Even COmet ISON is still a third of a billion miles out from the Sun the comet is coming to life spewing dust and probably carbon dioxide gas as seen in these pics from NASA:
These images from NASA's Spitzer Space Telescope of C/2012 S1 (Comet ISON) were taken on June 13, when ISON was 310 million miles (about 500 million kilometers) from the sun. Image credit: NASA/JPL-Caltech/JHUAPL/UCF
As of June 13 it is estimated that ISON is losing 120 million pounds of dust and 2.2 million pounds of CO2 gas a DAY! These number should increase as the cimet falls inward and ever closer to the Sun. Comet ISON's tail now stretches 186,400 miles and should continue to grow as well.
Estimates for the size of COmet ISON still range at less than 3 miles across with a weight of between 7 billion and 7 trillion pounds - but these numbers should solidify as the comet gets closer and more observations are made.
Late July-August is when the comet gets to the vicinity of Mars and other gases plus water should start coming off of the comet.