The study of terrestrial magnetism has revealed that all planets, with the exception of Venus, have or once generated magnetic fields. The research on planetary magnetic fields intensified after it was discovered that magnetic field plays a crucial role in navigation of the planets. The Earth has been central to the investigations of this nature since its magnetic data is easily accessible. Terrestrial magnetism uses magnetized rock samples to investigate historical records of planets. Therefore, the magnetic field, together with gravitational field, provides vital information about the interior compositions of the solar system. The launch of the US MAGSAT mission has widened the scope of the study of terrestrial magnetism particularly the earth. The technology provides high resolution maps of the interior of the earth, revealing the potential accumulation of historical data. Various models have been established to explain the nature of planetary magnetic fields. The Dynamo theory, which contends that planets and other objects produce magnetic fields through dynamo action, is the most accepted concept. This paper seeks to examine magnetic fields in the solar system and the function of the dynamos.
The solar system planets, basing on their physical properties and magnetic fields, are classified into terrestrial, ice giant, or gas giant (Schubert and Soderlund 92). These planets have distinct structures of layers. Terrestrial planets have a central iron-alloy core surrounded by a silicate mantle and an outer rocky crust. Jupiter and Saturn, which are the gas giants, have rocky cores with intermediate fluid mantles composed of helium and metallic hydrogen. Uranus and Neptune, which are the ice giants, also have rocky cores with an intermediate ionic waters but their outer crust is composed of molecular envelopes of helium, hydrogen, and ices (Schubert and Soderlund, 93). The earth, however, is the only planet with a well-constrained layer thickness. The formation of magnetic fields is explained by the Dynamo theory which was developed by Parker in 1955 (Hamouda et al. 147). Parker studied the origin of the earth’s magnetic field in terms of self-sustaining dynamo. Fluid motion in the outer core of the planets moves electrical conducting material across existing weak magnetic fields, therefore, producing electric currents (Hamouda et al 147). The connective motion is thought to be generated from the radioactive decay heat in the interior of the planets. The produced electric current, in turn, creates a magnetic field that that gets in contact with the fluid motion to generate a secondary magnetic field (Hamouda et al. 147). These magnetic fields are positioned in line with earth’s axis of rotation (Hamouda et al. 30). Dynamo action refers to the conversion of kinetic energy to magnetic energy.
While many planets and other objects generate magnetic fields through dynamo action in their cores, not all of them have a dynamo in their requisite electrically conducting fluid layers. These planets and objects are likely to have partially or completely molten metallic cores. They include Venus, Mars, Io, and the moon. Studies by Yoder et al. and Khan et al. (as cited in Schubert and Soderlund 92) have revealed that Mars is presently in a partial molten state and the dynamos that once existed ceased to function.
Earth. The earth’s magnetic field, as evident by the magnetized crustal rocks, is at least 3.5 billion years old (Schubert and Soderlund 93). According to the earth’s paleomagnetic records, its dynamos have always induced a strong axial dipole component with constant field strength. Presently, the earth’s magnetic field has a mean surface of strength of approximately 40µT while its dominant dipole component is offset 10° off the rotational axis (Schubert and Soderlund 93). The understanding of the earth’s magnetic field has greatly improved with technologies like MAGSAT, Orsted, CHAMP, and SAC-C satellites. These technologies provide measurements of the earth’s magnetic field across different attitudes for over a decade.
Mercury. Mercury’s intrinsic magnetic field is the lowest among other objects of the solar system, with a mean surface strength of approximately 0.3µT (Schubert and Soderlund 94). Mercury’s magnetic field was first measured in 1974 and 1975 by the Mariner10 flybys and for the second round in 2008 and 2009 by the MESSENGER flybys (Schubert and Soderlund 94). The observations by these satellites revealed that the magnetic fields are found along the rotation axis of the planet. The field is three times weaker on the surface than the Earth’s. The earth’s field strength at the core-mantle boundary is also 500 time stronger than Mercury core. While the origin of the field has not been accurately determined by the academic community, dynamo action is thought to be the driving force. This proposal is evident by gravity measurements provided by Smith et al. (as cited in Schubert and Soderlund 94) and libration measurements.
Jupiter. Jupiter’s magnetic field was detected in 1955, becoming the second discovery of planetary magnetic field after the earth (Schubert and Soderlund 95). This discovery was enhanced by the Pioneer 10 flyby and subsequent measurements were taken by the Voyager 1 and 2 flybys, the Ulysses flybys, the Galileo orbiter, and the New Horizons flyby. Data collected revealed that Mercury’s field is dominated by the axial dipole component and has the largest magnetic strength in the solar system, with a mean surface strength of 550µT. The magnetic field originates from connectively-driven dynamo action in the planet’s highly electrically conducting metallic hydrogen mantle. The magnetic field is also produced in the low electrically conducting area close to the base of the molecular envelope Stanley and Glatzmaier 621). The location and extent of the dynamo action is, however, poorly constrained.
Saturn. This planet’s magnetic field was detected in 1979 by the Pioneer 11 flyby and subsequent observations have been done by the Voyager 1, Voyager 2, and the Cassini spacecraft (Stanley and Glatzmaier 623). Saturn’s surface filed strength is 30 µT with the least dipole tilt of less tha1°.
Uranus and Neptune. These planets magnetic fields were first detected in 1986 and 1996 respectively by Voyager 2 flybys. The ice giants are multipolar and their offset is 59° for Pluto and 47° for Neptune. Both planets have the surface strengths of 30µT. The science community has not yet understood the reason behind the widely varied magnetic strengths. However, research has indicated that the fields could be originating from the ionic oceanic water of the planets (Schubert and Soderlund 95). The planets’ internal structures and dynamics, consistent with other planets apart from the Earth, are poorly constrained.
Mars. According to data by Mars Global Surveyor (MGS), Mars does not presently have a functional magnetic field but there are traces of magnetic anomalies related to the magnetization of the planet’s crustal rocks (Schubert and Soderlund 95). Studies by Folkner et al. and Konopliv et al. (as cited in Schubert and Soderlund 96) revealed that Mars has a metallic core although its physical properties of the cores are still unknown. The dynamo activity of Mars has been debated. Scholars like Acuna et al. (as cited in Schubert and Soderlund 96) have argued that the planet’s dynamo was active at the beginning of Noachian period and stopped functioning due to the Hellas impact 4 billion years ago. However, Schubert et al. (as cited in Shubert et al. 96) suggest that Mars’ dynamo activity could have postdated the Hellas period.
Venus is the only planet with no dynamos, according to the measurements by the Mariner spacecraft (1960s) and subsequently by Pioneer Venus Orbiter (Schubert and Soderlund 96). Explaining this occurrence, Zhang and Zhang (as cited in Stanley and Glatzmaier 630) proposed that Venus’ core is molten metallic and whose size is similar to the Earth’s core. Scientists have indicated a possibility of incomplete cooling process of the planet’s inner core, hence, cannot be operated by a thermally-driven dynamo (Schubert and Soderlund 97). Nimmo and Stevenson (11971) have also suggested that Venus lacks plate tectonics, therefore, unable to cool its core.
The Dynamo theory is influential in the study of planetary magnetic fields. The theory suggests that magnetic fields are generated in the electrically conducting fluid regions of solar objects and are induced by dynamo action. While most planets generate magnetic fields through dynamo action, some of the planets do not have dynamos in their electrically conducting layers. Planets with active dynamos include the Erath, Mercury, Jupiter, Saturn, Uranus, and Neptune. Mars’ dynamo is no longer functional while Venus does not have dynamos.
Hamouda, Samir, Alfadeel, Alsslam, Eman, and Mohamed, Belhasan, Mohamed. “Planetary Magnetic field and gravity in the solar system. International Journal of Research- Granthaalaya, vol. 5, no. 9, 145-155. Sep. 2017. oaji.net/pdf.html?n=2017/1330-1507278903.pdf. 28 Oct. 2018.
Nimmo, F. and Stevenson, D.J. “Influence of early plate tectonics on the thermal evolution and magnetic field of Mars” Journal of Geophysical Research, vol. 105, no. 5, pp. 11969-11979. 25 May 2000. https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/1999JE001216. 28 Oct. 2018.
Samir Ahmed Hamouda, Nada Eaz-Alden Emgau, Rabab Muftah Bohagar, Aisha Mohamed. Eissa. “Study of planetary magnetic fields”. International Journal of Research – Granthalaaya, vol. 5, no. 3 pp. 29-44. 2017. https://doi.org/10.5281/zenodo.439552. 28 Oct. 2018.
Schubert, G and Soderlund, K.M. “Planetary Magnetic Fields: Observations and Models”. Physics of the Earth and Planetary Interiors, vol. 187, pp. 92-108. June 11 2011
www.maths.gla.ac.uk/~rs/res/B/PlanetDyn/Schubert2011.pdf. 28 Oct. 2018
Stanley, Sabine and Glatzmaier, Garry, A. “Dynamo models for planets other than earth”. Space Science Reviews, vol. 152, no. 1-4, pp. 617-649. May 2010. https://link.springer.com/article/10.1007/s11214-009-9573-y. 28 Oct. 2018.