Mercury's magnetic field
Encyclopedia
Mercury's magnetic field is approximately a magnetic dipole
(meaning the field has only two magnetic poles) that is significant, and apparently global, on planet Mercury
. Data from Mariner 10
led to its discovery in 1974, and has 1.1% the strength of Earth's magnetic field
, as measured by the spacecraft. The origin of the magnetic field can be explained by dynamo theory
, and because the magnetic field is strong enough near the bow shock
, it slows down the solar wind
, which induces a magnetosphere
.
. At the Hermean equator, the relative strength of the magnetic field is around 300 nT
. Although much weaker than Earth's magnetic field (about 1/100 its magnitude) according to Mariner 10 data, the magnetic field is still strong enough to deflect solar wind
emissions, inducing a magnetosphere
. Because Mercury's magnetic field is weak while the interplanetary magnetic field
it interacts with in its orbit (a perihelion at 0.307 AU
and an aphelion at 0.467 AU) is relatively strong, the solar wind dynamic pressure at Mercury′s orbit on the average is also a factor of three larger than that at Earth, for example. Mercury's magnetic field being weaker than Earth's may be because its core had cooled and solidified more quickly than the Earth. Scientists have detected Mercury's magnetic field to be weaker than Jupiter
's moon
Ganymede's
.
Since its discovery in 1974 by Mariner 10, Mercury's magnetic dipole moment
has decayed. Dr. Robert Humphreys had predicted that in 2011, the magnetic moment should be between 4.5 and 4.6 x 1019 joules per tesla (ampere-square meters); but instead, when measured, the magnetic moment had declined from 4.8 to just over 3.8 x 1019 joules per tesla. Such a rapid decline would account for nearly 30 percent of the magnetic moment. Data from Mariner 10 and the MESSENGER spacecraft
illustrate that the strength and shape of the magnetic field seems stable.
; i.e., by the convection of electrically conductive molten iron in the planet's outer core
. A dynamo is generated by a large iron core that has sank 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 its discovery in 1974, it was thought that Mercury′s core had cooled over the years of evolution of the Solar System
, regarding the planet's small size. There are still difficulties with this dynamo theory, including the fact that Mercury has a slow, 59-day-long orbit 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. Because of the planet's slow rotation, the resulting magnetic field is dominated by small-scale components that fluctuate quickly with time.
Like Earth's, Mercury's magnetic field is tilted, 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. 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.
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). 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. The MESSENGER spacecraft will collect more than 500 million measurements of the planet's magnetic field, using its sensitive 'Magnetometer'.
. However, when Mariner 10 made a fly-by of Mercury (somewhere around April 1974), it detected a magnetic field that was about 1/100th 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. Since the discovery, Mercury's magnetic field has received a great deal of attention, primarily because of Mercury's small size and slow 59-day-long orbit.
Regarding the origins of the magnetic field, it is currently thought that it comes from the dynamo mechanism
, although whether this is responsible for the magnetic field or not has not been clarified well.
Scientists noted that Mercury's magnetic field can be extremely "leaky," 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. 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 metal
s.
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
(meaning the field has only two magnetic poles) that is significant, and apparently global, on planet Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
. Data from Mariner 10
Mariner 10
Mariner 10 was an American robotic space probe launched by NASA on November 3, 1973, to fly by the planets Mercury and Venus. It was launched approximately two years after Mariner 9 and was the last spacecraft in the Mariner program...
led to its discovery in 1974, and has 1.1% the strength of Earth's magnetic field
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
, as measured by the spacecraft. The origin of the magnetic field can be explained by dynamo theory
Dynamo theory
In geophysics, dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time...
, and because the magnetic field is strong enough near the bow shock
Bow shock
A bow shock is the area between a magnetosphere and an ambient medium. For stars, this is typically the boundary between their stellar wind and the interstellar medium....
, it slows down the 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...
, which induces a magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
.
Strength
The magnetic field is about 1.1% as strong as Earth'sEarth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
. At the Hermean equator, the relative strength of the magnetic field is around 300 nT
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
. Although much weaker than Earth's magnetic field (about 1/100 its magnitude) according to Mariner 10 data, the magnetic field is still strong enough to deflect 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...
emissions, inducing a magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
. Because Mercury's magnetic field is weak while the interplanetary magnetic field
Interplanetary Magnetic Field
The interplanetary magnetic field is the term for the solar magnetic field carried by the solar wind among the planets of the Solar System....
it interacts with in its orbit (a perihelion at 0.307 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
and an aphelion at 0.467 AU) is relatively strong, the solar wind dynamic pressure at Mercury′s orbit on the average is also a factor of three larger than that at Earth, for example. Mercury's magnetic field being weaker than Earth's may be because its core had cooled and solidified more quickly than the Earth. Scientists have detected Mercury's magnetic field to be weaker than Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
's moon
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
Ganymede's
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...
.
Since its discovery in 1974 by Mariner 10, Mercury's magnetic dipole moment
Magnetic moment
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
has decayed. Dr. Robert Humphreys had predicted that in 2011, the magnetic moment should be between 4.5 and 4.6 x 1019 joules per tesla (ampere-square meters); but instead, when measured, the magnetic moment had declined from 4.8 to just over 3.8 x 1019 joules per tesla. Such a rapid decline would account for nearly 30 percent of the magnetic moment. Data from Mariner 10 and the MESSENGER spacecraft
MESSENGER
The MErcury Surface, Space ENvironment, GEochemistry and Ranging space probe is a robotic NASA spacecraft in orbit around the planet Mercury. The spacecraft was launched aboard a Delta II rocket in August 2004 to study the chemical composition, geology, and magnetic field of Mercury...
illustrate that the strength and shape of the magnetic field seems stable.
Origins
The origins of the magnetic field can be explained by dynamo theoryDynamo theory
In geophysics, dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time...
; i.e., by the convection of electrically conductive molten iron in the planet's outer core
Planetary core
The planetary core consists of the innermost layer of a planet.The core may be composed of solid and liquid layers, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% to...
. A dynamo is generated by a large iron core that has sank to a planet's center of mass
Center of mass
In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...
, has not cooled over the years, an outer core that has not been completely solidified, and circulates around the interior. Before its discovery in 1974, it was thought that Mercury′s core had cooled over the years of evolution of the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, regarding the planet's small size. There are still difficulties with this dynamo theory, including the fact that Mercury has a slow, 59-day-long orbit that could not have made it possible to generate a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
.
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. Because of the planet's slow rotation, the resulting magnetic field is dominated by small-scale components that fluctuate quickly with time.
Magnetic poles and magnetic measurement
Magnetic moment
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
and the tilt of the dipole moment are completely unconstrained.
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). 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. The MESSENGER spacecraft will collect more than 500 million measurements of the planet's magnetic field, using its sensitive 'Magnetometer'.
Discovery
Before 1974, it was thought that Mercury could not generate a magnetic field because of its relatively small diameter and lack of an atmosphereAtmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
. However, when Mariner 10 made a fly-by of Mercury (somewhere around April 1974), it detected a magnetic field that was about 1/100th the total magnitude of Earth's magnetic field
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
. 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. Since the discovery, Mercury's magnetic field has received a great deal of attention, primarily because of Mercury's small size and slow 59-day-long orbit.
Regarding the origins of the magnetic field, it is currently thought that it comes from the dynamo mechanism
Dynamo theory
In geophysics, dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time...
, although whether this is responsible for the magnetic field or not has not been clarified well.
Field characteristics
Planetary core
The planetary core consists of the innermost layer of a planet.The core may be composed of solid and liquid layers, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% to...
, or at least partially liquid with iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and possibly other metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s.