A Late Martian Dynamo Cessation Time 3.77 Gy Ago

Introduction: Several global models ([1] and references therein) of the martian lithospheric magnetic field and/or of its associated magnetization depict similar first-order characteristics. Intense, localized magnetic anomalies are found in the southern hemisphere, while the northern lowlands, as well as the giant basins and the largest volcanic provinces are devoid of significant anomalies [2]. It was concluded that these structures were formed while the dynamo was not active. A cluster of giant basins, similar in age [3], was found with very different magnetic signature, and interpreted as an evidence that the Martian dynamo cessation occurred during the early Noachian between 4.115 and 4.13 Gy [4]. This timing is however incompatible with the magnetic signatures recorded over younger and smaller impact and younger structures. In this study we reconsider the problem of timing the cessation of the dynamo by characterizing the magnetic field (at MGS altitude) inside and outside craters larger than 200-km in diameter. We use the results of theoretical computations [5], which predict that 200-km diameter demagnetized impact basins possess magnetic signatures of the order of 1 nT at 400-km altitude. This implies that such craters can be characterized provided that the magnetic measurements are carefully selected. Our crater database is mainly based on the planetary nomenclature and on the Barlow’s catalog [6]. We complete our study with six Noachian volcanoes. We then compare our results to a time line for these structures. Data and method: Magnetic measurements acquired during the MGS Mapping Orbit cycles are used. We restrict our study to the last martian year of the MGS mission because this epoch is closer to the last solar minimum that occurred in 2008-2009. Only night side measurements are considered. Our selected measurement data set is further subdivided into four groups, each one representing one season of the martian year. Each crater is therefore characterized using three independent data sets which are used to infer the error associated with the statistics we derived. Magnetic measurements (Fig. 1a) are selected inside and outside (up to one radius) rims for each crater and for each of the four martian seasons. Measurements are then averaged onto 0.25 ̊x0.25 ̊bins to minimize small spatial scale variations. Three values are finally computed, the mean intensity inside (Bin) and outside (Bout) the crater rim, and the ratio of these two quantities (Bin/Bout).