ABSTRACT
The most compelling question in astrobiology, and arguably for the entire field of planetary exploration, is whether life exists beyond Earth. Strict definitions of life cannot be easily tested remotely and yield instead to investigations that seek to detect biomarkers and metabolism. The Viking landers obtained equivocal and still controversial evidence of metabolic activity in Martian soils and no organic molecules indicative of extinct or extant life. However, new knowledge of Mars’ aqueous history, compositional diversity, surface environment, and habitat suitability allows design of more compressive investigations of both types, as well as selection of exploration sites with greater astrobiological potential. Ocean worlds and ice-rich asteroidal bodies are also unquestionably of great interest, yet challenging targets. Flight-qualified scientific instrumentation has steadily become more sophisticated and versatile. Space missions still demand, however, major resources and devotion by the engineering and science teams that undertake them. Sample return missions may ultimately be necessary but are costlier and intrinsically entail greater risk. Numerous societal and scientific issues must also be resolved before returning to Earth samples that possibly contain alien forms of life.
