Scientists think a subsurface ocean could exist on Jupiter's moons Europa and Ganymede, and on Saturn's moons Titan and Enceladus. Cabrol develops science exploration strategies for these icy moons, where microbes may have evolved.
Unlike Mars, other bodies in the solar system would have had a more difficult time exchanging material.
"Mars and Earth could have a common root to their tree of life, but when you go beyond Mars, it's not that easy," Cabrol said. "If we were to discover life on those planets [and moons], it would be different from us."
Extraterrestrial life in the form of microbes, should it be found, may not lead to an equal exchange of intelligence, but primitive life can still answer questions about the existence of life, Cabrol said.
"Organic material is going to tell you about environment, about complexity, and about diversity. DNA, or any information carrier, is going to tell you about adaptation, about evolution, about survival, about planetary change, and about the transfer of information," she said. "All together, they are telling us why what started as a microbial pathway sometimes ends up as a civilization or sometimes ends up as a dead end."
Within our solar system, these questions could be answered in the not-too-distant future, she said.
"This can be achieved by our generation," Cabrol said. "This can be our legacy—but only if we dare to explore."
Water on Mars, so challenging to find today, may once have covered the planet with rivers and oceans. Where did all of the liquid water go? Why? Could some still remain?
Observations of the Red Planet indicate that rivers and oceans may have been prominent features in its early history. Billions of years ago, Mars was a warm and wet world that could have supported microbial life in some regions. But the planet is smaller than Earth, with less gravity and a thinner atmosphere. Over time, as liquid water evaporated, more and more of it escaped into space, allowing less to fall back to the surface of the planet.
Where is the water today?
Vast deposits of water appear to be trapped within the ice caps at the north and south poles of the planet. Each summer, as temperatures increase, the caps shrink slightly as their contents skip straight from solid to gas form, but in the winter, cooler temperatures cause them to grow to latitudes as low as 45 degrees, or halfway to the equator. The caps are an average of 2 miles (3 kilometers) thick and, if completely melted, could cover the Martian surface with about 18 feet (5.6 meters) of water.
Liquid water appears to flow from some steep, relatively warm slopes on the Martian surface. Researchers studying images taken by the High Resolution Imaging Science Experiment (HiRISE) camera aboard the Mars Reconnaissance Orbiter (MRO) noticed dark streaks that appear during warm weather but fade away when temperatures drop. Spectral analysis of these streaks, called recurring slope linae (RSL), lead scientists to conclude they are caused by salty liquid water.
More water may be frozen just beneath the surface, covered by the dry red dust that blankets the planet, scientists say. Some high-latitude regions seem to boast patterned ground-shapes that may have formed as permafrost in the soil freezes and thaws over time. The European Space Agency's Mars Express spacecraft captured images of sheets of ice in the cooler, shadowed bottoms of craters.