TESTING THE ORIGIN OF MARTIAN ‘BLUEBERRIES': MAGNETIC MEASUREMENTS MAY BE KEY TO THE ANSWER

Comparative studies of terrestrial Navajo Sandstone concretions show the striking resemblance to the hematite spherules (“blueberries”) imaged by the Mars Exploration Rover (MER) Opportunity. Microscale texture revealed by the microscopic imager (MI) showed well-sorted sand grains in the laminated rock units where the Mars spherules are abundant. Both the NASA MER work and terrestrial analog studies interpreted the hematite spherules to be concretions precipitated from diagenetic aqueous solutions in the sediment. Alternative hypotheses have argued for an impact surge origin for the hematite spherules on Mars. We suggest future Mars Rover and sample return missions should include instrumentation for in situ magnetic measurements of field intensity and susceptibility. Hematite is a well-studied magnetic mineral on Earth, and has magnetic properties well suited for resolving the origin of the spherules. Although the ferromagnetism of hematite is weak, it can still record ancient magnetic fields, and has the highest blocking-temperature recording the rock's remanence. It may, therefore, survive heating during metamorphism or impact heating that resets the magnetic remanence of other magnetic phases. Minute metal inclusions such as Ni will show distinct properties in magnetic measurements. If hematite spherules formed from an impact surge, then shock intensity may be inferred from the measurements. The bulk susceptibility is useful information for interpretation of paleo- and depositional environment as well as mineralogical analysis. Thus the magnetic signatures give us tools to test the origin of “blueberries”.

Magnetic analyses of Utah and Czech hematite concretions showed unusual and complex behavior of antiferromagnetic hematite-goethite composition. This gives a distinct magnetic signature tied to aqueous hematite formation in the sediment. Magnetite, hematite and goethite, or combinations of these can carry a record of past magnetization events. In situ magnetic analysis of spherules comparable to the terrestrial, may aid in the interpretation of their formation, and may also resolve histories of the impact magnetization/demagnetization processes with implications for magnetic anomalies on Mars.