Unlike other planetary sciences like astrobiology, astrochemistry, and planetary geology, it only began after the discovery of underground oceans in Saturn's moon Titan and Jupiter's moon Europa. This field remains speculative until further missions reach the oceans beneath the rock or ice layer of the moons. There are many theories about oceans or even ocean worlds of celestial bodies in the Solar System, from oceans made of liquid carbon with floating diamonds in Neptune to a gigantic ocean of liquid hydrogen that may exist underneath Jupiter's surface.
Chemical cycling describes systems of repeated circulation of chemicals between other compounds, states and materials, and back to their original state, that occurs in space, and on many objects in space including the Earth. Active chemical cycling is known to occur in stars, many planets and natural satellites.