The NASA experts said their hypothesis was based on favorable chemistry and episodes with thin films of liquid water during ongoing, long-term climate cycles.
Phoenix ended communications in November as the approach of the Martian winter depleted energy from its solar panels.
"Not only did we find water ice, as expected, but the soil chemistry and minerals we observed lead us to believe this site had a wetter and warmer climate in the recent past -- the last few million years -- and could again in the future," said Phoenix Principal Investigator Peter Smith of the University of Arizona-Tucson.
A paper about Phoenix water studies -- for which Smith is the lead author with 36 co-authors from six nations -- said evidence for water and potential nutrients "implies that this region could have previously met the criteria for habitability" during portions of continuing climate cycles.
Interpretations of data Phoenix returned during its five months of operation are reported in the journal Science.