Goal 2: Explore for past or present habitable environments, prebiotic chemistry and signs of life elsewhere in our Solar System.
Determine any chemical precursors of life and any ancient habitable climates in the Solar System, and characterize any extinct life, potential habitats, and any extant life on Mars and in the outer Solar System.
The exploration for habitable environments, life and/or prebiotic chemistry in the Solar System directly links basic research in astrobiology to NASA missions. Because little is presently known about habitable environments within our Solar System, the distribution and nature of potentially habitable environments should be determined on Mars, Titan, Europa, and other promising objects. As a corollary, we should understand the mechanisms of evolution of habitable environments throughout the Solar System. Although life elsewhere could have developed in ways different from life on Earth, our current knowledge of life and habitable environments serves as the starting point for our exploration strategy. Research in such widely divergent areas as planetary and Solar System evolution, the biology of extreme environments, and Precambrian paleontology has been instrumental in guiding NASA's search for evidence of life elsewhere in the Solar System. Earth-based analog studies and theoretical investigations, informed by data from previous Solar System missions, will assist astrobiologists to refine exploration strategies and scientific priorities for future missions.
Background
Understanding planetary habitability and the relationship between the occurrence of life and the evolution of planets is a primary organizing theme of NASA's Solar System Exploration Program. In the most basic sense, the strategy for the astrobiological exploration of the Solar System involves exploring for environments regarded as necessary for life to begin and/or persist, namely those having liquid water, energy sources that can sustain metabolism, conditions that promote the synthesis of complex organic molecules, and understanding the evolution of habitable environments on Solar System objects.
Advances in our understanding of the environmental limits of life on Earth have provided crucial information for refining our strategies to explore for life elsewhere in the Solar System. For example, a deep subsurface biosphere was discovered that included non-photosynthetic organisms that make organic compounds from hydrogen and other simple byproducts of aqueous weathering. This discovery has revolutionized our thinking about the potential for life on other planets like Mars or Europa, where surface conditions are fundamentally inhospitable to life. The necessity to explore the deep subsurface of other Solar System bodies has identified the need to develop robotic drilling systems that can penetrate 100's to 1000's of meters below the surface, where interior habitable zones of liquid water and a life-sustaining redox chemistry might exist. Of course, geological activity or meteorite impacts might have brought evidence of subsurface life to the surface, therefore the ability to identify and reach key sites with landers and rovers is also a high priority.
In preparing for future missions to explore for life and/or prebiotic chemistry in the Solar System, an important precursor activity for Astrobiology is to identify in situ instrumentation to support the search for complex organic molecules and life. As a starting point, there is a critical need for research to define unambiguous approaches to life detection over a broad range of environmental conditions that represent other planetary environments. Such research will also help address planetary protection issues, such as the effects of forward contamination of other planetary surfaces and the risks of back contamination associated with samples returned to Earth.
In pursuing the question of extraterrestrial life, humans have long held a fascination with Mars. Indeed, our robotic exploration of the red planet has provided compelling evidence for surface environments that could have supported life early in the planet's history. More recently, arguments have been made for the existence of a Martian groundwater system that could harbor an extant subsurface biota. Key questions include the following. If ever life arose on Mars, is it related to terrestrial life, or did Mars sustain an independent origin of life? If life never developed on Mars, is there a prebiotic chemical record preserved in ancient martian rock sequences that might contain clues about how life began on Earth?
Possibilities for subsurface habitable zones of liquid water have also been recognized in the outer Solar System. Induced magnetism, as well as surface geomorphology and chemistry, have provided compelling evidence for an ocean of liquid water (brine) beneath the icy crust of Europa. Similar conditions may also exist on two other Galilean satellites—Ganymede and Callisto. In addition, a complex prebiotic chemistry and zones of liquid water might exist on Titan. Some of these environments might resemble aspects of early Earth and thus they can teach us about our own origins. However, other environments could be quite different, and these might have hosted a prebiotic chemical evolution that led to an altogether different form of biology.
As the nature of the potentially habitable environments in our Solar System becomes better defined, the Astrobiology Program must interact with both observational astronomy and mission scientists to consider also the possibility of life in non-aqueous environments. Such a possibility can be explored during missions to places (like Titan) where liquid water is not predominant, and by developing the ability to recognize the biosignatures of life in non-aqueous environments.
Objective 2.1
Mars exploration
Through orbital and surface missions, explore Mars for potentially habitable environments, as evidenced by water or aqueous minerals. Study Martian meteorites to guide future Mars exploration. Develop the methods and supporting technologies for the in situ characterization of aqueous minerals, carbon chemistry and/or life.
Example investigations
- Target well-instrumented robotic rovers to
sites of past aqueous sedimentation to analyze rocks for geochemistry,
aqueous minerals, organic matter and fossil biosignatures.
- Develop flight-capable instrumentation for
the unambiguous detection of biosignatures preserved in surface and
subsurface rocks, soils and ices.
Objective 2.2
Outer Solar System exploration
Conduct basic research, develop instrumentation to support astrobiological exploration and provide scientific guidance for outer Solar System missions. Such missions should explore the Galilean moons Europa, Ganymede and Callisto for habitable environments where liquid water could have supported prebiotic chemical evolution or life. Explore Saturn's moon, Titan, for environments favorable for complex prebiotic synthesis or life.
Example investigations
- Explore the atmosphere and surface environments
of Titan for evidence of complex organic chemistry and water, to provide
a context for understanding potential habitability and prebiotic chemistry.
- Simulate the environment of Titan to aid
in designing in situ missions and to interpret data returned therefrom.
- Develop astrobiology instrumentation that
can survive the low temperature, high radiation environments of the
surface of Europa. Use in situ methods to test models that predict
the presence of energy sources for supporting life.