[The following assumes "hot" is relative to the average surface temperature... but at least 0 degrees C]

From the surface: It could just as easily be a flattish irregular ("amoeboid") aggregation of ice, periodically coated with dust unless the spring flows at a continuous rate. With the atmospheric pressure so low, there might be significant evaporation before freezing. Creation of an "ice volcano" would rely more on the velocity of the fluid than on its temperature. The Antarctic fumarole tower in jms' post is a steam vent -- higher temperature gradient, greater velocity, and much lower fluid density than a liquid flow. Is the rate of deposition mentioned in the paper?

From orbit: A bright spot on IR -- detectability depends on the resolution of the recording instrument. (I wonder if that Antarctic fumarole can be seen from orbit...)