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.

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[1] The objective of the Phoenix mission is to determine if Mars' polar region can support life. Since liquid water is a basic ingredient for life, as we know it, an important goal of the mission is to determine if liquid water exists at the landing site. It is believed that a layer of Martian soil preserves ice by forming a barrier against high temperatures and sublimation, but that exposed ice sublimates without the formation of the liquid phase. Here we show possible independent physical and thermodynamical evidence that besides ice, liquid saline water exists in areas disturbed by the Phoenix Lander. Moreover, we show that the thermodynamics of freeze-thaw cycles can lead to the formation of saline solutions with freezing temperatures lower than current summer ground temperatures on the Phoenix landing site on Mars' Arctic. Thus, we hypothesize that liquid saline water might occur where ground ice exists near the Martian surface. The ideas and results presented in this article provide significant new insights into the behavior of water on Mars.

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.

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It is thought that the Cerberus Fossae fissures on Mars were the source of both lava and water floods two to ten million years ago. Evidence for the resulting lava plains has been identified in eastern Elysium, but seas and lakes from these fissures and previous water flooding events were presumed to have evaporated and sublimed away. Here we present High Resolution Stereo Camera images from the European Space Agency Mars Express spacecraft that indicate that such lakes may still exist. We infer that the evidence is consistent with a frozen body of water, with surface pack-ice, around 5 degrees north latitude and 150 degrees east longitude in southern Elysium. The frozen lake measures about 800 x 900 km in lateral extent and may be up to 45 metres deep--similar in size and depth to the North Sea. From crater counts, we determined its age to be 5 +/- 2 million years old. If our interpretation is confirmed, this is a place that might preserve evidence of primitive life, if it has ever developed on Mars.

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Venus is known to have been volcanically resurfaced in the last third of solar system history and to have undergone a significant decrease in volcanic activity a few hundred million years ago. However, fundamental questions remain: Is Venus still volcanically active today, and if so, where and in what geological and geodynamic environment? Here we show evidence from the Venus Express Venus Monitoring Camera for transient bright spots that are consistent with the extrusion of lava flows that locally cause significantly elevated surface temperatures. The very strong spatial correlation of the transient bright spots with the extremely young Ganiki Chasma, their similarity to locations of rift‐associated volcanism on Earth, provide strong evidence for their volcanic origin and suggests that Venus is currently geodynamically active.