In last week’s blog about Earth’s magnetic field1 I wrote about recent research by three Australian astronomers that demonstrated the possible existence of life on a planet critically depends on that planet possessing a strong enduring magnetic dipole moment. Now, Harvard University astronomer Manasvi Lingam has published a paper in which he has determined the degree to which variations in the strength of Earth’s magnetic field impact life on Earth. He also shows the degree to which variations in the magnetic field strength of ancient Mars and of planets beyond our solar system impact the habitability of these planets.2
Evidence for Past Variations in Earth’s Magnetic Field
Earth’s magnetic field was not always as strong as it is today. Sometime in Earth’s past history the structure of its core transformed. A nucleation (formation) event occurred where a primordial amorphous core changed into a solid inner core and a liquid outer core comprised of the ferrous (magnetic) elements iron, nickel, and cobalt. The release of latent heat and the chemical differentiation that resulted from this transformation provided the necessary power for initiating Earth’s geodynamo that sustains Earth’s magnetic field (see figure 1).
Figure 1: Dynamo Mechanism That Generates Earth’s Magnetic Field. Convection currents within the liquid iron, nickel, and cobalt that comprise Earth’s outer core are driven by heat flow from Earth’s solid inner core and organized into rolls by the Coriolis force to generate circulating electric currents. These currents produce Earth’s magnetic field. Image credit: United States Geological Survey
Palaeomagnetic intensity measurements imply that the nucleation of Earth’s core occurred 1.0–1.5 billion years ago.3 Other palaeomagnetic measurements indicate that Earth’s geodynamo transitioned from: (1) multipolar to dipolar about 1.7 billion years ago, (2) to a weak-field dynamo about 1.0 billion years ago (a signature of the inner core nucleation event), and (3) to a strong-field dynamo 650 million years ago.4 Analysis of plagioclase and clinopyroxene crystals suggest that the nucleation event may have occurred as late as 565 million years ago.5
Earth’s geomagnetic field has undergone several hundred reversals over the past half billion years. A geomagnetic reversal is a polarity switch where the positions of magnetic north and magnetic south are interchanged. These geomagnetic reversals occur randomly and last from several decades to as long as a thousand years.6 During a geomagnetic reversal, Earth’s magnetic field strength weakens (see figure 2). It can weaken from about 40% of Earth’s present magnetic field strength level down to as little as 5%.7 During a geomagnetic reversal, Earth’s magnetic field strength weakens (see figure 2) from about 40% of Earth’s present magnetic field strength level down to as little as 5%.7
Figure 2: Reversal of Earth’s Magnetic Field. The four images progressively show (left to right) how Earth’s magnetic field undergoes a polarity shift as indicated by the gold and blue lines switching positions. Images credit: NASA; diagram credit: Hugh Ross
Carrington events (giant solar flares that impact Earth’s magnetosphere) such as the one that occurred on September 1–2, 18598 also temporarily weaken Earth’s magnetic field.9 During such events Earth’s magnetic field strength can be diminished to just 5–10% of its present strength.
Consequence of Changes in Earth’s Magnetic Field
In last week’s blog, I explained how a planet lacking a magnetic field or possessing a magnetic field much weaker than Earth’s would receive such an enormous flux of ionizing radiation impinging on its surface that the planet would experience a relatively rapid loss of its surface water. In addition to such a planet being completely desiccated, its surface would be bathed in cosmic radiation and radiation from its host star so intense and deadly that no life-form could possibly survive.
How then did life on Earth survive from 3.8–1.7 billion years ago when Earth’s magnetic field was weaker than it is now or the many episodes during the past 1.7 billion years when it briefly weakened? Lingam first demonstrated that, though Earth’s magnetic field was weaker when Earth’s geodynamo was multipolar rather than dipolar, it was, nevertheless, strong enough to prevent the loss of Earth’s surface water and to preserve the microbial life that exclusively made up Earth’s biodiversity at that time.
The Harvard researcher then determined the consequences of the later brief episodes when Earth’s magnetic field dropped to as low as 5–10% of its present strength level. He showed that, even for the longest of these episodes, the radiation dose received by Earth’s life increased by only a factor of three times and, typically, it was only double or less. This increase, Lingam showed, was too low to cause the extinction of any species of life.
Lingam also calculated that the atmospheric escape rates (for water) during the brief episodes when Earth’s magnetic field strength declined to 5–10% of its present level increased by no more than a factor of two times. Given the brevity of these episodes, this doubling of the atmospheric escape rates would result in no significant loss of water from Earth’s surface.
Consequences of Magnetic Field Changes for Mars and Exoplanets
The situation for Mars and planets beyond the solar system differs markedly. Mars’ dynamo shut down 4.0–4.1 billion years ago and has never restarted since then.10 Furthermore, the lower a planet’s atmospheric pressure, the more rapid its rate of water loss and the greater its radiation dose at the planet’s surface. While the shutdown of Mars’ dynamo might not have been immediately detrimental to possible life, the long-term effects of the dynamo shutdown would have been permanently catastrophic and would guarantee the loss of all Mars’s surface water.
Lingam’s research showed that a long-term shutdown of any planet’s dynamo makes it uninhabitable. For short-term shutdowns or for planets with magnetic fields weaker than 5% of Earth’s present level, there are two other important habitability factors. First, if the planet orbits a star less massive than the Sun (such stars comprise 95%11 of the total population of stars) at a distance where liquid water can possibly exist on its surface, it will be exposed to much more frequent and intense stellar flares and to much stronger stellar winds than is the case for Earth. Such exposure will deplete water from the planet at a more rapid rate and deliver a stronger radiation dose to the planet’s surface. Second, if the planet has a lower atmospheric pressure, water will likewise be depleted from the planet at a more rapid rate and the planet’s surface will receive a much stronger dose of radiation.
One design inference I see arising from Lingam’s research is that our planet and solar system possess several fine-tuned features pertaining to Earth’s magnetic field. These features (for example, planetary rotation rate, the thicknesses of the inner and outer cores, the relative abundances of elemental isotopes in the inner and outer core, the viscosity at the core-mantle boundary and at the inner core-outer core boundary, the Reynolds numbers in the inner and outer cores) ensure that throughout the past 3.8 billion years Earth’s magnetic field has either been strong enough or the duration when it is weak, brief enough, that life on Earth has not been seriously impacted. A second design inference conveys that life on Earth has consistently possessed the necessary features that allow it to tolerate brief time periods when Earth’s surface radiation dose is 2–3 times greater than it is now. Such design inferences imply the mind and work of a supernatural, super-intelligent Creator.