Martian Dynamo Change at ~4.1 Ga: Evidence from the Magnetic Measurements of the Iota Crater

The Martian dynamo evolution is critical for understanding the interior properties and climate change of Mars. It has been referred to shut down at ~4.1-4.0 Ga based on the magnetic signatures of large impact craters but be present at ~3.9 Ga and ~3.7 Ga from the paleomagnetic studies and magnetic fields above volcanic units. Here, we investigate the magnetic signatures of the Iota crater, located inside the CT3-G crater with central strong magnetic fields. The Iota crater shows a weak central magnetic field with an inside-outside magnetic field ratio of 0.29. A forward model is established and the results show that the magnetization strength of the retained materials beneath the Iota crater is about 40% of the surrounding, indicating that the dynamo strength at that time became weak. The different magnetic signatures of Iota and CT3-G reveal that the Martian dynamo changed at ~4.1 Ga, but did not stop completely.