Extended Data Fig. 5 — Measured surface and modeled subsurface conditions at the Phoenix landing site.

Using newly recalibrated environmental data from the Thermal and Electrical Conductivity Probe (TECP) on the Phoenix Lander along with a model of the subsurface, we studied the potential to form liquids at the Phoenix landing site. In (a), we plot the Phoenix measured temperature and relative humidity with respect to liquid (purple points) on the phase diagram of calcium perchlorate (following the color code in Extended Data Fig. 3). As can be seen, several measured conditions are within the liquid stability region of calcium perchlorate brines. Furthermore, in (b) we plot the simulated subsurface conditions during which a brine is (meta)stable, assuming an ice table depth of 10 cm. The models surface predictions for both temperature and relative humidity were validated against the TECP measurements. In purple scale, from light to dark, we show results for 2, 5, and 8 cm