Mt Holyoke AST 330 - Mars’ Magnetic Field

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Mars’ Magnetic Field:Crustal Magnetization as Evidence for an Ancient DynamoSolomon Granor12/8/02Astronomy 330Since the earliest Martian exploration, Mars’ magnetic field has perplexed muchof the scientific community. It has challenged theories of planetary magnetism andinspired much unjustified assumption and speculation in scientific papers. Overall, theworst setback arose from a near total lack of data. While over a dozen successful probemissions have visited Mars, only six of these carried magnetometers.The first human probe to visit Mars, Mariner 4, also made the first measurementsof Mars’ magnetic field. These measurements are significantly limited by the flybynature of the Mariner 4 mission; however, they are enough to limit any inherent Martiandipole to 3*10^-4 times the strength of that of Earth’s. The most relevant aspect of theMariner 4 data is a fluctuation of ~5*10^-5 Gauss, lasting for approximately 3 hours.This has both been interpreted as the probe entering Mars’ bow shock and as a totallyrandom fluctuation in the stellar magnetic field of the sort that the probe had encountereden route.1Much great controversy has arisen over the data from the Russian Mars 2, 3, and5 probes – the next probes to carry magnetometers to Mars. These data were heavilystudied and modeled in the late 1970’s and the 1980’s, as no other Martian magnetic datawould be taken until 1989.2 The data from the Mars series probes are far from ideal dueto the eccentricity of the probes’ orbits and their distance from the planet. Worse, theprobes’ magnetometers were not correctly calibrated, and could, thus, only return data onthe relative field strength at different locations. Nevertheless, each of these probesreturned at least one set of data magnetic data that has been interpreted as a disturbance insome way due to Mars. Data from Mars 2 were suggestive to some of a weak dipolemoment having its axis in the equatorial plane. A series of data from Mars 3 showedevidence of entry into a magnetosphere or magnetosheath. Finally, Mars 5 repeatedlyreturned data suggestive of passage through a magnetic tail on the night side of theplanet. It is worthwhile to note that only the first of these cannot be explained by effectsarising from changes in the interplanetary magnetic field due to the very presence of theplanet.3While much analysis and modeling were performed with the Mars 2, 3, and 5data, they alone never led to a coherent picture of Mars’ field, as the inferences from thedifferent data sets were always incompatible. Some of this confusion was resolved in1989 with data from the Russian Phobos 2 probe. During the two months before it waslost, Phobos 2 collected magnetic data, coming as close to Mars as 800 km in its 8-hourcircular orbit. This data strengthened the case for the effects seen being the result of theplanet’s interaction with the interplanetary magnetic field. The most compellingevidence for this strongly correlated the direction of Mars’ magnetic tail with theinterplanetary magnetic medium.2Finally, in 1997, with the arrival of Mars Global Surveyor into orbit, sufficientdata to fully characterize the Martian magnetic field began to arrive. Even the earliestdata received alone would have been enough to revolutionize the study of Mars’magnetic field, when, on September 15, MGS passed through and unequivocally detectedMars’ bow shock. Within a month, MGS had detected substantial enough evidence forthe Global Surveyor team to announce publicly that not only does Mars have a magneticfield, but that it is at least dominated by the magnetization of the planet’s crust.4Now, with several years’ more data, maps of Mars’ magnetic field 400 km abovethe surface (the altitude at which MGS orbits) have been created. As a sphere ofuniformly magnetized crust will give off no field, these maps are related to the relativemagnetizations of different areas of the crust, rather than to the actual magnetization.Comments here about such maps will be related specifically to the map in Connerney et.al. [2001] unless specifically noted otherwise. This map was created by taking themedian value from 18 mapping cycles for every block on the Martian surface; where alength of 1 degree of longitude defines a block. Only values taken on the night side ofthe planet during the 18 cycles were used in the process, in order to reduce the influenceof external fields.5The map shows Mars’ magnetization to be largely uniform with the vast majorityof deviations being along the equator and in the Terra Cimmeria/Terra Sirenum region ofthe southern hemisphere. In this later region, both the radial and colatitude componentsof the field show a striated pattern evocative of seafloor spreading on Earth. Themaximum variation in field strength over the striations is ~400 nT. This region couldvery well be what Mars 2 detected as evidence of a dipole moment.Except along the Tharsis bulge, where any magnetization seems to have beenerased, the northern boundary of the equatorial magnetic strip coincides strongly with theboundary between the southern highlands and the northern lowlands. This suggests thatthe magnetization is older than the northern surface, as the northern surface is known tobe younger than the southern.The other regions of significant interest on the map are the Hellas and Argyreimpact basins. Neither of these shows deviation from the background magnetization, butthere is net magnetization in some of the ejecta from Hellas. These patterns suggest thatboth of these impacts occurred after the crustal magnetization was formed. Since eitherof these impacts was large enough to raise the impacted surface past its Curietemperature, erasing any pre-existing magnetization, these impacts could not haveoccurred while the magnetization was forming, as the presence of a magnetic field strongenough to create the magnetizations seen in Terra Cimmeria and Terra Sirenum wouldhave left these impact basins as magnetic anomalies as well. Further, it is unlikely thatthe impacts occurred before the magnetization, as the ejecta from Hellas would not havebeen magnetized in that case. Thus, the lack of a magnetic signature in Hellas or Argyresuggests that the impacts occurred after the cessation of the strong magnetic field thatcreated the crustal magnetization.6The source of the magnetization is much more difficult to determine than itsimplications for the Martian timeline.


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