Do all planets have magnetospheres?

[physicallove]

do all planets have magnetospheres?

 

[magpies]

I'm not totally sure about this but I think Mars doesn't or if it does its very small compared to the Earth's.

 

[Astronuc]

Mars and Venus have very weak magnetic fields as compared to that of Earth.

A comparison of the magnetospheres of Mars, Venus and the earth
http://adsabs.harvard.edu/abs/1981AdSpR...1...5G

Modeling the response of the induced magnetosphere of Venus to changing IMF direction using MESSENGER and Venus Express observations
http://www.agu.org/pubs/crossref/2009/2008GL036718.shtml

 

[Saul]

Specifically what causes and why the planetary magnetic spheres are different is an interesting subject. As the field has advanced and new observational data has become available the number of anomalies has increased not decreased.

The hypothesized magnetic dynamo mechanism requires a conductive fluid that is in convection motion. What starts the magnetic dynamo is not clear and is not specified. (i.e. Vigorous stirring of a conductive fluid does not create a magnetic field.) An initial magnetic field is required to kick start the mechanism that then grows stronger as there is hypothesized to be a balance between the counter acting electromagnetic force/energy with the energy of the convection motion. Experiments with a laboratory apparatus that used liquid sodium to model the Earth where not able to generate a magnetic dynamo. That experiment used a strong electromagnetic to try to kick start the dynamo and a heating element to create the differential as well as a rotating sphere and pumps to enhance the convection circulation.

To drive heat convection the planet's core must lose heat to create a differential. (i.e. The core must be liquid and must be cooling.) The temperature loss from the Earth's liquid core to the surface is not sufficient to drive the Earth's dynamo. The theory is that the heat loss when the liquid core solidifies creates the necessary heat differential. The problem with that theory is that the Earth's solid core is believed to be at most a 1 billion years old and the Earth's magnetic field is known to have existed for at least 3.5 billion years. There is no current solution to the differential heat source problem for the earth.

The Earth's magnetic field protects the planet's water from being removed by the solar wind. The lack of magnetic field is believed to be the reason why Mars does not have any significant water.

http://geology.geoscienceworld.org/cgi/content/abstract/30/11/987

Why does Venus lack a magnetic field? Francis Nimmo

Venus and Earth have similar radii and estimated bulk compositions, and both have an iron core that is at least partially liquid. However, despite these similarities, Venus lacks an appreciable dipolar magnetic field. Here I examine the hypothesis that this absence is due to Venus's also lacking plate tectonics for the past 0.5 b.y. The generation of a global magnetic field requires core convection, which in turn requires extraction of heat from the core into the overlying mantle. Plate tectonics cools Earth's mantle; on the basis of elastic thickness estimates and convection models, it is argued here that the mantle temperature on Venus is currently increasing. This heating will reduce the heat flux out of the core to zero over 1 b.y., halting core convection and magnetic field generation. If plate tectonics was operating on Venus prior to ca. 0.5 Ga, a magnetic field may also have existed. On Earth, the geodynamo may be a consequence of plate tectonics; this connection between near-surface processes and core magnetism may also be relevant to the generation of magnetic fields on Mars, Mercury, and Ganymede.

 

[Saul]

Uranus and Neptune's magnetic field.

It was once believed that the planet's magnetic field would be aligned with the planet's rotational axis.

As noted below the data from Voyager showed that is not the case for both Uranus and Neptune. The Voyager data showed that both Neptune and Uranus's magnetic field were strongly offset from the planet's rotational axis and just as curiously were offset from the planet's center.

The New Solar System, 4th Edition by J. Beatth, C. Peterson, A Chaikin

As Voyager approached Uranus in January 1986, we wondered if our experiences with symmetric magnetic environments of Earth, Jupiter, and Saturn would true for a planet that is quite literally spinning on its side. (My comment in relationship to Uranus’ orbit about the sun.)

An empirical relationship that relates angular momentum and magnetic moments, the “Bode’s law” of planetary magnetism, suggested that the magnetic moment of Uranus would be about one-tenth of Saturn.

We knew that the rotational axis of Uranus would lie, in early 1986, within 8 degrees of the planet-Sun line. If Uranus’s magnetic and rotational axis were nearly parallel, as is the case for other magnetized planets, (my comment in planets in the solar system), one pole would be pointed almost directly at the Sun and the a very unusual magnetospheric shape would be expected.

The planet’s magnetic moment is nearly the same strength as that predicted, but orientation is very different from our expectations. Uranus’ magnetic axis is tilted at huge 59 degrees from Uranus’s rotational axis and offset from the planet’s center.

Figure 18
The magnetic fields of Uranus and Neptune are remarkably – and unexpectedly – alike. The large offset from centre means that the field strength … It also means that the fields source cannot lie in the cores but rather must in a turbulent liquid mantle where dynamo driving convection can be substained.

 

[Saul]

This is the paper that discusses the heat flux paradox for the earth. (i.e. The Earth's solid core is believed to be at most 1 billion years old. The latent heat of crystallization is required to create the necessary heat flux to drive convection in the Earth's core. The paradox is the geomagnetic field is known to have been in place and continuous for at least 3.5 billion years. Prior to the core crystallizing there is no known mechanism to create the necessary heat flux differential to drive the geomagnetic dynamo mechanism.)

Can the Earth’s Dynamo Run on Heat Alone?

http://eprints.whiterose.ac.uk/436/1/gubbinsd10.pdf

This is a different observational challenge for the dynamo mechanism. Paleomagnetic research has shown that in the last few million years the geomagnetic field intensity has dropped in intensity by a factor of 5 to 10 for roughly 20% of the time. The geomagnetic field direction abruptly changes direction during these magnetic excursions.

http://eprints.whiterose.ac.uk/416/

Is the geodynamo process intrinsically unstable?

Recent palaeomagnetic studies suggest that excursions of the geomagnetic field, during which the intensity drops suddenly by a factor of 5 to 10 and the local direction changes dramatically, are more common than previously expected. The `normal' state of the geomagnetic field, dominated by an axial dipole, seems to be interrupted every 30,000 to 100,000 kyr; it may not therefore be as stable as we thought.


Recent studies suggest that the Earth's magnetic field has fallen dramatically in magnitude and changed direction repeatedly since the last reversal 700 kyr ago (Langereis et al. 1997; Lund et al. 1998). These important results paint a rather different picture of the long-term behaviour of the field from the conventional one of a steady dipole reversing at random intervals: instead, the field appears to spend up to 20 per cent of its time in a weak, non-dipole state (Lund et al. 1998). One of us (Gubbins 1999) has suggested that this is evidence of a rapid natural timescale (500 yr) in the outer core, and that the magnetic field is usually prevented from reversing completely by the longer diffusion time of the inner core (2 to 5 kyr). This raises a number of important but difficult questions for geodynamo theory. How can the geomagnetic field change so rapidly and dramatically? Can slight variations of the geomagnetic field affect the dynamics of core convection significantly? If so, is the geodynamo process intrinsically unstable?