Mercury's weak core field can account for the crustal magnetism across most of its mapped north

Induced magnetization scales with the field doing the inducing and with the amount of magnetizable material sitting in that field. Mercury is short of both. Its core field is about a hundred times weaker than Earth's ^[6], and the crust is poor in iron, which keeps the induced signal small ^[7]. The study's route to a large enough total is depth. Assume the magnetization runs through the crustal column to roughly 30 km, and the integrated signal matches what was observed over more than 85 percent of the area north of 38 degrees north ^[4]. The model itself was built from the low-altitude field observations, with assumptions supplied for crustal thickness, magnetic mineralogy, iron content and the present-day core field ^[3].

One thing in the mapping pulls against the simple version of that story. Induced magnetization is a measure of where crustal iron is and how much of it there is ^[19], and yet the pattern of magnetization strength does not consistently match the near-surface iron abundance recorded by MESSENGER's spectroscopy ^[8]. The strongest magnetization is in the Caloris region, with localized signals elsewhere ^[10].

That 85 percent needs a denominator. The spherical cap north of 38 degrees is about 19 percent of a sphere's surface ^[1], so the area where induction suffices is at least about 16 percent of Mercury ^[2]. The crust south of that line is outside the claim.

The low-altitude data exist because of a change made while the spacecraft was flying. MESSENGER observed Mercury from 2011 to 2015 ^[1]. Lon Hood of the Lunar and Planetary Laboratory at the University of Arizona, who has worked extensively on Mercury's crustal magnetism with the mission data and was not involved in the new study, said a last-minute decision was made "to reduce the orbit altitude thereby allowing crustal fields to be detected" ^[11][12]. The expectation at the time, Hood said, was that crustal magnetism would be "partly due to remanent magnetization dating from the time when the crustal sources formed in the planetary magnetic field" ^[13]. Mercury and Earth are the only planets in the solar system with both a core-generated global field and a magnetized crust ^[20].

Telling the two magnetizations apart is the hard part, and on Earth it takes laboratory measurements of rock samples ^[14]. Mercury has yielded no samples, so the magnetic properties of its crust have to be inferred from satellite observations under assumptions about composition and magnetic behaviour ^[15]. Hood said the new delineation depends on "assumptions about magnetic mineralogy that cannot be verified due to the lack of returned samples" ^[16].

The thing this does not tell you is whether Mercury ever ran a dynamo early in its history. It says that across most of the mapped north, no remanent magnetization is needed to explain the observed crustal field ^[5]. Outside that majority, the study allows that a remanent contribution may still be required ^[18].