Horizon Hair of Extremal Black Holes and Measurements at Null Infinity

It is shown that the conserved charges on the event horizon and the Cauchy horizon associated with scalar perturbations on extremal black holes are externally measurable from null infinity. This suggests that these charges have the potential to serve as an observational signature. The proof of this result is based on obtaining precise late-time asymptotics for the radiation field of outgoing perturbations.