  As we begin to discover rocky planets in the habitable zone of nearby stars
with missions like TESS and CHEOPS, we will need quick advancements on
instrumentation and observational techniques that will enable us to answer key
science questions, such as What are the atmospheric characteristics of
habitable zone rocky planets? How common are Earth-like biosignatures in rocky
planets?} How similar or dissimilar are those planets to Earth? Over the next
decade we expect to have discovered several Earth-analog candidates, but we
will not have the tools to study the atmospheres of all of them in detail.
Ground-based ELTs can identify biosignatures in the spectra of these candidate
exo-Earths and understand how the planets' atmospheres compare to the Earth at
different epochs. Transit spectroscopy, high-resolution spectroscopy, and
reflected-light direct imaging on ELTs can identify multiple biosignatures for
habitable zone, rocky planets around M stars at optical to near-infrared
wavelengths. Thermal infrared direct imaging can detect habitable zone, rocky
planets around AFGK stars, identifying ozone and motivating reflected-light
follow-up observations with NASA missions like HabEx/LUVOIR. Therefore, we
recommend that the Astro2020 Decadal Survey Committee support: (1) the search
for Earth-like biosignatures on rocky planets around nearby stars as a key
science case; (2) the construction over the next decade of ground-based
Extremely Large Telecopes (ELTs), which will provide the large aperture and
spatial resolution necessary to start revealing the atmospheres of
Earth-analogues around nearby stars; (3) the development of instrumentation
that optimizes the detection of biosignatures; and (4) the generation of
accurate line lists for potential biosignature gases, which are needed as model
templates to detect those molecules.
