by neufer » Wed Jul 08, 2020 12:55 pm
Ann wrote: ↑Wed Jul 08, 2020 12:05 pm
Surely Earth should have a tail, too?
- Unlike Mars & Mercury, Earth has both a high escape velocity and a protective magnetic field.
https://en.wikipedia.org/wiki/Atmosphere_of_Mercury\ wrote:
<<Because of Mercury's proximity to the Sun, the pressure of solar light is much stronger than near Earth. Solar radiation pushes neutral atoms away from Mercury, creating a comet-like tail behind it. The main component in the tail is sodium, which has been detected beyond 24 million km (1000 RM) from the planet. This sodium tail expands rapidly to a diameter of about 20,000 km at a distance of 17,500 km.In 2009, MESSENGER also detected calcium and magnesium in the tail, although these elements were only observed at distances less than 8 RM.
Sodium in Mercury's exosphere was discovered in 1985 by Drew Potter and Tom Morgan, who observed its Fraunhofer emission lines at 589 and 589.6 nm. The average column density of this element is about 1 × 10
11 cm−2. Sodium is observed to concentrate near the poles, forming bright spots. Its abundance is also enhanced near the dawn terminator as compared to the dusk terminator. A year after the sodium discovery, Potter and Morgan reported that potassium (K) is also present in the exosphere of Mercury, though with a column density two orders of magnitude lower than that of sodium. The properties and spatial distribution of these two elements are otherwise very similar. In 1998 another element, calcium (Ca), was detected with column density three orders of magnitude below that of sodium. Observations by the MESSENGER probe in 2009 showed that calcium is concentrated mainly near the equator—opposite to what is observed for sodium and potassium. Further observations by Messenger reported in 2014 note the atmosphere is supplemented by materials vaporized off the surface by meteors both sporadic and in a meteor shower associated with Comet Encke.>>
https://en.wikipedia.org/wiki/Exosphere wrote:
<<The exosphere (Ancient Greek: ἔξω éxō "outside, external, beyond", Ancient Greek: σφαῖρα sphaĩra "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is too low for them to behave as a gas by colliding with each other. In the case of bodies with substantial atmospheres, such as Earth's atmosphere, the exosphere is the uppermost layer, where the atmosphere thins out and merges with interplanetary space. It is located directly above the thermosphere. Very little is known about it due to lack of research. Mercury, the Moon and three Galilean satellites of Jupiter have surface boundary exospheres, which are exospheres without a denser atmosphere underneath. The gases that can be found in the Earth's exosphere are mostly hydrogen and carbon dioxide.
In principle, the exosphere covers distances where particles are still gravitationally bound to Earth, i.e. particles still have ballistic orbits that will take them back towards Earth. The upper boundary of the exosphere can be defined as the distance at which the influence of solar radiation pressure on atomic hydrogen exceeds that of Earth's gravitational pull. This happens at half the distance to the Moon. The exosphere, observable from space as the geocorona, is seen to extend to at least 10,000 kilometres from Earth's surface.>>
https://en.wikipedia.org/wiki/Atmospheric_escape wrote:
<<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 (or suprathermal) 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 sun. 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.
..............................................................
Atmospheric escape of hydrogen on Earth is due to Jeans escape (~10 - 40%), charge exchange escape (~ 60 - 90%), and polar wind escape (~ 10 - 15%), currently losing about 3 kg/s of hydrogen. The Earth additionally loses approximately 50 g/s of helium primarily through polar wind escape. Escape of other atmospheric constituents is much smaller. A Japanese research team in 2017 found evidence of a small number of oxygen ions on the moon that came from the Earth. In 1 billion years, the Sun will be 10% brighter than it is now, making it hot enough for Earth to lose enough hydrogen to space to cause it to lose all of its water (See Future of Earth#Loss of oceans).
..............................................................
Recent models indicate that hydrogen escape on Venus is almost entirely due to suprathermal mechanisms, primarily photochemical reactions and charge exchange with the solar wind. Oxygen escape is dominated by charge exchange and sputtering escape. Venus Express measured the effect of coronal mass ejections on the rate of atmospheric escape of Venus, and researchers found a factor of 1.9 increase in escape rate during periods of increased coronal mass ejections compared with calmer space weather.
..............................................................
Primordial Mars also suffered from the cumulative effects of multiple small impact erosion events, and recent observations with MAVEN suggest that 66% of the
36Ar in the Martian atmosphere has been lost over the last 4 billion years due to suprathermal escape, and the amount of CO
2 lost over the same time period is around 0.5 bar or more. The MAVEN mission has also explored the current rate of atmospheric escape of Mars. Jeans escape plays an important role in the continued escape of hydrogen on Mars, contributing to a loss rate that varies between 160 - 1800 g/s. Oxygen loss is dominated by suprathermal methods: photochemical (~ 1300 g/s), charge exchange (~ 130 g/s), and sputtering (~ 80 g/s) escape combine for a total loss rate of ~ 1500 g/s. Other heavy atoms, such as carbon and nitrogen, are primarily lost due to photochemical reactions and interactions with the solar wind.
..............................................................
Saturn's moon Titan and Jupiter's moon Io have atmospheres and are subject to atmospheric loss processes. They have no magnetic fields of their own, but orbit planets with powerful magnetic fields, which protects these moons from the solar wind when its orbit is within the bow shock. However Titan spends roughly half of its transit time outside of the bow-shock, subjected to unimpeded solar winds. The kinetic energy gained from pick-up and sputtering associated with the solar winds increases thermal escape throughout the transit of Titan, causing neutral hydrogen to escape. The escaped hydrogen maintains an orbit following in the wake of Titan, creating a neutral hydrogen torus around Saturn. Io, in its transit around Jupiter, encounters a plasma cloud. Interaction with the plasma cloud induces sputtering, kicking off sodium particles. The interaction produces a stationary banana-shaped charged sodium cloud along a part of the orbit of Io.>>
[quote=Ann post_id=303904 time=1594209926 user_id=129702]
Surely Earth should have a tail, too?[/quote]
[list]Unlike Mars & Mercury, Earth has both a high escape velocity and a protective magnetic field.[/list]
[quote=https://en.wikipedia.org/wiki/Atmosphere_of_Mercury\]
[float=right][img3=""]https://upload.wikimedia.org/wikipedia/commons/thumb/2/28/Ca_and_Mg_tail_of_Mercury_%28PIA12366%29.jpg/800px-Ca_and_Mg_tail_of_Mercury_%28PIA12366%29.jpg[/img3][/float]
<<Because of Mercury's proximity to the Sun, the pressure of solar light is much stronger than near Earth. Solar radiation pushes neutral atoms away from Mercury, creating a comet-like tail behind it. The main component in the tail is sodium, which has been detected beyond 24 million km (1000 RM) from the planet. This sodium tail expands rapidly to a diameter of about 20,000 km at a distance of 17,500 km.In 2009, MESSENGER also detected calcium and magnesium in the tail, although these elements were only observed at distances less than 8 RM.
Sodium in Mercury's exosphere was discovered in 1985 by Drew Potter and Tom Morgan, who observed its Fraunhofer emission lines at 589 and 589.6 nm. The average column density of this element is about 1 × 10[sup]11[/sup] cm−2. Sodium is observed to concentrate near the poles, forming bright spots. Its abundance is also enhanced near the dawn terminator as compared to the dusk terminator. A year after the sodium discovery, Potter and Morgan reported that potassium (K) is also present in the exosphere of Mercury, though with a column density two orders of magnitude lower than that of sodium. The properties and spatial distribution of these two elements are otherwise very similar. In 1998 another element, calcium (Ca), was detected with column density three orders of magnitude below that of sodium. Observations by the MESSENGER probe in 2009 showed that calcium is concentrated mainly near the equator—opposite to what is observed for sodium and potassium. Further observations by Messenger reported in 2014 note the atmosphere is supplemented by materials vaporized off the surface by meteors both sporadic and in a meteor shower associated with Comet Encke.>>[/quote][quote=https://en.wikipedia.org/wiki/Exosphere]
[float=left][img3=The Earth and its hydrogen envelope, or geocorona, as seen from the Moon. This ultraviolet picture was taken in 1972 with a camera operated by Apollo 16 astronauts on the Moon.]https://upload.wikimedia.org/wikipedia/commons/thumb/9/9e/Earth%E2%80%99s_geocorona_from_the_Moon.jpg/1024px-Earth%E2%80%99s_geocorona_from_the_Moon.jpg[/img3][/float]
<<The exosphere (Ancient Greek: ἔξω éxō "outside, external, beyond", Ancient Greek: σφαῖρα sphaĩra "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is too low for them to behave as a gas by colliding with each other. In the case of bodies with substantial atmospheres, such as Earth's atmosphere, the exosphere is the uppermost layer, where the atmosphere thins out and merges with interplanetary space. It is located directly above the thermosphere. Very little is known about it due to lack of research. Mercury, the Moon and three Galilean satellites of Jupiter have surface boundary exospheres, which are exospheres without a denser atmosphere underneath. The gases that can be found in the Earth's exosphere are mostly hydrogen and carbon dioxide.
In principle, the exosphere covers distances where particles are still gravitationally bound to Earth, i.e. particles still have ballistic orbits that will take them back towards Earth. The upper boundary of the exosphere can be defined as the distance at which the influence of solar radiation pressure on atomic hydrogen exceeds that of Earth's gravitational pull. This happens at half the distance to the Moon. The exosphere, observable from space as the geocorona, is seen to extend to at least 10,000 kilometres from Earth's surface.>>[/quote][quote=https://en.wikipedia.org/wiki/Atmospheric_escape]
<<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 (or suprathermal) 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 sun. 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.
..............................................................
Atmospheric escape of hydrogen on Earth is due to Jeans escape (~10 - 40%), charge exchange escape (~ 60 - 90%), and polar wind escape (~ 10 - 15%), currently losing about 3 kg/s of hydrogen. The Earth additionally loses approximately 50 g/s of helium primarily through polar wind escape. Escape of other atmospheric constituents is much smaller. A Japanese research team in 2017 found evidence of a small number of oxygen ions on the moon that came from the Earth. In 1 billion years, the Sun will be 10% brighter than it is now, making it hot enough for Earth to lose enough hydrogen to space to cause it to lose all of its water (See Future of Earth#Loss of oceans).
..............................................................
Recent models indicate that hydrogen escape on Venus is almost entirely due to suprathermal mechanisms, primarily photochemical reactions and charge exchange with the solar wind. Oxygen escape is dominated by charge exchange and sputtering escape. Venus Express measured the effect of coronal mass ejections on the rate of atmospheric escape of Venus, and researchers found a factor of 1.9 increase in escape rate during periods of increased coronal mass ejections compared with calmer space weather.
..............................................................
Primordial Mars also suffered from the cumulative effects of multiple small impact erosion events, and recent observations with MAVEN suggest that 66% of the [sup]36[/sup]Ar in the Martian atmosphere has been lost over the last 4 billion years due to suprathermal escape, and the amount of CO[sub]2[/sub] lost over the same time period is around 0.5 bar or more. The MAVEN mission has also explored the current rate of atmospheric escape of Mars. Jeans escape plays an important role in the continued escape of hydrogen on Mars, contributing to a loss rate that varies between 160 - 1800 g/s. Oxygen loss is dominated by suprathermal methods: photochemical (~ 1300 g/s), charge exchange (~ 130 g/s), and sputtering (~ 80 g/s) escape combine for a total loss rate of ~ 1500 g/s. Other heavy atoms, such as carbon and nitrogen, are primarily lost due to photochemical reactions and interactions with the solar wind.
..............................................................
Saturn's moon Titan and Jupiter's moon Io have atmospheres and are subject to atmospheric loss processes. They have no magnetic fields of their own, but orbit planets with powerful magnetic fields, which protects these moons from the solar wind when its orbit is within the bow shock. However Titan spends roughly half of its transit time outside of the bow-shock, subjected to unimpeded solar winds. The kinetic energy gained from pick-up and sputtering associated with the solar winds increases thermal escape throughout the transit of Titan, causing neutral hydrogen to escape. The escaped hydrogen maintains an orbit following in the wake of Titan, creating a neutral hydrogen torus around Saturn. Io, in its transit around Jupiter, encounters a plasma cloud. Interaction with the plasma cloud induces sputtering, kicking off sodium particles. The interaction produces a stationary banana-shaped charged sodium cloud along a part of the orbit of Io.>>[/quote]