  We have developed a characterization of the geological evolution of the
Earths atmosphere and surface in order to model the observable spectra of an
Earth-like planet through its geological history. These calculations are
designed to guide the interpretation of an observed spectrum of such a planet
by future instruments that will characterize exoplanets. Our models focus on
spectral features that either imply habitability or are required for
habitability. These features are generated by H2O, CO2, CH4, O2, O3, N2O, and
vegetation-like surface albedos. We chose six geological epochs to
characterize. These epochs exhibit a wide range in abundance for these
molecules, ranging from a CO2 rich early atmosphere, to a CO2/CH4-rich
atmosphere around 2 billion years ago to a present-day atmosphere. We analyzed
the spectra to quantify the strength of each important spectral feature in both
the visible and thermal infrared spectral regions, and the resolutions required
to unambiguously observe the features for each epoch. We find a wide range of
spectral resolutions required for observing the different features. For
example, H2O and O3 can be observed with relatively low resolution, while O2
and N2O require higher resolution. We also find that the inclusion of clouds in
our models significantly affects both the strengths and resolutions required to
observe all spectral features.

Astrophysics;Earth and Planetary Astrophysics;Instrumentation and Methods for Astrophysics