Magnetic field morphology correlated with surface slopes at the Gerasimovich lunar magnetic anomaly

1 — Introduction

The Moon sustained a global magnetic field at some point in its past, likely driven by a dynamo. Although the global magnetic field no longer exists, multiple regions of magnetic anomalies are found in the Moon’s crust. Currently, there is no agreed-upon consensus for how any of these anomalies formed. Determining the formation mechanism of these anomalies would allow us to constrain the size of the source bodies and the timing of their emplacement. In turn, this would help constrain estimates of the Moon’s dynamo field strength and history.

The group of Gerasimovich magnetic anomalies, located west of the Orientale basin and near the Crisium basin antipode, exhibit some of the strongest magnetic fields observed from lunar orbit (Figure 1). Previous work has hypothesized that the high magnetic fields in the Gerasimovich region are due to magnetized ejecta from the impact that formed the Crisium basin. This work tests the antipodal ejecta hypothesis by analyzing the relationship between topography, surface slope, and magnetic field in the region of the Gerasimovich magnetic anomalies.

 

2 — Methods

Correlations between topography, slope, and magnetic field maps — First, we demonstrate that areas of relatively low slope in the Gerasimovich region are co-located with areas of high magnetic field by comparing topography, topographic slope, and magnetic field maps of the region. For our magnetic field data, we use the Tsunakawa et al. (2015) Surface Vector Magnetometer (SVM) model at a resolution of 5 pixels per degree (ppd). For our topographic data, we use Lunar Orbiter Laser Altimeter (LOLA) maps with a resolution of 512 ppd. We create surface slope maps by calculating the arctangent of the magnitude of the topography gradient.

Modeling magnetic field based on slope — We forward-model the magnetic field from source bodies whose magnitude is set by areas of low surface slope to demonstrate that it reproduces features in the observed magnetic data. We use a grid of 40,000 uniformly spaced dipoles with the same resolution as the SVM map, 5 ppd. We set the moments of the dipoles according to the locally normalized values of either topography or slope. For topographic elevation, we set the dipole moments to be inversely proportional to the topography values, and for topographic slope, we set the dipole moments to be inversely proportional to the surface slope values. In each case, the dipole moments are varied linearly between the two extremes. The peak moment was set to be 10¹¹ A⋅m², which was chosen such that the peak magnitude of the resulting magnetic field map matched the peak magnitude observed field in the area. Finally, we linearly decreased the magnetic moment of the dipoles as a function of distance from the center of the modeling space, to more accurately represent the finite, grouped nature of the magnetic anomalies.

Crater fill thickness and magnetization strength — Within the study area we assess the fill thicknesses (i.e., materials deposited inside the crater after their formation) for four craters older than Crisium. We measure the depths of these craters and compare them to the values expected from a statistically-derived depth-diameter relationship from Krüger et al. (2018). Assuming that the difference between the measured and expected crater depth values is due to magnetic fill, we then convert the fill thickness into a source body magnetization.

3 — Results

Correlations between topography, slope, and magnetic field maps — By plotting 100 and 150 nT magnetic field contours over surface slope maps (Figure 2), we found that the largest contiguous blocks of strong field are associated with low-to-moderate slopes, supporting our hypothesis that ejecta from the antipodal Crisium impact moved down pre-existing high-slope crater rims upon landing. We also find that the rim slopes of several relatively small craters, all mapped as Orientale secondaries, bound the area of high magnetic field, suggesting that any pre-existing magnetism beneath these craters’ footprints was destroyed by impacts.

 

Modeling magnetic field based on slope — We find that the model field map produced using slope values for dipole strength has good agreement with many of the actual field observations and is superior to the analogous model field map produced using topography values for dipole strength (Figure 3).

 

Crater fill thickness and magnetization strength — We find that all four of our study craters are anomalously shallow compared to their statistically expected depths. Additionally, profiles of their topography show they are shallower, or at least the same depth, as comparison craters elsewhere on the Moon of similar diameter and younger or similar age (Figure 4). For the crater Gerasimovich, we find a fill thickness of ~0.8km, which implies a source body magnetization of ~4.6 A/m.

 

4 — Conclusions

We have found that the areas of high surface magnetic field (>100 nT) around the Crisium antipode are collocated with areas of low surface slope. A model of magnetic field strength setting the rock magnetization inversely proportional to the surface slope reproduces key features in the observed magnetic field. Our interpretation is that ejecta from the Crisium-forming impact filled pre-existing craters and became magnetized after landing. Ejecta moved downslope upon landing and thereby avoided collecting on high-slope crater rims. Later smaller impacts also demagnetized the surficial magnetized layer, and their high-slope rims now bound the regions of strong magnetization. We inferred rock magnetization strength of ~4.6 A/m inside the Gerasimovich crater based on ~0.8 km of fill deposits inside it. Either way, all observations require a dynamo field existing during the time of Crisium basin formation, likely on the order of ten or even tens of microtesla in strength.

5 — References

• Krüger et al. (2018), Deriving Morphometric Parameters and the Simple-to-Complex Transition Diameter From a High-Resolution, Global Database of Fresh Lunar Impact Craters (D ≥ 3 km). JGR Planets 123, 2667–2690.
• Tsunakawa et al. (2015), Surface vector mapping of magnetic anomalies over the Moon using Kaguya and Lunar Prospector observations. JGR Planets 120, 1160–1185.