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Direct-imaging observations of terrestrial exoplanets will enable theiratmospheric characterization and habitability assessment. Considering theEarth, the key atmospheric signatures for the biosphere is O$_2$ and thephotochemical product O$_3$. However, this O$_2$-O$_3$ biosignature is notdetectable in the visible wavelengths for most of the time after the emergenceof oxygenic photosynthesis life (i.e., the Proterozoic Earth). Here wedemonstrate spectroscopic observations in the ultraviolet wavelengths fordetecting and characterizing O$_2$ and O$_3$ in Proterozoic Earth-like planets,using ExoReL$^\Re$. For an O$_2$ mixing ratio 2 to 3 orders of magnitude lessthan the present-day Earth, and an O$_3$ mixing ratio of $10^{-7}-10^{-6}$, wefind that O$_3$ can be detected and its mixing ratio can be measured precisely(within $~1$ order of magnitude) in the ultraviolet ($0.25-0.4\ \mu$m) inaddition to visible-wavelength spectroscopy. With modest spectral resolution($R=7$) and S/N ($\sim10$) in the ultraviolet, the O$_3$ detection is robustagainst other potential gases absorbing in the ultraviolet (e.g., H$_2$S andSO$_2$), as well as the short-wavelength cutoff between 0.2 and 0.25 $\mu$m.While the O$_3$ detection does not rely on the near-infrared spectra, extendingthe wavelength coverage to the near-infrared ($1-1.8\ \mu$m) would provideessential information to interpret the O$_3$ biosignature, including the mixingratio of H$_2$O, the cloud pressure, as well as the determination of thedominant gas of the atmosphere. The ultraviolet and near-infrared capabilitiesshould thus be evaluated as critical components for future missions aiming atimaging and characterizing terrestrial exoplanets, such as the Habitable WorldsObservatory.

Reflected spectroscopy of small exoplanets III: probing the UV band to measure biosignature gasses