Mercury's magnetic field

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Magnetosphere of Mercury
Mercury Magnetic Field NASA.jpg
Graph showing relative strength of Mercury's magnetic field.
Discovery [1]
Discovered by Mariner 10
Discovery dateApril 1974
Internal field [2] [3]
Radius of Mercury2,439.7 ± 1.0 km
Magnetic moment 2 to 6 × 1012 T•m3
Equatorial field strength 300 nT
Dipole tilt0.0° [4]
Solar wind parameters [5]
Speed400 km/s
Magnetospheric parameters [6] [7]
TypeIntrinsic
Magnetopause distance1.4 RM
Magnetotail length10–100 RM
Main ions Na+, O+, K+, Mg+, Ca+, S+, H2S+
Plasma sources Solar wind
Maximum particle energyup to 50 keV
Aurora

Mercury's magnetic field is approximately a magnetic dipole, apparently global, [8] on the planet of Mercury. [9] Data from Mariner 10 led to its discovery in 1974; the spacecraft measured the field's strength as 1.1% that of Earth's magnetic field. [10] The origin of the magnetic field can be explained by dynamo theory. [11] The magnetic field is strong enough near the bow shock to slow the solar wind, which induces a magnetosphere. [12]

Contents

Strength

The magnetic field is about 1.1% as strong as Earth's. [10] At the Hermean equator, the relative strength of the magnetic field is around 300 nT, which is weaker than that of Jupiter's moon Ganymede. [13] Mercury's magnetic field is weaker than Earth's because its core had cooled and solidified more quickly than Earth's. [14] Although Mercury's magnetic field is much weaker than Earth's magnetic field, it is still strong enough to deflect the solar wind, inducing a magnetosphere. Because Mercury's magnetic field is weak while the interplanetary magnetic field it interacts with in its orbit is relatively strong, the solar wind dynamic pressure at Mercury's orbit is also three times larger than at Earth.

Whether the magnetic field changed to any significant degree between the Mariner 10 mission and the MESSENGER mission remains an open question. A 1988 J.E.P. Connerney and N.F. Ness review of the Mariner magnetic data noted eight different papers in which were offered no less than fifteen different mathematical models of the magnetic field derived from spherical harmonic analysis of the two close Mariner 10 flybys, with reported centered magnetic dipole moments ranging from 136 to 350 nT-RM3 (RM is a Mercury radius of 2436 km). In addition they pointed out that "estimates of the dipole obtained from bow shock and/or magnetopause positions (only) range from approximately 200 nT-RM3 (Russell 1977) to approximately 400 nT-RM3 (Slavin and Holzer 1979b)." They concluded that "the lack of agreement among models is due to fundamental limitations imposed by the spatial distribution of available observations." [15] Anderson et al. 2011, using high-quality MESSENGER data from many orbits around Mercury – as opposed to just a few high-speed flybys – found that the dipole moment is 195 ± 10 nT-RM3. [16]

Discovery

Data from Mariner 10 led to the discovery of Mercury's magnetic field. Mariner 10.jpg
Data from Mariner 10 led to the discovery of Mercury's magnetic field.

Before 1974, it was thought that Mercury could not generate a magnetic field because of its relatively small diameter and lack of an atmosphere. However, when Mariner 10 made a fly-by of Mercury (somewhere around April 1974), it detected a magnetic field that was about 1/100 the total magnitude of Earth's magnetic field. But these passes provided weak constraints on the magnitude of the intrinsic magnetic field, its orientation and its harmonic structure, in part because the coverage of the planetary field was poor and because of the lack of concurrent observations of the solar wind number density and velocity. [3] Since the discovery, Mercury's magnetic field has received a great deal of attention, [17] primarily because of Mercury's small size and slow 59-day-long rotation.

The magnetic field itself is thought to originate from the dynamo mechanism, [11] [18] although this is uncertain as yet.

Origins

The origins of the magnetic field can be explained by the dynamo theory; [11] i.e., by the convection of electrically conductive molten iron in the planet's outer core. [19] A dynamo is generated by a large iron core that has sunk to a planet's center of mass, has not cooled over the years, an outer core that has not been completely solidified, and circulates around the interior. Before the discovery of its magnetic field in 1974, it was thought that because of Mercury's small size, its core had cooled over the years. There are still difficulties with this dynamo theory, including the fact that Mercury has a slow, 59-day-long rotation that could not have made it possible to generate a magnetic field.

This dynamo is probably weaker than Earth's because it is driven by thermo-compositional convection associated with inner core solidification. The thermal gradient at the core–mantle boundary is subadiabatic, and hence the outer region of the liquid core is stably stratified with the dynamo operating only at depth, where a strong field is generated. [20] Because of the planet's slow rotation, the resulting magnetic field is dominated by small-scale components that fluctuate quickly with time. Due to the weak internally generated magnetic field it is also possible that the magnetic field generated by the magnetopause currents exhibits a negative feedback on the dynamo processes, thereby causing the total field to weaken. [21] [22]

Magnetic poles and magnetic measurement

Mercury's magnetic field tends to be stronger at the equator than at other areas of Mercury. Mercury in color - Prockter07 centered.jpg
Mercury's magnetic field tends to be stronger at the equator than at other areas of Mercury.

Like Earth's, Mercury's magnetic field is tilted, [9] [23] meaning that the magnetic poles are not located in the same area as the geographic poles. As a result of the north-south asymmetry in Mercury's internal magnetic field, the geometry of magnetic field lines is different in Mercury's north and south polar regions. [24] In particular, the magnetic "polar cap" where field lines are open to the interplanetary medium is much larger near the south pole. This geometry implies that the south polar region is much more exposed than in the north to charged particles heated and accelerated by solar wind–magnetosphere interactions. The strength of the quadrupole moment and the tilt of the dipole moment are completely unconstrained. [3]

There have been various ways that Mercury's magnetic field has been measured. In general, the inferred equivalent internal dipole field is smaller when estimated on the basis of magnetospheric size and shape (~150–200 nT R3). [25] Recent Earth-based radar measurements of Mercury's rotation revealed a slight rocking motion explaining that Mercury's core is at least partially molten, implying that iron "snow" helps maintain the magnetic field. [26] The MESSENGER spacecraft was expected to make more than 500 million measurements of Mercury's magnetic field using its sensitive magnetometer. [19] During its first 88 days in orbit around Mercury, MESSENGER made six different sets of magnetic field measurements as it passed through Mercury's magnetopause. [27]

Field characteristics

The MESSENGER spacraft noted that Mercury's magnetic field is responsible for several magnetic "tornadoes" - twisted bundles of magnetic fields connecting the planetary field to interplanetary space - that are some 800 km wide or a third the total radius of the planet. MESSENGER.jpg
The MESSENGER spacraft noted that Mercury's magnetic field is responsible for several magnetic "tornadoes" – twisted bundles of magnetic fields connecting the planetary field to interplanetary space – that are some 800 km wide or a third the total radius of the planet.

Scientists noted that Mercury's magnetic field can be extremely "leaky," [28] [29] [30] because MESSENGER encountered magnetic "tornadoes" during its second fly-by on October 6, 2008, which could possibly replenish the atmosphere (or "exosphere", as referred to by astronomers). When Mariner 10 made a fly-by of Mercury back in 1974, its signals measured the bow shock, the entrance and exit from the magnetopause, and that the magnetospheric cavity is ~20 times smaller than Earth's, all of which had presumably decayed during the MESSENGER flyby. [31] Even though the field is just over 1% as strong as Earth's, its detection by Mariner 10 was taken by some scientists as an indication that Mercury's outer core was still liquid, or at least partially liquid with iron and possibly other metals. [32]

BepiColombo mission

BepiColombo is a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) to Mercury. [33] It is launched in October 2018. [34] Part of its mission objectives will be to elucidate Mercury's magnetic field. [35] [36]

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 ancient 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), which is about 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">Magnetopause</span> Abrupt boundary between a magnetosphere and the surrounding plasma

The magnetopause is the abrupt boundary between a magnetosphere and the surrounding plasma. For planetary science, the magnetopause is the boundary between the planet's magnetic field and the solar wind. The location of the magnetopause is determined by the balance between the pressure of the dynamic planetary magnetic field and the dynamic pressure of the solar wind. As the solar wind pressure increases and decreases, the magnetopause moves inward and outward in response. Waves along the magnetopause move in the direction of the solar wind flow in response to small-scale variations in the solar wind pressure and to Kelvin–Helmholtz instabilities.

<span class="mw-page-title-main">Magnetosphere</span> Region around an astronomical object in which its magnetic field affects charged particles

In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynamo.

<span class="mw-page-title-main">Solar wind</span> Stream of charged particles from the Sun

The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV. The composition of the solar wind plasma also includes a mixture of particle species found in the solar plasma: trace amounts of heavy ions and atomic nuclei of elements such as carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron. There are also rarer traces of some other nuclei and isotopes such as phosphorus, titanium, chromium, and nickel's isotopes 58Ni, 60Ni, and 62Ni. Superimposed with the solar-wind plasma is the interplanetary magnetic field. The solar wind varies in density, temperature and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field. The boundary separating the corona from the solar wind is called the Alfvén surface.

<span class="mw-page-title-main">Earth's magnetic field</span> Magnetic field that extends from the Earths outer and inner core to where it meets the solar wind

Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic field is generated by electric currents due to the motion of convection currents of a mixture of molten iron and nickel in Earth's outer core: these convection currents are caused by heat escaping from the core, a natural process called a geodynamo.

<i>MESSENGER</i> NASA mission to Mercury

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">BepiColombo</span> ESA/JAXA mission to study Mercury in orbit (2018–present)

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 November 2026, 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">Planetary core</span> Innermost layer(s) of a planet

A planetary core consists of the innermost layers of a planet. Cores may be entirely liquid, or a mixture of solid and liquid layers as is the case in the Earth. In the Solar System, core sizes range from about 20% to 85% of a planet's radius (Mercury).

<span class="mw-page-title-main">Bow shock</span> Shock wave caused by blowing stellar wind

In astrophysics, bow shocks are shock waves in regions where the conditions of density and pressure change dramatically due to blowing stellar wind. Bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at which the speed of the stellar wind abruptly drops as a result of its approach to the magnetopause. For stars, this boundary is typically the edge of the astrosphere, where the stellar wind meets the interstellar medium.

<span class="mw-page-title-main">Interplanetary medium</span> Material which fills the Solar System

The interplanetary medium (IPM) or interplanetary space consists of the mass and energy which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planets, asteroids, and comets, move. The IPM stops at the heliopause, outside of which the interstellar medium begins. Before 1950, interplanetary space was widely considered to either be an empty vacuum, or consisting of "aether".

<span class="mw-page-title-main">Magnetosheath</span> Region of a magnetosphere which cannot fully deflect charged particles

The magnetosheath is the region of space between the magnetopause and the bow shock of a planet's magnetosphere. The regularly organized magnetic field generated by the planet becomes weak and irregular in the magnetosheath due to interaction with the incoming solar wind, and is incapable of fully deflecting the highly charged particles. The density of the particles in this region is considerably lower than what is found beyond the bow shock, but greater than within the magnetopause, and can be considered a transitory state.

<span class="mw-page-title-main">Geology of Mercury</span>

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">Magnetosphere of Saturn</span> Cavity in the solar wind the sixth planet creates

The magnetosphere of Saturn is the cavity created in the flow of the solar wind by the planet's internally generated magnetic field. Discovered in 1979 by the Pioneer 11 spacecraft, Saturn's magnetosphere is the second largest of any planet in the Solar System after Jupiter. The magnetopause, the boundary between Saturn's magnetosphere and the solar wind, is located at a distance of about 20 Saturn radii from the planet's center, while its magnetotail stretches hundreds of Saturn radii behind it.

<span class="mw-page-title-main">Magnetosphere of Jupiter</span> Cavity created in the solar wind

The magnetosphere of Jupiter is the cavity created in the solar wind by Jupiter's magnetic field. Extending up to seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction, Jupiter's magnetosphere is the largest and most powerful of any planetary magnetosphere in the Solar System, and by volume the largest known continuous structure in the Solar System after the heliosphere. Wider and flatter than the Earth's magnetosphere, Jupiter's is stronger by an order of magnitude, while its magnetic moment is roughly 18,000 times larger. The existence of Jupiter's magnetic field was first inferred from observations of radio emissions at the end of the 1950s and was directly observed by the Pioneer 10 spacecraft in 1973.

<span class="mw-page-title-main">Magnetic field of the Moon</span>

The magnetic field of the Moon is very weak in comparison to that of the Earth; the major difference is the Moon does not have a dipolar magnetic field currently, so that the magnetization present is varied and its origin is almost entirely crustal in location; so it's difficult to compare as a percentage to Earth. But, one experiment discovered that lunar rocks formed 1 - 2.5 billion years ago were created in a field of about 5 microtesla (μT), compared to present day Earth's 50 μT. During the Apollo program several magnetic field strength readings were taken with readings ranging from a low of 6γ (6nT) at the Apollo 15 site to a maximum of 313γ (0.31μT) at the Apollo 16 site, note these readings were recorded in gammas(γ) a now outdated unit of magnetic flux density equivalent to 1nT.

<span class="mw-page-title-main">Exploration of Mercury</span> Sending probes to the smallest planet

The exploration of Mercury has a minor role in the space interests of the world. It is the least explored inner planet. As of 2015, the Mariner 10 and MESSENGER missions have been the only missions that have made close observations of Mercury. MESSENGER made three flybys before entering orbit around Mercury. A third mission to Mercury, BepiColombo, a joint mission between the Japan Aerospace Exploration Agency (JAXA) and the European Space Agency, is to include two probes. MESSENGER and BepiColombo are intended to gather complementary data to help scientists understand many of the mysteries discovered by Mariner 10's flybys.

<span class="mw-page-title-main">Stellar magnetic field</span> Magnetic field generated inside a star

A stellar magnetic field is a magnetic field generated by the motion of conductive plasma inside a star. This motion is created through convection, which is a form of energy transport involving the physical movement of material. A localized magnetic field exerts a force on the plasma, effectively increasing the pressure without a comparable gain in density. As a result, the magnetized region rises relative to the remainder of the plasma, until it reaches the star's photosphere. This creates starspots on the surface, and the related phenomenon of coronal loops.

<span class="mw-page-title-main">Atmosphere of Mercury</span> Composition and properties of the atmosphere of the innermost planet of the Solar System

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 containing hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level of about 10−14 bar. 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.

Magnetometer (<i>Juno</i>) Scientific instrument on the Juno space probe

Magnetometer (MAG) is an instrument suite on the Juno orbiter for planet Jupiter. The MAG instrument includes both the Fluxgate Magnetometer (FGM) and Advanced Stellar Compass (ASC) instruments. There two sets of MAG instrument suites, and they are both positioned on the far end of three solar panel array booms. Each MAG instrument suite observes the same swath of Jupiter, and by having two sets of instruments, determining what signal is from the planet and what is from spacecraft is supported. Avoiding signals from the spacecraft is another reason MAG is placed at the end of the solar panel boom, about 10 m and 12 m away from the central body of the Juno spacecraft.

<span class="mw-page-title-main">Dungey Cycle</span>

The Dungey cycle, officially proposed by James Dungey in 1961, is a phenomenon that explains interactions between a planet's magnetosphere and solar wind. Dungey originally proposed a cyclic behavior of magnetic reconnection between Earth's magnetosphere and flux of solar wind. This reconnection explained previously observed dynamics within Earth's magnetosphere. The rate of reconnection in the beginning of the cycle is dependent on the orientation of the interplanetary magnetic field as well as the resultant plasma conditions at the site of reconnection. On Earth, the reconnection cycle takes around 1 hour, but this differs from planet to planet.

References

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