Hi there! The Moon is so close and so familiar to us, but I realize this is my first post on it. Today I present you a paper entitled South Pole Aitken Basin magnetic anomalies: Evidence for the true polar wander of Moon and a lunar dynamo reversal, by Jafar Arkani-Hamed and Daniel Boutin, which will be published soon in Journal of Geophysical Research: Planets. The idea is to track the variations of the magnetic field of the Moon along its history, as a signature of the motion of its rotation pole, i.e. of a polar wander.

Our Moon’s facts

The Moon is a fascinating object, as it is the only known natural satellite of the Earth, and we see it as large as the Sun in our sky. It orbits around the Earth at a distance of almost 400,000 km in 27.3 days. It shows us always the same face, as a result of a tidal locking of its rotation, making it synchronous, i.e. its spin period is equal to its orbital period.

Something interesting is its pretty large size, i.e. its radius is one fourth of the one of the Earth. It is widely admitted that the Moon and the Earth have a common origin, i.e. either a proto-Earth has been impacted by a Mars-sized impactor, which split it between the Earth and the Moon, or the Earth-Moon system results from the collision of two objects of almost the same size. In both cases, the Earth and the Moon would have been pretty hot just after the impact, which also means active… and this has implications for the magnetic field.

A very weak magnetic field has been detected for the Moon, but which is very different from the Earth’s. The magnetic field of the Earth, or geomagnetic field, has the signature of a dipolar one, in the sense that it has a clear orientation. This happens when the rotating core acts as a dynamo. The north magnetic pole is some 10° shifted from the spin pole of the Earth, and has an amplitude between 25 and 65 μT (micro-teslas). However, the magnetic field of the Moon, measured at its surface, does not present a clear orientation, and never reaches 1 μT. Its origin is thus not obvious, even if we could imagine that the early Moon was active enough to harbor a dynamo, from which the measured magnetic field would be a signature… But the absence of preferred orientation is confusing.

The core dynamo

The core of the Earth spins, it is surrounded by liquid iron, which is conductive, and there is convection in this fluid layer, which is driven by the heat flux diffusing from the core to the surface of the Earth. This process creates and maintains a magnetic field.

For the Earth, the core dynamo is assumed to account for 80 to 90% of the total magnetic field. This results in a preferred orientation. Other processes that could create a magnetic field are a global asymmetry of the electric charges of the planet, or the presence of an external magnetic field, for instance due to a star.

A dynamo could be expected for many planetary objects, which would be large enough to harbor a global fluid layer. It is usually thought that the detection of a magnetic field is a clue for the presence of a global ocean. Such a magnetic field has been detected for Jupiter’s moon Ganymede, which is probably due to an outer liquid layer coating its iron core.

The Moon has probably no dynamo, but could have had one in the past. The measured magnetic field could be its signature. A question is: what could have driven this dynamo? The early Moon was hotter than the current one, so a magnetic field existed at that time. And after that, the Moon experienced intense episodes of bombardment, like the Late Heavy Bombardment. The resulting impacts affected the orientation of the Moon, its shape, and also its temperature. This could have itself triggered a revival of the magnetic field, particularly for the biggest impact.

The study I present today deals with measurements of the magnetic field in the South Pole-Aitken Basin, not to be confused with the Aitken crater, which is present in its region. The South Pole-Aitken Basin is one of the largest known impact crater in the Solar System, with a diameter of 2,500 km and a depth of 13 km. This basin contains other craters, which means that it is older than all of them, its age is estimated to be 4.1 Gyr (gigayears, i.e. billions of years). Measurements of the magnetic field in each of these craters could give its evolution over the ages. But why is it possible?

The magnetic field as a signature of the history

When a material is surrounded by a magnetic field, it can become magnetic itself. This phenomenon is known as induced magnetization, and depends on the magnetic susceptibility of the material, i.e. the efficiency of this process depends on the material. Once the surrounding magnetic field has disappeared, the material might remain magnetic anyway, i.e. have its remanent magnetic field. This is what has been measured by the Lunar Prospector mission, whose data originated this study.
An issue is the temperature. The impact should be hot enough to trigger the magnetic field, which implies that the material would be hot, but it cannot be magnetized if it is too hot. Below a Curie temperature, the process of induced magnetization just does not work. You can even demagnetize a material in heating it. For the magnetite, which is a mineral containing iron and present on the Moon, the Curie temperature is 860 K, i.e. 587°C, or 1089°F.

Lunar Prospector

This study uses data of the Lunar Prospector mission. This NASA mission has been launched in January 1998 from Cape Canaveral and has orbited the Moon on a polar orbit during 18 months, until July 1999. It made a full orbit in a little less than 2 hours, at a mean altitude of 100 km (60 miles). This allowed to cover the whole surface of the Moon, and to make measurements with 6 instruments, related to gamma rays, electrons, neutrons, gravity… and the magnetic field.

Results of this study

This study essentially consists of two parts: a theoretical study of the temperature evolution of the Moon over its early ages, including after impacts, and the interpretation of the magnetic field data. These data are 14 magnetic anomalies in the South Pole-Aitken Basin, which the theoretical study helps to date. And the data show two orientations of the magnetic field in the magnetic in the past, giving an excursion of more than 100° over the ages.

Now, if we consider that in the presence of a core dynamo, the magnetic field should be nearly aligned with the spin pole, this means that the Moon has experienced a polar wander of more than 100° in its early life. More precisely, the two orientations are temporally separated by the creation of the Imbrium basin, 3.9 Gyr ago. In other words, the Moon has been tilted. This is not the only case in the Solar System, see e.g. Enceladus.

To know more

• The study. You can also find a shortened version as a conference abstract, here.
• The web page of Jafar Arkani-Hamed.

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.