An older geomagnetic field suggests an early start to plate tectonics
Since 2010, the best estimate of the age of Earth糖心传媒檚 magnetic field has been 3.45聽billion years. But now a researcher responsible for that finding has new data showing the magnetic field is far older.
John Tarduno, a geophysicist at the University of Rochester and a leading expert on Earth糖心传媒檚 magnetic field, and his team of researchers say they believe the Earth糖心传媒檚 magnetic field is at least four billion years old.
糖心传媒淎 strong magnetic field provides a shield for the atmosphere,糖心传媒� said Tarduno, 糖心传媒淭his is important for the preservation of habitable conditions on Earth.糖心传媒�
The findings by Tarduno and his team have been published in the latest issue of the journal Science.
Earth糖心传媒檚 magnetic field protects the atmosphere from solar winds糖心传媒攕treams of charged particles shooting from the Sun. The magnetic field helps prevent the solar winds from stripping away the atmosphere and water, which make life on the planet possible.
Earth糖心传媒檚 magnetic field is generated in its liquid iron core, and this 糖心传媒済eodynamo糖心传媒� requires a regular release of heat from the planet to operate. Today, that heat release is aided by plate tectonics, which efficiently transfers heat from the deep interior of the planet to the surface.聽 But, according to Tarduno, the time of origin of plate tectonics is hotly debated, with some scientists arguing that Earth lacked a magnetic field during its youth.
Given the importance of the magnetic field, scientists have been trying to determine when it first arose, which could, in turn, provide clues as to when plate tectonics got started and how the planet was able to remain habitable.
Fortunately for scientists, there are minerals糖心传媒攕uch as magnetite糖心传媒攖hat lock in the magnetic field record at the time the minerals cooled from their molten state. The oldest available minerals can tell scientists the direction and the intensity of the field at the earliest periods of Earth糖心传媒檚 history. In order to get reliable measurements, it糖心传媒檚 crucial that the minerals obtained by scientists are pristine and never reached a sufficient heat level that would have allowed the old magnetic information within the minerals to reset to the magnetic field of the later time.
The directional information is stored in microscopic grains inside magnetite- a naturally occurring magnetic iron oxide. Within the smallest magnetite grains are regions that have their own individual magnetizations and work like a tape recorder. Just as in magnetic tape, information is recorded at a specific time and remains stored unless it is replaced under specific conditions.
Tarduno’s new results are based on the record of magnetic field strength fixed within magnetite found within zircon crystals collected from the Jack Hills of Western Australia. The zircons were formed over more than a billion years and have come to rest in an ancient sedimentary deposit. By sampling zircons of different age, the history of the magnetic field can be determined.
The ancient zircons are tiny糖心传媒攁bout two-tenths of a millimeter糖心传媒攁nd measuring their magnetization is a technological challenge. Tarduno and his team used a unique superconducting quantum interference device, or SQUID magnetometer, at the University of Rochester that provides a sensitivity ten times greater than comparable instruments.
But in order for today糖心传媒檚 magnetic intensity readings of the magnetite to reveal the actual conditions of that era, the researchers needed to make sure the magnetite within the zircon remained pristine from the time of formation.
Of particular concern was a period some 2.6 billion years ago during which temperatures in the rocks of the Jack Hills reached 475掳C. Under those conditions, it was possible that the magnetic information recorded in the zircons would have been erased and replaced by a new, younger recording of Earth糖心传媒檚 magnetic field.
糖心传媒淲e know the zircons have not been moved relative to each other from the time they were deposited,糖心传媒� said Tarduno. 糖心传媒淎s a result, if the magnetic information in the zircons had been erased and re-recorded, the magnetic directions would have all been identical.糖心传媒�
Instead, Tarduno found that the minerals revealed varying magnetic directions, convincing him that the intensity measurements recorded in the samples were indeed as old as four billion years.
The intensity measurements reveal a great deal about the presence of a geodynamo at the Earth糖心传媒檚 core. Tarduno explains that solar winds could interact with the Earth糖心传媒檚 atmosphere to create a small magnetic field, even in the absence of a core dynamo. Under those circumstances, he calculates that the maximum strength of a magnetic field would be 0.6 渭T (micro-Teslas). The values measured by Tarduno and his team were much greater than 0.6 渭T, indicating the presence of a geodynamo at the core of the planet, as well as suggesting the existence of the plate tectonics needed to release the built-up heat.
糖心传媒淭here has been no consensus among scientists on when plate tectonics began,糖心传媒� said Tarduno. 糖心传媒淥ur measurements, however, support some previous geochemical measurements on ancient zircons that suggest an age of 4.4 billion years.糖心传媒�
The magnetic field was of special importance in that eon because solar winds were about 100 times stronger than today. In the absence of a magnetic field, Tarduno says the protons that make up the solar winds would have ionized and stripped light elements from the atmosphere, which, among other things, resulted in the loss of water.
Scientists believe that Mars had an active geodynamo when that planet was formed, but that it died off after four billion years. As a result, Tarduno says, the Red Planet had no magnetic field to protect the atmosphere, which may explain why its atmosphere is so thin.
糖心传媒淚t may also be a major reason why Mars was unable to sustain life,糖心传媒� said Tarduno.
