The Phoenix mission investigated patterned ground and weather in the northern arctic region of Mars for 5 months starting 25 May 2008 (solar longitude between 76.5 degrees and 148 degrees ). A shallow ice table was uncovered by the robotic arm in the center and edge of a nearby polygon at depths of 5 to 18 centimeters. In late summer, snowfall and frost blanketed the surface at night; H(2)O ice and vapor constantly interacted with the soil. The soil was alkaline (pH = 7.7) and contained CaCO(3), aqueous minerals, and salts up to several weight percent in the indurated surface soil. Their formation likely required the presence of water.

Abstract. Using a validated general circulation model, we determine where and for how long the surface pressure and surface temperature on Mars meet the minimum requirements for the existence of liquid water in the present climate system: pressures and temperatures above the triple point of water but below the boiling point. We find that for pure liquid water, there are five "favorable" regions where these requirements are satisfied: between 0 ø and 30øN in the plains of Amazonis, Arabia, and Elysium; and in the Southern Hemisphere impact basins of Hellas and Argyre. The combined area of these regions represents 29% of the planet's surface area. In the Amazonis region these requirements are satisfied for a total integrated time of 37 sols each Martian year. In the Hellas basin the number of degree days above zero is 70, which is well above those experienced in the dry valley lake region of Antarctica. These regions are remarkably well correlated with the location of Amazonian paleolakes mapped by Cabrol and Grin [2000] but are poorly correlated with the seepage gullies found by Malin and Edgett [2000]. In both instances, obliquity variations may play a role. For brine solutions the favorable regions expand and could potentially include most of the planet for highly concentrated solutions. Whether liquid water ever forms in these regions depends on the availability of ice and heat and on the evaporation rate. The latter is poorly understood for low-pressure CO2 environments but is likely to be so high that melting occurs rarely, if at all. However, even rare events of liquid water formation would be significant since they would dominate the chemistry of the soil and would have biological implications as well. It is therefore worth reassessing the potential for liquid water formation on present day Mars, particularly in light of recent Mars Global Surveyor observations.

The light detection and ranging instrument on the Phoenix mission observed water-ice clouds in the atmosphere of Mars that were similar to cirrus clouds on Earth. Fall streaks in the cloud structure traced the precipitation of ice crystals toward the ground. Measurements of atmospheric dust indicated that the planetary boundary layer (PBL) on Mars was well mixed, up to heights of around 4 kilometers, by the summer daytime turbulence and convection. The water-ice clouds were detected at the top of the PBL and near the ground each night in late summer after the air temperature started decreasing. The interpretation is that water vapor mixed upward by daytime turbulence and convection forms ice crystal clouds at night that precipitate back toward the surface.

A primary objective of the Phoenix mission was to examine the characteristics of high latitude ground ice on Mars. We report observations of ground ice, its depth distribution and stability characteristics, and examine its origins and history. High latitude ground ice was explored through a dozen trench complexes and landing thruster pits, over a range of polygon morphological provinces. Shallow ground ice was found to be abundant under a layer of relatively loose ice‐free soil with a mean depth of 4.6 cm, which varied by more than 10x from trench to trench. These variations can be attributed mainly to slope effects and thermal inertia variations in the overburden soil affecting ground temperatures. The presence of ice at this depth is consistent with vapor‐diffusive equilibrium with respect to a mean atmospheric water content of 3.4 × 1019 m−3, consistent with the present‐day climate. Significant ice heterogeneity was observed, with two major forms: ice‐cemented soil and relatively pure light toned ice. Ice‐cemented soils, which comprised about 90% of the icy material exposed by trenching, are best explained as vapor deposited pore ice in a matrix supported porous soil. Light toned ice deposits represent a minority of the subsurface and are thought to consist of relatively thin near surface deposits. The origin of these relatively pure ice deposits appears most consistent with the formation of excess ice by soil ice segregation, such as would occur by thin film migration and the formation of ice lenses, needle ice, or similar ice structures.

The thermal and electrical conductivity probe (TECP), a component of the Microscopy, Electrochemistry, and Conductivity Analyzer (MECA), was included on the Phoenix Lander to conduct in situ measurements of the exchange of heat and water in the Martian polar terrain. TECP measured regolith thermal conductivity, heat capacity, temperature, electrical conductivity, and dielectric permittivity throughout the mission. A relative humidity sensor returned the first in situ humidity measurements from the Martian surface. The dry overburden above the ground ice is a good thermal insulator (average κ = 0.085 W m−1 K−1 and average Cρ = 1.05 × 106 J m−3 K−1). Surface thermal inertia (I) calculated from these values agrees well with daytime orbital determinations, but differences in the spatial and temporal scale of heat transport lead to very different measurements at night. Electrical conductivity was consistent with open circuit throughout the mission; an upper limit conductivity of 2 nS cm−1 is derived. Bulk dielectric permittivity (ɛb) shows several puzzling signals but also a systematic increase overnight in the latter half of the mission, contemporaneous with H2O adsorption. The magnitude of the increase is difficult to reconcile with expected changes in unfrozen water. Atmospheric H2O averages around 1.8 Pa during the day, corresponding to a RH < 5%. At night, much of the H2O disappears from the atmosphere, and RH increases to ∼100%. Temperature and H2O partial pressure data suggest that adsorption on mineral surfaces plays a major role in scrubbing H2O, with a possible contribution from perchlorate salts.