Planetary magnetic fields- The summary (part 1)

Introduction

The solar system can be defined as the sun with all the planets, satellites and asteroids that move around it. The bodies in the solar system which have radius greater than 1000 km are likely to have dynamos that can generate magnetic fields (Jones, 2007). These bodies are the gas giant planets: Jupiter and Saturn, the ice rich planets Uranus and Neptune and the terrestrial planets, which contain iron rich cores, Mercury, Venus, Earth and Mars. Before explaining the solar system’ bodies one by one how they can drive their self-sustaining dynamos, first let we see the dynamo theory explaining that the magnetic field can be obtained through the conversion of mechanical energy (Merril, McElhinny & McFadden, 1998). From the Navier-Stokes equation we know that the Coriolis force has an important role to play in solving the dynamo problem. Therefore, the planets which have slow rotation may be difficult to drive a dynamo.

Moreover, the convection processes are also very important, the planets that have only gases as their main constituent may be much easier in producing magnetic field than those which have solid cores. Recent studies (Stevenson, 2003) suggest that some planets have dynamos namely Earth, Jupiter, Saturn, Uranus, Neptune and Mercury although in this planet the magnetic field is very weak, while Mars has strong remanent magnetism in the Martian crust suggest that in the past Mars had a dynamo. Even though Venus is predicted to have a liquid outer core but there is no dynamo.

Observation of planetary magnetic fields

Many observations to extract information about the planets with their magnetic fields have been conducted since forty years ago through the space exploration or even in ancient time for the Earth. The first magnetic phenomena that was observed in the Earth is conducted by the Greek philosopher Thales of Miletus in the sixth century B.C. although the explanation of this was in animistic term (Merril et al., 1998). He stated that loadstone has a soul or life to attract irons (Fowler, 1997). Advanced discovery around Earth’s magnetic field was observed 2000 years ago by ancient Chinese which found the magnetic compass and followed by the discovery of declination (the angle between Earth magnetic and north direction in geographic map. The inclination itself was discovered in Europe in the 16^th (Fearn and Roberts, 2007). Further a scientific statement that the earth acts as a giant magnet given by William Gilbert in his famous book “De Magnete” in 1600 (Fowler, 1997).

The second observation was Jupiter’s magnetic field through its radio emission tracked by two radio-astronomers Ken Franklin and Bernie Burke in 1955. This discovery successfully opened a new window to explore planetary magnetic fields. After the discovery of the Earth’s magnetic belt, Frank Drake discovered that radio emission from Jupiter is caused by electrons trapped in a strong magnetic field. Drake’s statement then was supported and strengthened by data from Pioneer 10 spacecraft that found a huge magnetic field in Jupiter. An interesting invention related with Jupiter’s radio emission observed by Voyager 1 is the interaction between Jupiter and its satellite Io causing the signal of radio emission which is received at the Earth to up and down (Stern, 2007).

Other observations to detect existence of planetary magnetic fields were done through the space missions. In 1974 Mariner 10 was successfully discovered magnetic field in Mercury and this made most scientists surprised because Mercury is a small planet without geological activities (Stevenson, 1987). Early Data about Venus’s magnetic dipole moment were interpreted from U.S. and USSR spacecraft in 1976 by Russel (Stevenson, Spohn & Schubert, 1983). The Viking in 1979 detected a small permanent field in Mars by using potential analyzer (Stevenson et al.,1983), at the same time two US spacecraft Voyager 1 and 2 reached Jupiter and continued to Saturn and Voyager 2 traveled to Uranus and reached to Neptune in 1989 (Jones, 2007). continue to part 2 click here