Atmosphere of Mercury

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Atmosphere of Mercury
Mercury profile MDIS MESSENGER.jpg
Mercury's surface, with the atmosphere too thin to be visible.
General information
Chemical species Column density cm−2; Surface density cm−3 [1]
Composition
Hydrogen ~ 3 × 109; ~ 250
Molecular hydrogen < 3 × 1015; < 1.4 × 107
Helium < 3 × 1011; ~ 6 × 103
Oxygen < 3 × 1011; ~ 4 × 104
Molecular oxygen < 9 × 1014; < 2.5 × 107
Sodium ~ 2 × 1011; 1.7–3.8 × 104
Potassium ~ 2 × 109; ~ 4000
Calcium ~ 1.1 × 108; ~ 3000
Magnesium ~ 4 × 1010; ~ 7.5 × 103
Argon ~ 1.3 × 109; < 6.6 × 106
Water < 1 × 1012; < 1.5 × 107

Mercury, being the closest to the Sun, with a weak magnetic field and the smallest mass of the recognized terrestrial planets, has a very tenuous and highly variable atmosphere (surface-bound exosphere) containing hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level of about 10−14 bar (1 nPa). [2] The exospheric species originate either from the Solar wind or from the planetary crust. Solar light pushes the atmospheric gases away from the Sun, creating a comet-like tail behind the planet.

Contents

The existence of a Mercurian atmosphere was contentious until 1974, although by that time a consensus had formed that Mercury, like the Moon, lacked any substantial atmosphere. This conclusion was confirmed in 1974 when the unmanned Mariner 10 spaceprobe discovered only a tenuous exosphere. Later, in 2008, improved measurements were obtained by the MESSENGER spacecraft, which discovered magnesium in the Mercurian exosphere.

Composition

The Mercurian exosphere consists of a variety of species originating either from the Solar wind or from the planetary crust. [3] The first constituents discovered were atomic hydrogen (H), helium (He) and atomic oxygen (O), which were observed by the ultraviolet radiation photometer of the Mariner 10 spaceprobe in 1974. The near-surface concentrations of these elements were estimated to vary from 230 cm−3 for hydrogen to 44,000 cm−3 for oxygen, with an intermediate concentration of helium. [3] In 2008 the MESSENGER probe confirmed the presence of atomic hydrogen, although its concentration appeared higher than the 1974 estimate. [4] Mercury's exospheric hydrogen and helium are believed to come from the Solar wind, while the oxygen is likely to be of crustal origin. [3]

Ca and Mg in the tail Ca and Mg tail of Mercury (PIA12366).jpg
Ca and Mg in the tail

The fourth species detected in Mercury's exosphere was sodium (Na). It was discovered in 1985 by Drew Potter and Tom Morgan, who observed its Fraunhofer emission lines at 589 and 589.6 nm. [5] The average column density of this element is about 1 × 1011 cm−2. Sodium is observed to concentrate near the poles, forming bright spots. [6] Its abundance is also enhanced near the dawn terminator as compared to the dusk terminator. [7] Some research has claimed a correlation of the sodium abundance with certain surface features such as Caloris or radio bright spots; [5] however these results remain controversial. 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. [8] In 1998 another element, calcium (Ca), was detected with column density three orders of magnitude below that of sodium. [9] 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. [10] 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. [11]

In 2008 the MESSENGER probe's Fast Imaging Plasma Spectrometer (FIPS) discovered several molecular and different ions in the vicinity of Mercury, including H2O+ (ionized water vapor) and H2S+ (ionized hydrogen sulfide). [12] Their abundances relative to sodium are about 0.2 and 0.7, respectively. Other ions such as H3O+ (hydronium), OH (hydroxyl), O2+ and Si+ are present as well. [13] During its 2009 flyby, the Ultraviolet and Visible Spectrometer (UVVS) channel of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on board the MESSENGER spacecraft first revealed the presence of magnesium in the Mercurian exosphere. The near-surface abundance of this newly detected constituent is roughly comparable to that of sodium. [10]

Properties

Mariner 10's ultraviolet observations have established an upper bound on the exospheric surface density at about 105 particles per cubic centimeter. This corresponds to a surface pressure of less than 10−14  bar (1  nPa). [14]

The temperature of Mercury's exosphere depends on species as well as geographical location. For exospheric atomic hydrogen, the temperature appears to be about 420 K, a value obtained by both Mariner 10 and MESSENGER. [4] The temperature for sodium is much higher, reaching 750–1,500 K on the equator and 1,500–3,500 K at the poles. [15] Some observations show that Mercury is surrounded by a hot corona of calcium atoms with temperature between 12,000 and 20,000 K. [9] In the early 2000s, a simulation of Mercury's Na exosphere and its temporal variation was conducted to identify the source process that supplied crustal species to the exosphere. Processes like; evaporation, diffusion from the interior, sputtering by photons and energetic ions, chemical sputtering by photons, and meteoritic vaporization were tested. However, evaporation provides the strongest match when comparing the changes in the sodium exosphere with solar distance and time of day to the 2001 observations of Mercury's sodium tail. [16]

Tails

Sodium tail, photographed by an amateur in Italy Mercury Sodium Tail.png
Sodium tail, photographed by an amateur in Italy
Sodium tail Mercury Sodium tail (PIA11076).jpg
Sodium tail

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. [17] The main component in the tail is sodium, which has been detected beyond 24 million km (1000 RM) from the planet. [18] This sodium tail expands rapidly to a diameter of about 20,000 km at a distance of 17,500 km. [19] In 2009, MESSENGER also detected calcium and magnesium in the tail, although these elements were only observed at distances less than 8 RM. [17]

Observation difficulties

Mercury is the least explored planet of the inner Solar System due to the many difficulties of observation. The position of Mercury as seen from Earth is always very close to the Sun, which causes challenges when trying to observe it. The Hubble Space Telescope and other Earth-based space imaging systems have highly sensitive sensors so they can observe deep space objects. They must not be directed toward the Sun, lest its powerful radiation destroy the sensors. [16]

Instead, flyby and orbital missions to Mercury can study the planet and receive accurate data. Even though Mercury is closer to Earth than Pluto is, the transfer orbit from Earth to Mercury requires more energy. Mercury being so close to the Sun, space probes going there are accelerating as they approach, due to the Sun's gravitational pull. This requires the use of retrorockets, which use fuel that the probe must carry instead of better instruments. [20]

See also

Related Research Articles

<span class="mw-page-title-main">Mercury (planet)</span> First planet from the Sun

Mercury is the first planet from the Sun and the smallest in the Solar System. In English, it is named after the Roman god Mercurius (Mercury), god of commerce and communication, and the messenger of the gods. Mercury is classified as a terrestrial planet, with roughly the same surface gravity as Mars. The surface of Mercury is heavily cratered, as a result of countless impact events that have accumulated over billions of years. Its largest crater, Caloris Planitia, has a diameter of 1,550 km (960 mi) and one-third the diameter of the planet. Similarly to the Earth's Moon, Mercury's surface displays an expansive rupes system generated from thrust faults and bright ray systems formed by impact event remnants.

<span class="mw-page-title-main">Jupiter</span> Fifth planet from the Sun

Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, and slightly less than one one-thousandth the mass of the Sun. Jupiter orbits the Sun at a distance of 5.20 AU (778.5 Gm) with an orbital period of 11.86 years. Jupiter is the third brightest natural object in the Earth's night sky after the Moon and Venus, and it has been observed since prehistoric times. It was named after Jupiter, the chief deity of ancient Roman religion.

<span class="mw-page-title-main">Rhea (moon)</span> Moon of Saturn

Rhea is the second-largest moon of Saturn and the ninth-largest moon in the Solar System, with a surface area that is comparable to the area of Australia. It is the smallest body in the Solar System for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium. It was discovered in 1672 by Giovanni Domenico Cassini.

The exosphere 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 so low that the molecules are essentially collision-less. 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 outer space. It is located directly above the thermosphere. Very little is known about it due to a lack of research. Mercury, the Moon, Ceres, Europa, and Ganymede have surface boundary exospheres, which are exospheres without a denser atmosphere underneath. The Earth's exosphere is mostly hydrogen and helium, with some heavier atoms and molecules near the base.

<span class="mw-page-title-main">Atmosphere</span> Layer of gases surrounding an astronomical body held by gravity

An atmosphere is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. 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.

<span class="mw-page-title-main">Caloris Planitia</span> Crater on Mercury

Caloris Planitia is a plain within a large impact basin on Mercury, informally named Caloris, about 1,550 km (960 mi) in diameter. It is one of the largest impact basins in the Solar System. "Calor" is Latin for "heat" and the basin is so-named because the Sun is almost directly overhead every second time Mercury passes perihelion. The crater, discovered in 1974, is surrounded by the Caloris Montes, a ring of mountains approximately 2 km (1.2 mi) tall.

<i>MESSENGER</i> Seventh mission of the Discovery program; orbital reconnaissance of the planet Mercury (2004–2015)

MESSENGER was a NASA robotic space probe that orbited the planet Mercury between 2011 and 2015, studying Mercury's chemical composition, geology, and magnetic field. The name is a backronym for "Mercury Surface, Space Environment, Geochemistry, and Ranging", and a reference to the messenger god Mercury from Roman mythology.

<span class="mw-page-title-main">HD 209458 b</span> Gas giant exoplanet orbiting HD 209458

HD 209458 b is an exoplanet that orbits the solar analog HD 209458 in the constellation Pegasus, some 157 light-years from the Solar System. The radius of the planet's orbit is 0.047 AU, or one-eighth the radius of Mercury's orbit. This small radius results in a year that is 3.5 Earth-days long and an estimated surface temperature of about 1,000 °C. Its mass is 220 times that of Earth and its volume is some 2.5 times greater than that of Jupiter. The high mass and volume of HD 209458 b indicate that it is a gas giant.

<span class="mw-page-title-main">Comet Encke</span> Periodic comet with 3-year orbit

Comet Encke, or Encke's Comet, is a periodic comet that completes an orbit of the Sun once every 3.3 years. Encke was first recorded by Pierre Méchain on 17 January 1786, but it was not recognized as a periodic comet until 1819 when its orbit was computed by Johann Franz Encke. Like Halley's Comet, it is unusual in its being named after the calculator of its orbit rather than its discoverer. Like most comets, it has a very low albedo, reflecting only 4.6% of the light its nucleus receives, although comets generate a large coma and tail that can make them much more visible during their perihelion. The diameter of the nucleus of Encke's Comet is 4.8 km.

<span class="mw-page-title-main">BepiColombo</span> European–Japanese satellites heading to Mercury

BepiColombo is a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) to the planet Mercury. The mission comprises two satellites launched together: the Mercury Planetary Orbiter (MPO) and Mio. The mission will perform a comprehensive study of Mercury, including characterization of its magnetic field, magnetosphere, and both interior and surface structure. It was launched on an Ariane 5 rocket on 20 October 2018 at 01:45 UTC, with an arrival at Mercury planned for on 5 December 2025, after a flyby of Earth, two flybys of Venus, and six flybys of Mercury. The mission was approved in November 2009, after years in proposal and planning as part of the European Space Agency's Horizon 2000+ programme; it is the last mission of the programme to be launched.

<span class="mw-page-title-main">Geology of Mercury</span> Geologic structure and composition of planet Mercury

The geology of Mercury is the scientific study of the surface, crust, and interior of the planet Mercury. It emphasizes the composition, structure, history, and physical processes that shape the planet. It is analogous to the field of terrestrial geology. In planetary science, the term geology is used in its broadest sense to mean the study of the solid parts of planets and moons. The term incorporates aspects of geophysics, geochemistry, mineralogy, geodesy, and cartography.

<span class="mw-page-title-main">Ice giant</span> Giant planet primarily consisting of compounds with freezing points exceeding 100°K

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<span class="mw-page-title-main">Atmosphere of the Moon</span> Very scant presence of gases around the Moon

The atmosphere of the Moon is a very scant presence of gases surrounding the Moon. For most practical purposes, the Moon is considered to be surrounded by vacuum. The elevated presence of atomic and molecular particles in its vicinity compared to interplanetary medium, referred to as "lunar atmosphere" for scientific objectives, is negligible in comparison with the gaseous envelopes surrounding Earth and most planets of the Solar System. The pressure of this small mass is around 3×10−15 atm (0.3 nPa), varying throughout the day, and in total mass less than 10 metric tonnes. Otherwise, the Moon is considered not to have an atmosphere because it cannot absorb measurable quantities of radiation, does not appear layered or self-circulating, and requires constant replenishment due to the high rate at which its gases are lost into space.

<span class="mw-page-title-main">Extraterrestrial atmosphere</span> Area of astronomical research

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<span class="mw-page-title-main">Atmosphere of Uranus</span> Layer of gases surrounding the planet Uranus

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<span class="mw-page-title-main">Comet tail</span> Dust or gases blown off a comet by solar wind in the inner solar system, leaving a visible trail

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<span class="mw-page-title-main">Inter-crater plains on Mercury</span>

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Michael Mendillo is a professor of astronomy at Boston University. His primary research interests include space physics, planetary atmospheres, observations and models. He is a Fellow of the American Geophysical Union and a longtime member of the American Astronomical Society.

References

Notes

  1. ↑ Killen 2007, p. 456, Table 5
  2. ↑ "NASA—Mercury". Archived from the original on 2005-01-05. Retrieved 2009-09-26.
  3. 1 2 3 Killen, 2007, pp. 433–434
  4. 1 2 McClintock 2008, p. 93
  5. 1 2 Killen, 2007, pp. 434–436
  6. ↑ Killen, 2007, pp. 438–442
  7. ↑ Killen, 2007, pp. 442–444
  8. ↑ Killen, 2007, pp. 449–452
  9. 1 2 Killen, 2007, pp. 452–453
  10. 1 2 McClintock 2009, p. 612–613
  11. ↑ Rosemary M. Killen; Joseph M. Hahn (December 10, 2014). "Impact Vaporization as a Possible Source of Mercury's Calcium Exosphere". Icarus. 250: 230–237. Bibcode:2015Icar..250..230K. doi:10.1016/j.icarus.2014.11.035.
  12. ↑ "MESSENGER Scientists 'Astonished' to Find Water in Mercury's Thin Atmosphere". The Planetary Society. 2008-07-03. Archived from the original on 6 April 2010. Retrieved 2010-03-28.
  13. ↑ Zurbuchen 2008, p. 91, Table 1
  14. ↑ Domingue, 2007, pp. 162–163
  15. ↑ Killen, 2007, pp. 436–438
  16. 1 2 Solomon, Sean C (2003). "Mercury: the enigmatic innermost planet". Earth and Planetary Science Letters. 216 (4): 441–455. Bibcode:2003E&PSL.216..441S. doi:10.1016/S0012-821X(03)00546-6.
  17. 1 2 McClintock 2009, p. 610–611
  18. ↑ Schmidt 2010, p. 9–16
  19. ↑ Killen, 2007, p. 448
  20. ↑ Benkhoff, Johannes (2010). "BepiColombo—Comprehensive exploration of Mercury: Mission overview and science goals". Planetary and Space Science. 58 (1–2): 2–20. Bibcode:2010P&SS...58....2B. doi:10.1016/j.pss.2009.09.020.

Bibliography

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