Regional Mapping of MAVEN Orbital Magnetometer Data: Implications for the Nature of Crustal Field Sources and the Duration of the Mars Dynamo

Previous results of global mapping have shown that Mars crustal magnetism is generally stronger south of the crustal dichotomy boundary (where the crust is thicker) and is strongest in one-third of the Southern Hemisphere.  It is generally weak over ancient impact basins (e.g., Hellas, Argyre,) and is weakest over young volcanic provinces (Tharsis, Elysium).  However, a lack of clear correlations of orbital anomalies with surface geology has inhibited a full understanding of the nature of crustal magnetic field sources.  Here, we present preliminary regional mapping results for the Claritas Fossae region south of Tharsis that shows a more detailed correlation than found before of magnetic anomalies with areas of ancient magmatic activity and uplift.  The possible existence of a magnetic anomaly over Claritas Fossae was first reported by Dohm et al. (2009), based on MGS magnetometer data at higher altitudes.  However, the correlation of anomalies with the Claritas rise is much clearer using the MAVEN data.  The simplest interpretation is that the anomaly sources consist of magmatic intrusions magnetized thermoremanently in the Mars core dynamo magnetic field during the Noachian.  By extension, most or all crustal magnetic anomaly sources on Mars may consist of magmatic intrusions.

While it is accepted that a Mars core dynamo existed during the Early to Middle Noachian when the southern highlands formed and did not exist during the Late Hesperian and Amazonian when the younger volcanic constructs formed, the timing of the final termination of dynamo generation (Middle Noachian, Late Noachian, or Early Hesperian) remains uncertain. Preliminary regional mapping of anomalies over volcanic constructs whose final eruptions occurred in Late Hesperian or later times confirms that crustal fields are relatively weak over the main calderas.  Hadriacus Mons, with a Late Noachian or Early Hesperian model age, has previously been reported to have a magnetization signature based on MGS electron reflectometry data (Lillis et al., 2006).  Preliminary ESD mapping of MAVEN data confirms that an anomaly is present over the central caldera of Hadriacus Mons and its southern flank.  It extends southwestward along the direction of pyroclastic flows into the outer Hellas basin.  Formation of the large valley that dissects Hadriacus Mons’ flanks (Dao Vallis) has been attributed to melting of subsurface ice by magmatic heat, producing a large ‘’outburst flood’’. This interpretation is consistent with the hypothesis that acquisition of strong thermoremanence in Fe-rich volcanic materials occurred mainly in an oxidizing environment.  It is proposed here that this region is a good candidate for future low-altitude magnetometer data acquisition. If such measurements confirm that anomalies are associated with the pyroclastic flow deposits, which have a model age of 3.7 to 3.9 Gyr, it would follow that dynamo activity continued into the Late Noachian or Early Hesperian.