The magnetic field of Mars is the

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 ...

generated from

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...

' interior. Today, Mars does not have a global

. However, Mars did power an early dynamo that produced a strong magnetic field 4 billion years ago, comparable to Earth's present surface field. After the early dynamo ceased, a weak late dynamo was reactivated (or persisted up to) ~3.8 billion years ago. The distribution of Martian crustal magnetization is similar to the

Martian dichotomy The most conspicuous feature of Mars is a sharp contrast, known as the Martian dichotomy, between the Southern and the Northern hemispheres. The two hemispheres' geography differ in elevation by 1 to 3 km. The average thickness of the Marti ...

(i.e., the Martian hemispheric topographic dichotomy). Whereas the Martian northern lowlands are largely unmagnetized, the southern hemisphere possesses strong remanent magnetization, showing alternating stripes. Our understanding of the evolution of the magnetic field of Mars is based on the combination of satellite measurements, Martian ground-based magnetic data

Crustal magnetism

Satellite data

The reconstruction of the Martian global crustal magnetism is mainly based on magnetic field measurements from the

Mars Global Surveyor ''Mars Global Surveyor'' (MGS) was an American robotic space probe developed by NASA's Jet Propulsion Laboratory and launched November 1996. MGS was a global mapping mission that examined the entire planet, from the ionosphere down through the a ...

(MGS) magnetic field experiment/electron reflectometer (MAG/ER) and Mars Atmosphere and Volatile Evolution (MAVEN) magnetic-field data. However, these satellites are located at altitudes of 90–6000 km and have spatial resolutions of ≥160 km, so the measured magnetization cannot observe crustal magnetic fields at shorter length scales. Mars currently does not sustain an active

dynamo file:DynamoElectricMachinesEndViewPartlySection USP284110.png, "Dynamo Electric Machine" (end view, partly section, ) A dynamo is an electrical generator that creates direct current using a commutator (electric), commutator. Dynamos were the f ...

based on the

(MGS) and Mars Atmosphere and Volatile Evolution (MAVEN)

measurements. However, the satellite data show that the older(~4.2–4.3 Ga) southern-hemisphere crust records strong remanent magnetization (~22 nT), but the younger northern lowlands have a much weaker or zero remanent magnetization. In contrast, the large basins formed during the

Late Heavy Bombardment The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. According to the hypoth ...

(LHB) (~ 4.1–3.9 Ga) (e.g., Argyre, Hellas, and Isidis) and volcanic provinces (e.g.,

Elysium Elysium (, ), otherwise known as the Elysian Fields ( grc, Ἠλύσιον πεδίον, ''Ēlýsion pedíon'') or Elysian Plains, is a conception of the afterlife that developed over time and was maintained by some Greek religious and philos ...

Olympus Mons Olympus Mons (; Latin for Mount Olympus) is a large shield volcano on Mars. The volcano has a height of over 21.9 km (13.6 mi or 72,000 ft) as measured by the Mars Orbiter Laser Altimeter (MOLA). Olympus Mons is about two and a h ...

Tharsis Montes The Tharsis Montes () are three large shield volcanoes in the Tharsis region of the planet Mars. From north to south, the volcanoes are Ascraeus Mons, Pavonis Mons and Arsia Mons. Mons (plural ''montes'') is the Latin word for mountain; it is a ...

, and

Alba Patera Alba Mons (formerly and still occasionally known as Alba Patera, a term that has since been restricted to the volcano's summit caldera; also initially known as the Arcadia ring) is a volcano located in the northern Tharsis region of the planet Ma ...

) lack magnetic signatures, but the younger

Noachian The Noachian is a geologic system and early time period on the planet Mars characterized by high rates of meteorite and asteroid impacts and the possible presence of abundant surface water. The absolute age of the Noachian period is uncerta ...

and

Hesperian The Hesperian is a system (stratigraphy), geologic system and geologic timescale, time period on the planet Mars characterized by widespread Volcanology of Mars, volcanic activity and catastrophic flooding that carved immense outflow channels acr ...

volcanoes (e.g., Tyrrhenus Mons and Syrtis Major) have crustal remanence.

Mars lander observation

The Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (

InSight Insight is the understanding of a specific cause and effect within a particular context. The term insight can have several related meanings: *a piece of information *the act or result of understanding the inner nature of things or of seeing intu ...

) mission measured the crustal field at the Insight landing site located in Elysium Planitia to be ~2 µT. This detailed ground-level data is an order of magnitude higher than satellite-based estimates of ~200 nT at the

landing site. The source of this high magnetization is suggested to be

basement (~3.9 Ga) beneath the Early Amazonian and

flows (~3.6 and 1.5 Ga).

Paleomagnetism

Paleomagnetic evidence

Martian meteorites enable estimates of Mar's paleofield based on the thermal remanent magnetization (or TRM) (i.e., the remanent magnetization acquired when the meteorite cooled below the

Curie temperature In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...

in the presence of the ambient

). The thermal remanent magnetization of carbonates in meteorite

ALH84001 Allan Hills 84001 (ALH84001) is a fragment of a Martian meteorite that was found in the Allan Hills in Antarctica on December 27, 1984, by a team of American meteorite hunters from the ANSMET project. Like other members of the shergottite–nakhl ...

revealed that the early (4.1–3.9 Ga) Martian magnetic field was ~50 µT, much higher than the modern field, suggesting that a Martian dynamo was present until at least this time. Younger (~1.4 Ga) Martian

Nakhlite Nakhlites are a group of Martian meteorites, named after the first one, Nakhla meteorite. Nakhlites are igneous rocks that are rich in augite and were formed from basaltic magma about 1.3 billion years ago. They contain augite and olivine crys ...

meteorite Miller Range (MIL) 03346 recorded a paleofield of only ~5 µT. However, given the possible source locations of the Nakhlite meteorite, this paleointensity still suggests that the surface magnetization is stronger than the magnetic fields estimated from satellite measurements. The ~5 µT paleofield of this meteorite can be explained either by a late active dynamo or the field generated from lava flows emplaced in the absence of a late Martian dynamo.

Martian meteorites as paleomagnetic recorders

Martian meteorites contain a wide range of magnetic minerals that can record ancient remanent magnetism, including

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...

, titano-magnetite,

pyrrhotite Pyrrhotite is an iron sulfide mineral with the formula Fe(1-x)S (x = 0 to 0.2). It is a nonstoichiometric variant of FeS, the mineral known as troilite. Pyrrhotite is also called magnetic pyrite, because the color is similar to pyrite and it is ...

, and

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

. The magnetic mineralogy includes single domain (SD), pseudo single domain (PSD)-like, multi-domain (MD) states. However, only limited Martian meteorites are available to reconstruct the Martian paleofield due to aqueous, thermal, and

shock Shock may refer to: Common uses Collective noun *Shock, a historic commercial term for a group of 60, see English numerals#Special names * Stook, or shock of grain, stacked sheaves Healthcare * Shock (circulatory), circulatory medical emergen ...

overprints that make many Martian meteorites unsuitable for these studies. Paleomagnetic studies of Martian meteorites are listed in the table below:

Martian dynamo

Timeline of Martian dynamo

The exact timing and duration of the Martian

remain unknown, but there are several constraints from satellite observations and

paleomagnetic Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain magnetic minerals in rock ...

studies. The strong crustal magnetization in the southern hemisphere and the paleomagnetic evidence of

indicate that Mars sustained a strong magnetic field between ~4.2–4.3 Ga. The absence of crustal magnetic signatures in the upper lowlands and large impact basins implies dynamo termination prior to the formation of these basins (~4.0–3.9 Ga).

Magnetic anomalies In geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying ...

from two young volcanoes (e.g., Tyrrhenus Mons, Syrtis Major) may reflect the presence of a Martian magnetic field with possible magnetic reversals during the late

and

period.

Hemispheric magnetic dichotomy

One unresolved question is why the Martian crustal hemispheric

dichotomy A dichotomy is a partition of a whole (or a set) into two parts (subsets). In other words, this couple of parts must be * jointly exhaustive: everything must belong to one part or the other, and * mutually exclusive: nothing can belong simulta ...

correlates to the magnetic dichotomy (and whether the origin of this dichotomy is an

exogenic In a variety of contexts, exogeny or exogeneity () is the fact of an action or object originating externally. It contrasts with endogeneity or endogeny, the fact of being influenced within a system. Economics In an economic model, an exogeno ...

endogenic Endogenous substances and processes are those that originate from within a living system such as an organism, tissue, or cell. In contrast, exogenous substances and processes are those that originate from outside of an organism. For example, es ...

process). One exogenic explanation is that the

Borealis Borealis (Latin for ''northern'' or ''of the north'') may refer to: Astronomy * Borealis Basin or North Polar Basin, a basin on the planet Mars * Borealis quadrangle, an area on the planet Mercury ** Borealis Planitia, a basin within the quadran ...

impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...

resulted in thermal

demagnetization In classical electromagnetism, magnetization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material. Movement within this field is described by direction and is either Axial or Di ...

of an initially magnetized northern hemisphere, but the proposed age of this event (~4.5 Ga) is long before the Martian dynamo termination (~4.0–4.1 Ga). An alternate model suggests that degree-1 mantle convection (i.e., a convective structure in which mantle upwelling dominates in one hemisphere but downwelling takes in the other hemisphere) can produce a single-hemisphere dynamo.

Alternating stripes

One striking feature in Martian crustal magnetism is the long E–W trending alternating stripes on the southern hemisphere (

Terra Cimmeria Terra Cimmeria is a large Martian region, centered at and covering at its broadest extent. It covers latitudes 15 N to 75 S and longitudes 170 to 260 W. It lies in the Eridania quadrangle. Terra Cimmeria is one part of the heavily cratered, s ...

and

Terra Sirenum Terra Sirenum is a large region in the southern hemisphere of the planet Mars. It is centered at and covers 3900 km at its broadest extent. It covers latitudes 10 to 70 South and longitudes 110 to 180 W. Terra Sirenum is an upland area nota ...

). It has been proposed that these bands are formed by

plate tectonic Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...

activity similar to the alternating

magnetic polarity A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nicke ...

caused by seafloor crust spreading on Earth or the results of repeated

dike Dyke (UK) or dike (US) may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), ''Dikē'', the Greek goddess of moral justice * Dikes ...

intrusions. However, careful selection of the data analysis method is required to interpret these alternating stripes. Using sparse solutions (e.g., L1 regularization) of crustal-field measurements instead of smoothing solutions (e.g., L2 regularization) shows highly magnetized local patches (with the rest of the crust unmagnetized) instead of stripes. These patches might be formed by localized events such as volcanism or heating by impact events, which may not require continuous fields (e.g., intermittent dynamo).

Dynamo mechanisms

The dynamo mechanism of Mars is poorly understood but expected to be similar to the Earth's dynamo mechanism.

Thermal convection Convection (or convective heat transfer) is the transfer of heat from one place to another due to the movement of fluid. Although often discussed as a distinct method of heat transfer, convective heat transfer involves the combined processes o ...

due to the high

thermal gradient A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a Dimensional analysis, dimensional quantity express ...

s in the hot, initial core was likely the primary mechanism for driving a

early in Mars' history. As the mantle and core cooled over time, inner-core crystallization (which would provide latent heat) and chemical convection may have played a major role in driving the dynamo. Following inner-core formation, light elements migrated from the inner-core boundary into the liquid outer core and drove

convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...

by buoyancy. However, even

lander data could not confirm the presence of Mars' solid inner core, and we cannot exclude the possibility that there was no core crystallization (only thermal convection without chemical convection). Also, the possibility that magnetic fields may have been generated by a

magma ocean Magma oceans exist during periods of Earth's or any planet's accretion when the planet is completely or partly molten. In the early Solar System, magma oceans were formed by the melting of planetesimals and planetary impacts. Small planetesimals a ...

cannot be ruled out. It is also unclear when and by what mechanism the Martian dynamo shut down. Perhaps a change in the cooling rate of the mantle may have caused the cessation of the Martian dynamo. One theory is giant impacts during the early and mid-

periods stopped the dynamo by decreasing global heat flow at the core-mantle boundary. The seismic measurements from the

lander revealed that the Martian outer core is in a liquid state and larger than expected. In one model, a partially crystallized Martian core explains the current state of Mars (i.e., lack of magnetic field despite liquid outer core), and this model predicts that the magnetic field has the potential to be reactivated in the future.

References

{{Reflist Mars Magnetospheres Planetary science