First, let’s review some technical background. The state of a planet’s magnetic field can be “frozen” into a magnetically susceptible rock once it cools below the “Curie Point”. This is the temperature below which an induced magnetic field becomes permanent (at least until heated again). On Earth the most common rocks with fossil fields are basalts formed on land or on the ocean floor. Many other common types of volcanic rock, such as granite, can also become magnetized and carry a fossil field. Thus, the basic prerequisites for producing paleomagnetism are a planetary magnetic field of sufficient strength and magnetically susceptible material that is heated above, and then cools below its Curie point. A related form of paleomagnetism called detitral remanent magnetism forms in sedimentary environments. To be preserved, these sediments must remain below the Curie point of the magnetic minerals in the sediment once they have formed. Accessibility to rocks with paleomagnetism depends on several factors that we will discuss below, but rocks at the surface will be more accessible than those that are deeply buried.
How many bodies in the Solar System fulfill these requirements? To answer this question we need to understand the forms that planets and moons take. In broadest terms, planets can be divided up into terrestrial planets and giant planets. Giant planets can be divided into gas giants like Jupiter and Saturn and ice giants like Uranus and Neptune. Giant planets do not have a low temperature solid surface or layer wherein their magnetic fields can be locked in. Terrestrial planets and large moons do have solid surfaces, except for terrestrial planets significantly larger than Earth, which are probably covered by deep oceans and thick atmospheres (no such example exists in the Solar System).
Large moons, such as those around Jupiter and Saturn, generally have thick ice crusts. Io, which has lost its volatiles from tidal heating, frequently resurfaces itself. Sulfur and its compounds dominate Io’s surface. Earth’s Moon lacks volatiles partly because it formed in a dry area of the Solar System, and partly from the way it formed.
Of these planets and large moons (9 planets plus 8 moons), only Earth and Mercury retain significant global magnetic fields and can record them on their surfaces. Venus erased its past history with a resurfacing event about 700 million years ago, presently lacks a global magnetic field, and would probably not be able preserve fossil fields on its surface rocks today even if it had a magnetic field. If Venus had a paleomagnetic record of an early field, possibly just like we find on Mars, then it was erased during the global resurfacing event. Mars once had a global field, but it faded early on.
A global magnetic field protects life on the surface of an Earth-like planet or large moon in two ways. Over the long term, a planetary magnetic field protects life from some particle radiation from the Sun and from beyond the Solar System. It also slows the rate of atmospheric loss (especially of water if the surface is not frozen).
The value of a global magnetic field to science is that paleomagnetic fields can communicate information to us about a planet’s geological history. Geologists acquire this information in two ways. First, they can measure paleomagnetic fields in rock cores retrieved on or near the surface. Second, scientists can measure it remotely. The effectiveness of the first method depends mostly on the strength of the global field at the time it was fossilized into the rock. A stronger field is more likely to leave its mark in magnetizable rock. The effectiveness of the second method depends on several factors, including the strength of the global field at the time of measurement and the observer’s distance from the paleomagnetic rocks. A stronger field makes it more difficult to measure fossil fields remotely.
Thus, we can divide up the scientific usefulness of the paleomagnetic record into two aspects. The “integrative” aspect includes the total sum of the paleomagnetic records produced by a planetary body on its surface during the time it had a strong global field. The longer the integrative record, the more information that is conveyed about a planet’s past. The other aspect is the present strength of a planet’s global field. As long as the original field was strong enough, its strength when it’s measured has no effect on the ability to measure the integrative record that is easily accessible on land. The same cannot be said for remotely sensed paleomagnetism. In addition, the paleomagnetic record is more informative if the global field oscillates on a geologically short time-scale.
For instance, consider the paleomagnetic records on Earth and Mars. Mars presently lacks a global magnetic field, but it had one early in its history as evidenced by the paleomagnetic field patterns measured by an orbiter. If Mars still had a strong magnetic field, it would have been more difficult or even impossible to measure the weak remnant fields from an orbiter. Thus, for remote sensing of paleomagnetic fields, Mars is more measurable than Earth. But, the total scientific value of paleomagnetic fields is the combination of the information obtained from remote sensing and direct measurement and the length of the integrative record. Earth’s magnetic field has been active far longer than that of Mars, giving it a longer integrative record.
Added to this mix are the four coincidences noted by the geophysicist David Sandwell, whom we quoted in Chapter 3. The combination of Earth’s mid-ocean spreading rates, magnetic reversal intervals, ocean depth, and crustal thickness conspire to optimize the remote measurement of the paleomagnetic field on the ocean floor from a ship. Without this combination of parameters, the paleomagnetic field of the ocean floor would have been much more difficult to map in the presence of Earth’s global field.
What are the implications of all this for our argument? The presence of a long-lasting magnetic field has made Earth more habitable. The magnetic field has provided long-duration accessible magnetic records, but it also limits the effectiveness of remote sensing of the less accessible records. All other planetary bodies in the Solar System are less habitable than Earth, and most offer obviously less paleomagnetic information. Mars is the next most habitable planet in the Solar System. So it’s not surprising that the overall scientific value of the paleomagnetic records on Mars is comparable to that of Earth’s records. Mars generated its paleomagnetic record during its most habitable epoch. (Whether Mercury offers paleomagnetic records as scientifically useful as Earth’s will have to await the next missions to this planet.)
So, one aspect of the information we gain from paleomagnetism--namely, remote sensing--would be easier for Martians than for Earthlings. This is similar to the locations we mention in the book that are better than a habitable environment for one type of scientific measurement. For instance, we could access the cosmic background radiation more easily if we were situated somewhere in intergalactic space. But such a location would be vastly inferior overall to our Earthly location for many other types of important scientific discoveries. In the same way, the overall value of Mars for scientific discovery is significantly inferior to Earth. For instance, Mars contains less paleoclimatic and paleomagnetic information, has fewer earthquakes, has a dustier atmosphere, probably has less concentrated ores, and lacks a large moon.
All of this is consistent with the argument we develop in The Privileged Planet. We do not argue that habitable environments like Earth are superior to less habitable environments for every type of scientific measurement and discovery. Such an argument would be obviously incorrect. (Recall the superiority of intergalactic space for measuring the background radiation.) Rather, we argue that habitable environments provide the best overall setting for a wide range of diverse and often competing set of prerequisites for such discovery.
We would like to thank Casey Luskin for digging deeper into paleomagnetism as it relates to our argument in The Privileged Planet. We hope his work encourages others to explore this and other aspects of our hypothesis.