Wouldn’t frozen CO2 (dry ice) also disappear though? Do we know it was WATER ice and not dry ice?
Per the NASA press release today, it’s not cold enough for dry ice to last as long as this ice did:
http://phoenix.lpl.arizona.edu/06_20_pr.php
“The key new evidence is that chunks of bright material exposed by digging on June 15 and still present on June 16 had vaporized by June 19. “This tells us we’ve got water ice within reach of the arm, which means we can continue this investigation with the tools we brought with us,” said Mark Lemmon of Texas A&M University, College Station, lead scientist for Phoenix’s Surface Stereo Imager camera. He said the disappearing chunks could not have been carbon-dioxide ice at the local temperatures because that material would not have been stable for even one day as a solid.”
Same question asked on the NASA photo blog. The answer is that the air temperature in the Martian summer (which is now) is much too high for solid CO2 to exist---it has long already vaporized--so what is left pretty much HAS to be water ice.