Bandfield suggests a near-unity value for Apollinaris Patera from TES data (0.98 +_ 0.02 or so)

Icarus 202 (2009), 414–428

The exact value of the Mars Global Average Emissivity is not constant, as it can vary depending on the location, time, and frequency of observation. However, a commonly cited value for the MGAE is around 0.9, which suggests that the Martian surface is relatively efficient at emitting thermal radiation.