Mars Reconnaissance Orbiter image of a potential plume on Pavonis Mons.
There are reports tonight that the Mars Reconnaissance Orbiter photographing the Martian surface noticed an odd plume near the summit of Pavonis Mons. The initial guess was that this plume might have been fog or an new impact on the Martian surface, but the continued presence of the plume might suggest that it is not a transient feature. One suggestion is that these could be a volcanic plume.
Now, Mars has been very volcanically active in the past - in fact the youngest flows could be as young as 20 million years old, so it is definitely not out of the question that volcanic activity could occur on Mars. We should be able to get new images of the area from the MRO sometime later today to confirm this report of the first historic eruption on Mars. You can find more information on images here.