Cowan NB, Robinson T, Livengood TA, Deming D, Agol E, A'Hearn MF, Charbonneau D, Lisse CM, Meadows VS, Seager S, et al. Rotational Variability of Earth's Polar Regions: Implications for Detecting Snowball Planets. \apj. 2011;731 (1) :76.