Planetary magnetic fields- The summary (part 2)

How they can drive dynamos

We know that all information of planetary magnetic fields based on indirect measurements which are mostly obtained from the space missions. We also make Earth’s dynamo process as reference to understand other planet magnetic fields, although for the Earth itself there is no certain theory how it can generate a dynamo at least 3.5 Ga (Stevenson, 2003).

a. Mercury

This planet is a quite small with a diameter only 4880 km with less geological activities. Even though we have already made exploration to this planet via Mariner 10 which mapped 40 % of it over 3 flybys (Thomas, 2006 ), we still lack of detail information about its magnetic fields. The only interesting feature of the magnetic field in this planet is when the probe caught a sudden spam which was supposed as energized particles (Stern, 2007). This make the explanation of the permanent magnetism and dynamos theory more complicated because of the magnitude of its magnetic field (Stevenson, 1987). However, since Mercury is predicted to have a liquid outer core then it is likely to drive a self sustain dynamo (Stevenson, 2003).

The magnetic field in Mercury is very weak and not strong enough to trap many particles (Stern, 2007). This can be explained by considering physical parameters of Mercury to calculate the Elsasser number as an indicator of the field strength (Fearn and Roberts, 2007). So, Stevenson et al. (1987) suggested that the possible explanation regarding Mercury’s energy sources is through the thermoelectric effect.

b. Venus

Although this planet is probably to have a liquid outer core, it does not have a dynamo at present. Some people believe that there is no existence a dynamo in Venus because it has slow rotation (Levy, 1995). However, Stevenson (2003) states that the rotation of Venus is good enough to drive a dynamo. So, for Venus there is no certain theory that can successfully explain why it does not have a dynamo, logically if it has a liquid core to do mantle convection like Earth’s, it should have a dynamo. There are some possibilities regarding this case for example the liquid core of Venus can not afford to make convection because there is no inner core (Stevenson et al., 1983).

c. Earth

The dynamo theory may be the best theory that can explain how the Earth generates a self sustaining dynamo. This theory states that Earth’s inner core condition is sufficient for convection to drive a self sustaining dynamo. Since the Earth’s inner core is the fluid iron which has a significant role to drive the geodynamo, its rotation leads to dynamo effect. This convection process in the outer core of the Earth can generate an effective current loop which gives contribution to the magnetic dipole type magnetic field of the Earth (Nave, 2005).

In addition, There are some contributions of energy sources in running the geodynamo namely: radioactivity contribution, latent heat, specific heat, gravitational contribution and adiabatic contribution (Nimmo , Brodholt & Gubbins, 2003). However, it still can not afford to explain how it can drive a dynamo at least 3.5 Ga. We know that inner core had not formed at that time (Labrosse, Poirier & Mouel, 2001) and paleomagnetic evidences suggest that the Earth’s magnetic field has existed at 3.5 Ga. So, what kind of energy sources that maintained to run the geodynamo if there was no inner core (Jones, 2007). Some people believe (e.g Nimmo et al., 2003) that the radioactive energy and a cooling process in the liquid core produce adequate energy to drive the geodynamo. They modelled the thermal history of core and mantle by using the entropy and a convection scheme to generate the geodynamo as function of time. They found that the Earth’s core must contain a few hundred ppm potassium to obtain a good model. This is quite helpful in explaining why the Earth can maintain the geodynamo even though it did not have inner core.

d. Mars

This planet is predicted to have a dynamo in the past from the evidence of strong magnetism on Mars’s ancient crust. There are some speculations why the magnetic field disappeared from Mars (Stevenson, 2003).

1. There was a decreasing core cooling that caused lack of heat convection.
2. The mantle and core stopped cooling which shut down convection and the dynamo.
3. The core of Mars had completely frozen so that it could not afford to maintain a dynamo.

The origin of magnetization on Mars itself is still uncertain. One possibility scientific explanation is that the Mars’s magnetic field was driven by a volcanic mechanism (Stevenson, 2001).

e. Jupiter and Saturn

The structures of these planets can be divided into three homogenous regions: a small icy and rocky core, a fluid metallic H and He and an insulator region which composes primarily of molecular H₂ and He. The dynamo in these planets is believed from convecting metallic H and He (Merrill et al, 1998).

The interaction between Jupiter’s magnetic field and its larger satellites results in an interesting feature. For example, the interaction between its ionosphere and the relative motion between Io (one of the Jupiter’s satellites) and Jupiter’s magnetosphere generates huge currents flowing around them. This also gives effect to the Jupiter’s radio emission propagating to the Earth as up and down signals (Stern, 2007).

f. Uranus and Neptune

The structure of these planets are very similar and they also have similar the magnetic field inclination by around 60^◦ to their rotation axes (Stern, 2007). This evidence indicates that the rotation has an important role to play in generating the dynamo on these planets (Merrill et al, 1998). Nevertheless, the magnetic fields in these planets are very unusual, in general the other planets have dipolar fields while these planet have large non dipolar fields. One of the possible explanations of this case is that their sources are far away from the center of the planet and they are not relatively symmetric. This lead to magnetic fields with large deviation of the dipole axis to the rotation axes (Merrill et al, 1998). Lack of data about these planets makes the explanation of unusual Uranus’ and Neptune’s magnetic field still in the scope of speculation theories.

Conclusions

We have already known the existence of the planetary magnetic fields which mostly come from space missions, but the limitation of data and observation of them make scientists just suggest speculation theories about how the planetary magnetic fields formed and how they could be drove. Even we still do not understand or have a certain theory that can explain how the earth maintained the magnetic field before its inner core formed. Einstein himself states that the Earth’s magnetic field is one of the most important unsolved problems in physics (Baker, 1999).

There is no doubt that the planetary magnetic fields are an interesting subject to be acquired. Human beings, who are thirsty on sciences, always try to broad their horizon to find a new thing in the universe. So, to accommodate our curiosity on the planetary magnetic fields at least we need more observations and data of the planets and computer simulations so that we can solve unanswered questions that we met in studying this subject. James Cameroon says that “ We stand on the edge of glorious new age of exploration. The future is ready and willing – if we are” (Burke, 2008).

References

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