It is unknown how and why Mars transitioned from once being warm and wet to now cold and dry. A longstanding hypothesis is that an early thick atmosphere was lost due to the decline of a dynamo once generated in its churning metallic core. Here, we describe how future laboratory measurements of returned samples like those being collected by the Perseverance rover can test this idea by establishing the lifetime, intensity, and direction of the ancient magnetic field. These measurements can also constrain other key processes in Martian evolution including how the field was generated, the possibility of plate tectonics, the mineralogy of the crust, how water and lavas flowed on the surface, and even whether the samples preserve fossils.
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