  Long-standing unexplained Venus atmosphere observations and chemical
anomalies point to unknown chemistry but also leave room for the possibility of
life. The unexplained observations include several gases out of thermodynamic
equilibrium (e.g. tens of ppm O2, the possible presence of PH3 and NH3, SO2 and
H2O vertical abundance profiles), an unknown composition of large, lower cloud
particles, and the "unknown absorber(s)". Here we first review relevant
properties of the Venus atmosphere and then describe the atmospheric chemical
anomalies and how they motivate future astrobiology missions to Venus.
