Water Ice Permafrost on Mars and on the Moon

Abstract

The Moon and Mars are the most explored planetary bodies in the solar system. For the more than 60 years of the space era, dozens of science robotic missions have explored the Moon and Mars. The primary scientific goal for many of these missions was declared to be a search for surface or ground water/water ice and gaining an understanding of its distribution and origin. Today, for the Moon, the focus of scientific exploration has moved to the lunar polar regions and permanently shadowed regions (PSRs). PSRs do not receive any direct sunlight and are frozen at very low temperatures (< 120 K), acting as cold traps. They are considered to be a storehouse that preserves records of the solar system’s evolution by trapping water ice and potentially other volatile deposits brought by comets and asteroids over billions of years. For Mars, the water/water ice search was part of an attempt to find traces of ancient extraterrestrial life and possibly to understand how life appeared on Earth. Current Mars is cold and dry, but its high latitudes and some equatorial regions are enriched with surface and subsurface water ice. Scientists argue that oceans could have existed on ancient Mars if it was warm and wet and that different life forms could have originated similar to Earth’s. If this is the case, then biomarkers could be preserved in the Martian ground ice depositions. Another popular idea that ties water ice permafrost on the Moon and Mars is related to the expected future human expansion to deep space. The Moon and Mars are widely considered to be the first destinations for future manned space-colony missions or even space-colony missions. In this scenario, the long-term presence and survival of astronauts on the lunar or Martian surface strongly depend on in situ resource utilization (ISRU). Water ice is at the top of the ISRU list because it could be used as water for astronauts’ needs. Its constituents, oxygen and hydrogen, could be used for breathing and for rocket fuel production, respectively. The Moon is the closest body to Earth and discussion about presence of water ice on the Moon has both scientific and practical interest, especially for planning manned space missions. The focus further in space is on how subsurface water ice is distributed on Mars. A related topic is the debates about whether ancient Mars was wet and warm or if, for most of its history, the Martian surface was covered with glaciers. Finally, there are fundamental questions that should be answered by upcoming Mars and Moon missions.