Low-altitude magnetic field measurements by MESSENGER reveal Mercury’s ancient crustal field Catherine Johnson et al. 2015

North-South asymmetry in magnetic field Figure 1 from APL 2012 report Indicates North-South asymmetry in dynamo

North-South asymmetry in magnetic field Figure 3 from APL 2012 report Red shading shows areas subject to solar wind

What is MESSENGER measuring? Measurements dominated by magnetopause and magnetotail. – Subtracted with magnetospheric models developed with MESSENGER data. Remnant long-wavelength (~ km) signals of ~30 nT magnitude, change substantially with each orbit. – Subtracted out with each orbit’s data and a high pass filter

What is MESSENGER trying to measure? Short-wavelength, small-amplitude signals coming from planet’s interior. – Waves are allowed through high pass filter Source depth estimates of 7 to 45 km, suggesting magnetized rocks are source of higher frequency signal.

Considerations Peak amplitudes of last Figure (20nT) observed at lowest flight altitude (15km). Constraining time of magnetization is difficult due to varying ages of surface topography. – Smooth plains give lower limit on age of magnetization (3.7 to 3.9 Ga). Means the magnetic field is at least this old. Chemically reduced characteristics of surface suggest iron metal, iron alloys, and iron sulfides. – Troilite may be more stable than pyrrhotite on Mercury, but this is unconfirmed

Results Magnetization values of Am -1, similar to that of the Moon. Level of magnetization requires a ~4500 nT field for magnetization depth of 25 km. – This result is a minima, as it is inferred from the lower bound of magnetization while assuming all iron is magnetized. Even Earth-like fields (~50,000 nT) are possible with 0.1-5% of iron magnetized.

Implications for dynamo Presents new challenge for models. – Must form core dynamo at time of smooth plains emplacement via: Superadiabatic (expansion) cooling of liquid core. – Typically ends by 3.9 Ga in models Cooling and compositional convection due to formation of solid inner core. – Typically does not start until well after 3.7 Ga in models

Questions?

References APL, Mercury’s Oddly Offset Magnetic Field, Johnson, C. et al., Low-altitude magnetic field measurements by MESSENGER reveal Mercury’s ancient crustal field, Science, 348, 6237, 2015.