  A long-term goal of exoplanet studies is the identification and detection of
biosignature gases. Beyond the most discussed biosignature gas O$_2$, only a
handful of gases have been considered in detail. Here we evaluate phosphine
(PH$_3$). On Earth, PH$_3$ is associated with anaerobic ecosystems, and as such
is a potential biosignature gas on anoxic exoplanets.
  We simulate CO$_2-$ and H$_2-$dominated habitable terrestrial planet
atmospheres. We find that PH$_3$ can accumulate to detectable concentrations on
planets with surface production fluxes of 10$^{10}$-10$^{14}$ cm$^{-2}$
s$^{-1}$ (corresponding to surface concentrations of 10s of ppb to 100s of
ppm), depending on atmospheric composition and UV flux. While high, such
surface fluxes are comparable to the global terrestrial production rate of
CH$_4$ (10$^{11}$ cm$^{-2}$ s$^{-1}$) and below the maximum local terrestrial
PH$_3$ production rate (10$^{14}$ cm$^{-2}$ s$^{-1}$). As with other gases,
PH$_3$ can more readily accumulate on low-UV planets, e.g. planets orbiting
quiet M-dwarfs or with a photochemical UV shield.
  If detected, PH$_3$ is a promising biosignature gas, as it has no known
abiotic false positives on terrestrial planets that could generate the high
fluxes required for detection. PH$_3$ also has 3 strong spectral features such
that in any atmosphere scenario 1 of the 3 will be unique compared to other
dominant spectroscopic molecules. PH$_3$'s weakness as a biosignature gas is
its high reactivity, requiring high outgassing for detectability. We calculate
that 10s of hours of JWST time are required for a potential detection of
PH$_3$. Yet because PH$_3$ is spectrally active in the same wavelength regions
as other atmospherically important molecules (e.g., H$_2$O and CH$_4$),
searches for PH$_3$ can be carried out at no additional observational cost to
searches for other molecules relevant to exoplanet habitability.
