Mercury's magnetic field is approximately a magnetic
dipole
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
*An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system i ...
(meaning the field has only two magnetic poles) apparently global, on
planet Mercury.
Data from ''
Mariner 10
''Mariner 10'' was an American Robotic spacecraft, robotic space probe launched by NASA on 3 November 1973, to fly by the planets Mercury (planet), Mercury and Venus. It was the first spacecraft to perform flybys of multiple planets.
''Ma ...
'' led to its discovery in 1974; the spacecraft measured the field's strength as 1.1% that of
Earth's magnetic field
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 f ...
.
The origin of the magnetic field can be explained by
dynamo theory
In physics, the dynamo theory proposes a mechanism by which a celestial body such as Earth or a star generates a magnetic field. The dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid can ...
.
The magnetic field is strong enough near the
bow shock
In astrophysics, a 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 ...
to slow the
solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
, which induces a
magnetosphere
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 dynam ...
.
Strength
The magnetic field is about 1.1% as strong as
Earth's.
At the Hermean equator, the relative strength of the magnetic field is around 300
nT, which is weaker than that of
Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, 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, but ...
's moon
Ganymede. Mercury's magnetic field is weaker than Earth's because its core had cooled and solidified more quickly than Earth's. Although Mercury's magnetic field is much weaker than Earth's magnetic field, it is still strong enough to deflect the
solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
, inducing a
magnetosphere
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 dynam ...
. Because Mercury's magnetic field is weak while the
interplanetary magnetic field
The interplanetary magnetic field (IMF), now more commonly referred to as the heliospheric magnetic field (HMF), is the component of the solar magnetic field that is dragged out from the solar corona by the solar wind flow to fill the Solar Syst ...
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
''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, Geoche ...
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-R
M3 (R
M 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-R
M3 (Russell 1977) to approximately 400 nT-R
M3 (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." 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-R
M3.
Discovery
![Mariner 10](https://upload.wikimedia.org/wikipedia/commons/f/fe/Mariner_10.jpg)
Before 1974, it was thought that Mercury could not generate a magnetic field because of its relatively small diameter and lack of an
atmosphere
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 s ...
. 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, 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 f ...
. 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 rotation.
The magnetic field itself is thought to originate from the
dynamo mechanism,
although this is uncertain as yet.
Origins
The origins of the magnetic field can be explained by the
dynamo theory
In physics, the dynamo theory proposes a mechanism by which a celestial body such as Earth or a star generates a magnetic field. The dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid can ...
;
i.e., by the convection of electrically conductive molten iron in the planet's outer
core
Core or cores may refer to:
Science and technology
* Core (anatomy), everything except the appendages
* Core (manufacturing), used in casting and molding
* Core (optical fiber), the signal-carrying portion of an optical fiber
* Core, the centra ...
.
A dynamo is generated by a large iron core that has sunk to a planet's
center of mass
In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...
, 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
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
.
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. 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.
Magnetic poles and magnetic measurement
![Mercury in color - Prockter07 centered](https://upload.wikimedia.org/wikipedia/commons/3/30/Mercury_in_color_-_Prockter07_centered.jpg)
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
A quadrupole or quadrapole is one of a sequence of configurations of things like electric charge or current, or gravitational mass that can exist in ideal form, but it is usually just part of a multipole expansion of a more complex structure ref ...
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 ''R''
3). 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 was expected to make more than 500 million measurements of Mercury's magnetic field using its sensitive
magnetometer
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, o ...
.
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.
Field characteristics
![MESSENGER](https://upload.wikimedia.org/wikipedia/commons/3/3c/Messenger.jpg)
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
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 s ...
(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
A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...
, or at least partially liquid with
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
and possibly other
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
s.
''BepiColombo'' mission
''
BepiColombo
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'' ...
'' is a joint mission of the
European Space Agency
, owners =
, headquarters = Paris, Île-de-France, France
, coordinates =
, spaceport = Guiana Space Centre
, seal = File:ESA emblem seal.png
, seal_size = 130px
, image = Views in the Main Control Room (1205 ...
(ESA) and the
Japan Aerospace Exploration Agency
The is the Japanese national air and space agency. Through the merger of three previously independent organizations, JAXA was formed on 1 October 2003. JAXA is responsible for research, technology development and launch of satellites into orb ...
(JAXA) to Mercury.
It is launched in October 2018.
Part of its mission objectives will be to elucidate Mercury's magnetic field.
References
{{magnetospherics
Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
Mercury (planet)