  A major focus of the planetary science and astrobiology community is the
understanding of planetary habitability, including the myriad factors that
control the evolution and sustainability of temperate surface environments such
as that of Earth. The few substantial terrestrial planetary atmospheres within
the Solar System serve as a critical resource in studying these habitability
factors, from which models can be constructed for application to extrasolar
planets. The recent Astronomy and Astrophysics and Planetary Science and
Astrobiology Decadal Surveys both emphasise the need for an improved
understanding of planetary habitability as an essential goal within the context
of astrobiology. The divergence in climate evolution of Venus and Earth
provides a major, accessible basis for understanding how the habitability of
large rocky worlds evolves with time and what conditions limit the boundaries
of habitability. Here, we argue that Venus can be considered an "anchor point"
for understanding planetary habitability within the context of terrestrial
planet evolution. We discuss the major factors that have influenced the
respective evolutionary pathways of Venus and Earth, how these factors might be
weighted in their overall influence, and the measurements that will shed
further light on their impacts of these worlds' histories. We further discuss
the importance of Venus with respect to both of the recent decadal surveys, and
how these community consensus reports can help shape the exploration of Venus
in the coming decades.
