Abstract
Venus today is inhospitable at the surface, its average temperature of
750 K being incompatible to the existence of life as we know it.
However, the potential for past surface habitability and upper
atmosphere (cloud) habitability at the present day is hotly debated, as
the ongoing discussion regarding a possible phosphine signature coming
from the clouds shows. We review current understanding about the
evolution of Venus with special attention to scenarios where the planet
may have been capable of hosting microbial life. We compare the
possibility of past habitability on Venus to the case of Earth by
reviewing the various hypotheses put forth concerning the origin of
habitable conditions and the emergence and evolution of plate tectonics
on both planets. Life emerged on Earth during the Hadean when the planet
was dominated by higher mantle temperatures (by about
200$^\circ$C), an uncertain tectonic regime that
likely included squishy lid/plume-lid and plate tectonics, and proto
continents. Despite the lack of well-preserved crust dating from the
Hadean-Paleoarchean eons, we attempt to resume current understanding of
the environmental conditions during this critical period based on zircon
crystals and geochemical signatures from this period, as well as studies
of younger, relatively well-preserved rocks from the Paleoarchean. For
these early, primitive life forms, the tectonic regime was not critical
but it became an important means of nutrient recycling, with possible
consequences to the global environment on the long-term, that was
essential to the continuation of habitability and the evolution of life.
For early Venus, the question of stable surface water is closely related
to tectonics. We discuss potential transitions between stagnant lid and
(episodic) tectonics with crustal recycling, as well as consequences for
volatile cycling between Venus’ interior and atmosphere. In particular,
we review insights into Venus’ early climate and examine critical
questions about early rotation speed, reflective clouds, and silicate
weathering, and summarize implications for Venus’ long-term
habitability. Finally, the state of knowledge of the venusian clouds and
the proposed detection of phosphine is covered.