Magnetic field of Mars

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Martian Dynamo. The schematic illustration of the ancient dipolar magnetic field of Mars generated by a core dynamo process. Martian Dynamo.jpg
Martian Dynamo. The schematic illustration of the ancient dipolar magnetic field of Mars generated by a core dynamo process.

The magnetic field of Mars is the magnetic field generated from Mars's interior. Today, Mars does not have a global magnetic field. 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 magnetism is similar to the Martian dichotomy. Whereas the Martian northern lowlands are largely unmagnetized, the southern hemisphere possesses strong remanent magnetization, showing alternating stripes. Scientific understanding of the evolution of the magnetic field of Mars is based on the combination of satellite measurements and Martian ground-based magnetic data.

Contents

Crustal magnetism

Satellite data

Map of Martian crustal magnetism. Cylindrical projection map of crustal magnetism on Mars observed by MGS satellite at 400 km altitude. Colors represent intensities of the median value of the radial magnetic field components contoured over two orders of magnitude variation. Mars Crustal Magnetism MGS.png
Map of Martian crustal magnetism. Cylindrical projection map of crustal magnetism on Mars observed by MGS satellite at 400 km altitude. Colors represent intensities of the median value of the radial magnetic field components contoured over two orders of magnitude variation.

The reconstruction of the Martian global crustal magnetism is mainly based on magnetic field measurements from the Mars Global Surveyor (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, [1] so the measured magnetization cannot observe crustal magnetic fields at shorter length scales. [2]

Mars currently does not sustain an active dynamo based on the Mars Global Surveyor (MGS) and Mars Atmosphere and Volatile Evolution (MAVEN) magnetic field measurements. The satellite data show that the older (~4.2–4.3 billion years, Ga) southern-hemisphere crust records strong remanent magnetization (~22  nT), but the younger northern lowlands have a much weaker or zero remanent magnetization. [3] The large basins formed during the Late Heavy Bombardment (LHB) (~ 4.1–3.9 Ga) (e.g., Argyre, Hellas, and Isidis) and volcanic provinces (e.g., Elysium, Olympus Mons, Tharsis Montes, and Alba Patera) lack magnetic signatures, but the younger Noachian and Hesperian volcanoes (e.g., Tyrrhenus Mons and Syrtis Major) have crustal remanence. [4]

Mars lander observation

The Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission measured the crustal field at the Insight landing site located in Elysium Planitia to be ~2  μT. [2] This detailed ground-level data is an order of magnitude higher than satellite-based estimates of ~200 nT at the InSight landing site. The source of this high magnetization is suggested to be Noachian basement (~3.9 Ga) beneath the Early Amazonian and Hesperian flows (~3.6 and 1.5 Ga). [2]

Paleomagnetism

Paleomagnetic evidence

Martian meteorites enable estimates of Mars'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 the presence of the ambient magnetic field). The thermal remanent magnetization of carbonates in meteorite ALH84001 [5] 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 meteorite Miller Range (MIL) 03346 recorded a paleofield of only ~5 μT. [6] [7] 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. [7] The ~5 μT paleofield of this meteorite can be explained either by a late active dynamo [6] [7] or the field generated from lava flows emplaced in the absence of a late Martian dynamo. [7]

Martian meteorites as paleomagnetic recorders

Martian meteorites contain a wide range of magnetic minerals that can record ancient remanent magnetism, including magnetite, titano-magnetite, pyrrhotite, and hematite. 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 overprints that make many Martian meteorites unsuitable for these studies. [7] Paleomagnetic studies of Martian meteorites are listed in the table below:

TypeCrystallization Age Shock events Paleointensity SourcesReferences
Shergottites (Shergotty)~343 Mamultiple shock events2 μT, 0.25–1 μT shock demagnetization [8]
Shergottites (Tissint)~600 Mamultiple shock events2 μTremagnetized by impact events [9]
Nakhlite ~1.3–1.4 Ga-4 μTlate dynamo ? [6]
Nakhlite ~1.4 Gano significant shock event5 μTold source rock or late dynamo ? [7]
ALH84001 ~4.5 Ga~4.0 Ga (major impact)50 μTactive early dynamo [5]
ALH84001 ~4.5 Ga~4.0 Ga (major impact) [10]

Martian dynamo

Timeline of Martian dynamo

The exact timing and duration of the Martian dynamo remain unknown, but there are several constraints from satellite observations and paleomagnetic studies. The strong crustal magnetization in the southern hemisphere and the paleomagnetic evidence of ALH84001 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 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 Noachian and Hesperian period. [4]

Timing of the Martian dynamo. Grey shading represents possible age constraints (in Ga years) for the early and late dynamo. Stars indicate new age constraints from MAVEN data. [a] Early dynamo before the formation of Hellas, Isidis, and Argyre. [b] The cessation of the early dynamo based on large basin population. [c] The age of ALH84001. [d] Late dynamo after the formation of the major basins. Timing of the Martian Dynamo.jpg
Timing of the Martian dynamo. Grey shading represents possible age constraints (in Ga years) for the early and late dynamo. Stars indicate new age constraints from MAVEN data. [a] Early dynamo before the formation of Hellas, Isidis, and Argyre. [b] The cessation of the early dynamo based on large basin population. [c] The age of ALH84001. [d] Late dynamo after the formation of the major basins.

Hemispheric magnetic dichotomy

One unresolved question is why the Martian crustal hemispheric dichotomy correlates to the magnetic dichotomy (and whether the origin of this dichotomy is an exogenic or endogenic process). One exogenic explanation is that the Borealis impact event resulted in thermal demagnetization of an initially magnetized northern hemisphere, [11] but the proposed age of this event (~4.5 Ga) is long before the Martian dynamo termination (~4.0–4.1 Ga). [11] [12] 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. [13]

Alternating stripes

One striking feature in Martian crustal magnetism is the long E–W trending alternating stripes on the southern hemisphere (Terra Cimmeria and Terra Sirenum). [14] It has been proposed that these bands are formed by plate tectonic activity similar to the alternating magnetic polarity caused by seafloor crust spreading on Earth [14] or the results of repeated dike intrusions. [15] However, careful selection of the data analysis method is required to interpret these alternating stripes. [16] 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. [16] These patches might be formed by localized events such as volcanism or heating by impact events, [16] which may not require continuous fields (e.g., intermittent dynamo). [11]

Dynamo mechanisms

The dynamo mechanism of Mars is poorly understood but expected to be similar to the Earth's dynamo mechanism. [17] [18] Thermal convection due to the high thermal gradients in the hot, initial core was likely the primary mechanism for driving a dynamo early in Mars's history. [17] [18] 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 by buoyancy. [18] However, even InSight lander data could not confirm the presence of Mars's solid inner core, [19] and we cannot exclude the possibility that there was no core crystallization (only thermal convection without chemical convection). [17] [18] Also, the possibility that magnetic fields may have been generated by a magma ocean cannot be ruled out. [17]

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. [17] One theory is giant impacts during the early and mid-Noachian periods stopped the dynamo by decreasing global heat flow at the core-mantle boundary. [20]

The seismic measurements from the InSight lander revealed that the Martian outer core is in a liquid state and larger than expected. [19] 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. [18]

Possible dynamo mechanisms
Dynamo sourcesDynamo mechanismsNotesReferences
ThermalThermal convection- requires high temperature, high sulfur content

- no solid inner core

[17] [18]
Magma ocean- requires conductive silicate-dominated melts [17]
ThermocompositionalChemical convection

(Top-down crystallization)

- requires low temperature, low thermal expansivity, low sulfur content

- possible future dynamo reactivation

[18]
Chemical convection

(Bottom-up crystallization or iron snow)

- requires low temperature, high thermal expansivity, high sulfur content

- powers dynamo based on the light element partitioning coefficient

[18]
MechanicalImpact events- reduces global heat flow at the core mantle boundary and stops dynamo [20]

See also

Related Research Articles

<span class="mw-page-title-main">Earth's outer core</span> Fluid layer composed of mostly iron and nickel between Earths solid inner core and its mantle

Earth's outer core is a fluid layer about 2,260 km (1,400 mi) thick, composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle. The outer core begins approximately 2,889 km (1,795 mi) beneath Earth's surface at the core-mantle boundary and ends 5,150 km (3,200 mi) beneath Earth's surface at the inner core boundary.

<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.

<span class="mw-page-title-main">Paleomagnetism</span> Study of Earths magnetic field in past

Paleomagnetism is the study of prehistoric Earth's magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called paleomagnetists.

<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">Rock magnetism</span> The study of magnetism in rocks

Rock magnetism is the study of the magnetic properties of rocks, sediments and soils. The field arose out of the need in paleomagnetism to understand how rocks record the Earth's magnetic field. This remanence is carried by minerals, particularly certain strongly magnetic minerals like magnetite. An understanding of remanence helps paleomagnetists to develop methods for measuring the ancient magnetic field and correct for effects like sediment compaction and metamorphism. Rock magnetic methods are used to get a more detailed picture of the source of the distinctive striped pattern in marine magnetic anomalies that provides important information on plate tectonics. They are also used to interpret terrestrial magnetic anomalies in magnetic surveys as well as the strong crustal magnetism on Mars.

A geomagnetic reversal is a change in a planet's dipole magnetic field such that the positions of magnetic north and magnetic south are interchanged. The Earth's magnetic field has alternated between periods of normal polarity, in which the predominant direction of the field was the same as the present direction, and reverse polarity, in which it was the opposite. These periods are called chrons.

<span class="mw-page-title-main">Earth's inner core</span> Innermost part of Earth, a solid ball of iron-nickel alloy

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<span class="mw-page-title-main">Terra Sirenum</span>

Terra Sirenum is a large region in the southern hemisphere of the planet Mars. It is centered at 39.7°S 150°W 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 notable for massive cratering including the large Newton Crater. Terra Sirenum is in the Phaethontis quadrangle and the Memnonia quadrangle of Mars. A low area in Terra Sirenum is believed to have once held a lake that eventually drained through Ma'adim Vallis.

<span class="mw-page-title-main">Magnetofossil</span> Fossils produced by magnetotactic bacteria

Magnetofossils are the fossil remains of magnetic particles produced by magnetotactic bacteria (magnetobacteria) and preserved in the geologic record. The oldest definitive magnetofossils formed of the mineral magnetite come from the Cretaceous chalk beds of southern England, while magnetofossil reports, not considered to be robust, extend on Earth to the 1.9-billion-year-old Gunflint Chert; they may include the four-billion-year-old Martian meteorite ALH84001.

Natural remanent magnetization is the permanent magnetism of a rock or sediment. This preserves a record of the Earth's magnetic field at the time the mineral was laid down as sediment or crystallized in magma and also the tectonic movement of the rock over millions of years from its original position. Natural remanent magnetization forms the basis of paleomagnetism and magnetostratigraphy.

<span class="mw-page-title-main">Vine–Matthews–Morley hypothesis</span> Concept in plate tectonics

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<span class="mw-page-title-main">Martian dichotomy</span> Geomorphological feature of Mars

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 Martian crust is 45 km, with 32 km in the northern lowlands region, and 58 km in the southern highlands.

<span class="mw-page-title-main">Volcanism on Mars</span>

Volcanic activity, or volcanism, has played a significant role in the geologic evolution of Mars. Scientists have known since the Mariner 9 mission in 1972 that volcanic features cover large portions of the Martian surface. These features include extensive lava flows, vast lava plains, and the largest known volcanoes in the Solar System. Martian volcanic features range in age from Noachian to late Amazonian, indicating that the planet has been volcanically active throughout its history, and some speculate it probably still is so today. Both Mars and Earth are large, differentiated planets built from similar chondritic materials. Many of the same magmatic processes that occur on Earth also occurred on Mars, and both planets are similar enough compositionally that the same names can be applied to their igneous rocks.

<span class="mw-page-title-main">Phaethontis quadrangle</span> Map of Mars

The Phaethontis quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The Phaethontis quadrangle is also referred to as MC-24.

Plate reconstruction is the process of reconstructing the positions of tectonic plates relative to each other or to other reference frames, such as the Earth's magnetic field or groups of hotspots, in the geological past. This helps determine the shape and make-up of ancient supercontinents and provides a basis for paleogeographic reconstructions.

<span class="mw-page-title-main">Composition of Mars</span> Branch of the geology of Mars

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<span class="mw-page-title-main">Tectonics of Mars</span>

Like the Earth, the crustal properties and structure of the surface of Mars are thought to have evolved through time; in other words, as on Earth, tectonic processes have shaped the planet. However, both the ways this change has happened and the properties of the planet's lithosphere are very different when compared to the Earth. Today, Mars is believed to be largely tectonically inactive. However, observational evidence and its interpretation suggests that this was not the case further back in Mars's geological history.

<span class="mw-page-title-main">Crustal magnetism</span>

Crustal magnetism is the magnetic field of the crust of a planetary body. The crustal magnetism of Earth has been studied; in particular, various magnetic crustal anomalies have been studied. Two examples of crustal magnetic anomalies on Earth that have been studied in the Americas are the Brunswick magnetic anomaly (BMA) and East Coast magnetic anomaly (ECMA). Also, there can be a correlation between physical geological features and certain readings from crustal magnetism on Earth. Below the surface of the Earth, the crustal magnetism is lost because the temperature rises above the curie temperature of the materials producing the field.

<span class="mw-page-title-main">Gravity of Mars</span> Gravitational force exerted by the planet Mars

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Subir Kumar Banerjee is an Indian-American geophysicist, known for research on rock magnetism, palaeomagnetism, and environmental magnetism.

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