However, inside Mars, which is roughly half the diameter of Earth, the geodynamo process cooled rapidly and convection ceased as heat leaked out. This event had significant consequences for the future evolution of Mars. Without a global magnetic field, Mars would not have been able to repel the solar winds that were dissipating its atmosphere, or prevent surface waters from evaporating and harmful cosmic rays from hitting the surface.
Planetary scientists thought that Mars’ magnetic field disappeared about 4.1 billion years ago. The reason for this conclusion was that the large impact basins that formed during the bombardment period between 4.1 billion and 3.7 billion years ago did not preserve any trace of the magnetic field in their rocks.
During a strong collision, ferromagnetic minerals in molten rock can align themselves with the nearby magnetic field. As hot rocks cool slowly upon impact, this level of ferromagnetic minerals can become trapped within them, leading scientists to study the ancient magnetic field billions of years later. However, the evidence from the largest Martian collisions shows that the magnetic field was not involved when these collisions occurred.
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But Steele and his colleagues, including Harvard’s Roger Fu, believe that planetary scientists have misinterpreted the signs. Their analysis of parts of the famous Martian meteorite Allen Hills 84001 in 2023 shows evidence of magnetic field reversals in ferromagnetic minerals within the meteorite. In the 1990s, researchers claimed the presence of microfossils in this meteorite, which was a controversial claim at that time.
Now, researchers have strengthened the claim of magnetic field reversal with computer modeling, which shows that the absence of magnetic field recorded during the formation of impact basins is not due to the disappearance of magnetic forces, but the main reason is the polarity reversal. This happens every few hundred thousand years on Earth, so that the north and south magnetic poles move; In this situation, ferromagnetic materials do not have a clear orientation, and as a result, the magnetic field seems to be weakened or absent. If Steele’s team is right, Mars’ global magnetic field did not disappear about 4.1 billion years ago; Rather, this happened at least 3.9 billion years ago.
Although these time intervals are very ancient, the extra 200 million years could have serious implications for the possibility of life on ancient Mars. The reason for this issue is the overlap with the period during which the surface of Mars was covered with water, the evidence of which has been published by NASA’s Mars rover; Therefore, if the magnetic field existed at that time, life probably had the opportunity to grow in water environments without being killed by cosmic rays.
The possible survival of the magnetic field on Mars for a longer period of time than previously thought could also have affected the rate at which the atmosphere is lost. Scientists can track the rate of water and atmospheric loss and infer what Mars’ thin atmosphere and water surface looked like in the past. If the magnetic field disappeared later, then the atmospheric loss occurred later, and the time course of Mars’ changing conditions needs to be revised.
The research findings have been published in the journal Nature.
