Mercury’s Magnetic FieldKerri Donaldson HannaImage Courtesy of NASAParts of a Magnetic FieldMariner 10 Observations• Took measurements on the 1st and 3rdflyby ~43 minutes worth of datacollected• Signals measured the bow shock, andentrance and exit from themagnetopause Has an Earth-like magnetosphere• Magnetospheric cavity ~20 timessmaller than the Earth’s• Field strength ~ 300 - 400 nT (T =Telsa the unit to measure magneticintensity) 1% of Earth’s magnetic fieldstrength• Dipole field with the same polarity asEarth’s present magnetic fieldComparing to Earth’s Magnetic Field• Earth’s magnetic field well-known• Can build a model with Mariner 10 data and what weknow about the Earth’s magnetic field• Mercury fills a greater portion of its magnetosphere thanthe Earth• The cusp regions, where the field lines intersect the surfaceof the planet are probably at lower latitudes on Mercurythan on the Earth• The magnetosphere probably fluctuates more rapidly andmore often as a result of perturbations from the solar wind -I.e. flares and coronal mass ejections (CMEs)• Mariner 10 results could be biased due to comparisonswith Earth’s model• Still can’t determine if field is a remanent field orgenerated by an active dynamoMagnetic Field via Active Dynamo1. Rapid planetaryrotation to getconducting materialmoving about2. Convection of anelectricallyconducting fluid inthe liquid outer coreCourtesy of https://www.physast.uga.eduFor Active Dynamo Need:As conducting fluid flows across an existing magnetic field, electric currents are induced, which in turn creates or sustains a magnetic fieldActive Dynamo on Mercury?• If Mercury’s magnetic field is dipolar and there is afluid outer core a physical mechanism to maintainhigh core temperatures is needed:1. provide more internal heat by adding more radioactiveelements to the core such as uranium or thorium2. retain heat longer by reducing the thermal diffusivity of themantle3. add light alloying element to lower the melting point of Fe 3. is the most likely case with sulfur being the likely culprit,would only need 7% S for the entire core to be fluid at thepresent time Need further knowledge on the elemental surfacecompositionProblems with an Active Dynamo on Mercury• An Earth-like dynamo would be expected to produce amuch stronger field than what is observed at Mercury• Two methods used to estimate magnetic field strength:energy balance and magnetostrophic balance• Need to consider whether Mercury’s dynamo operatesin a thin- or thick-shell core geometry• Thin-shell - thin fluid shell surrounding a large solid inner core• Thick-shell - thicker fluid shell surrounding a smaller solidinner core• Sensitive function of core sulfur content and initial coretemperatures Magnetic field measurements by MESSENGER shouldbe able to distinguish between a thin- or thick-shellcore geometryRemanent Field on Mercury?• Remanent field could have been induced by a largeexternal magnetic field (solar or nebular) or by aninternal dynamo that existed earlier in the planet’sevolution• Remanent fields exist on Mars and the Moon• Would require a thick layer of abundant magneticminerals (possibly greater than 30 km deep)• Magnetic minerals responsible for crustal magneticremanence must be able to acquire and preservemagnetic fields• On the Moon it is metallic Fe, on Mars it istitanomagnetite, titanohematite, or pyrrhotite• On Mercury pyrrhotite, iron sulfide, might be likely -- it could be a source of sulfur in the atmosphereand deposits in high-latitude cold craters with highradar backscatter signalsProblems with a Remanent Magnetic Fieldon Mercury• At first it wasn’t believed that surface regions thathad high enough remanent magnetization toexplain the field strength measured existed• But on the Moon and on Mars localized regionshave been measured to have several tens of nTup to 200 nT in field strength• Not enough iron on Mercury’s surface formagnetic minerals• However material could be buried• Or iron sulfides such as pyrrhotite could be themagnetic source materialWhat could MESSENGER tell us?• MESSENGER will be able to determine if field isdue to an active dynamo, a remanent field orboth• If magnetic field structure correlates with gravity dataindicating topography at the core-mantle boundary --likely dynamo• If it is a dynamo, could determine the thickness of thefluid outer core in relation to the size of the solidifiedinner core• If there is small-scale magnetic structure with ashallow source depth -- likely remanent field• If there is a remanent field, could determine whatminerals are on the surface or buried that wouldpreserve the remanent fieldMESSENGER 1st Flyby Results• MAG measuredmagnetic fieldstrength• MAG identified allboundary locationsin themagnetosphere• Found thatMercury’smagnetospherewas a morequiescent systemthan observed byMariner 10• Results consistentwith the FIPS andEPPS instrumentsCredit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of WashingtonMESSENGER 1st Flyby Results, Cont.• This is a simulatedmodel not measuredduring flyby• Measured during quietsolar conditions andequatorial pass• Dipole field nearlyaligned with planet’sspin axis• Field strength weakerby 1/3 than thatmeasured by Mariner10 - likely due todifferences intrajectories• Dipolar fieldconsistent with anactive dynamo• Could not determine ifmagnetic crustalanomalies existCredit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of