NASA's MESSENGER spacecraft has revealed evidence of magnetization of ancient crustal rocks on Mercury.
The spacecraft crashed onto Mercury last week after running out of fuel, but the mission provided a trove of new information on the planet closest to the Sun. About 4 billion years ago, Mercury's magnetic field could have been much stronger than today, as indicated by low-altitude observations.
Catherine Johnson, senior scientist at the Planetary Science Institute and lead author said with MESSENGER orbiting Mercury closer than 100 kilometers from the planet's surface, the spacecraft's magnetometer instrument that measures magnetic field strength and detection was able to resolve signals too small to be detected earlier at higher altitudes.
The observed decrease in signal strength measured with changes in altitude from 15 to 80 kilometers confirms that the signals are due to the presence of magnetized crustal rocks.
Mercury is the only inner solar system body other than Earth that currently possesses a global magnetic field generated by a dynamo in a fluid metallic outer core. In Mercury, as in Earth, the outer core is molten iron.
Johnson said that magnetized rocks recorded the history of the magnetic field of a planet, a key ingredient in understanding its evolution. They already knew that around 3.7 to 3.9 billion years ago Mercury was volcanically and tectonically active, and had a magnetic field at around that time.
Without the recent very low-altitude observations, it would have been impossible to discover these signals, she added.
The paper is published in the journal Science.