  Mars exploration motivates the search for extraterrestrial life, the
development of space technologies, and the design of human missions and
habitations. Here we seek new insights and pose unresolved questions relating
to the natural history of Mars, habitability, robotic and human exploration,
planetary protection, and the impacts on human society. Key observations and
findings include:(1)high escape rates of early Mars' atmosphere, including loss
of water, impact present-day habitability;(2)putative fossils on Mars will
likely be ambiguous biomarkers for life;(3)microbial contamination resulting
from human habitation is unavoidable;(4)based on Mars' current planetary
protection category, robotic payload(s) should characterize the local martian
environment for any life-forms prior to human habitation. Some of the
outstanding questions are:(1)which interpretation of the hemispheric dichotomy
of the planet is correct;(2)to what degree did deep-penetrating faults
transport subsurface liquids to Mars' surface;(3)in what abundance are
carbonates formed by atmospheric processes;(4)what properties of martian
meteorites could be used to constrain their source locations;(5)the origin(s)
of organic macromolecules;(6)was/is Mars inhabited;(7)how can missions designed
to uncover microbial activity in the subsurface eliminate potential false
positives caused by microbial contaminants from Earth;(8)how can we ensure that
humans and microbes form a stable and benign biosphere;(9)should humans relate
to putative extraterrestrial life from a biocentric viewpoint (preservation of
all biology), or anthropocentric viewpoint of expanding habitation of space?
Studies of Mars' evolution can shed light on the habitability of extrasolar
planets. In addition, Mars exploration can drive future policy developments and
confirm (or put into question) the feasibility and/or extent of human
habitability of space.
