2004 Annual Science Report

Virtual Planetary Laboratory (JPL/CalTech) Reporting  |  JUL 2003 – JUN 2004

Characterization of Terrestrial Planets From Disk-Averaged Spectra: Earths Around Other Stars

Project Summary

In this task, we have attempted to understand the nature and detectability of biosignatures and planetary characteristics for Earth-like planets around other stars

4 Institutions
3 Teams
0 Publications
0 Field Sites
Field Sites

Project Progress

In this task, we have attempted to understand the nature and detectability of biosignatures and planetary characteristics for Earth-like planets around other stars. In our initial experiment, we used a coupled climate-photochemical model to simulate Earth-like planets with different amounts of atmospheric oxygen, in orbit around the Sun (a G2V star), an F2V star, and a K2V star. For the solar case, Proterozoic Earth was also included in the simulations. Three significant results were derived from this research: 1) O2 can be detected at visible wavelengths (~0.765 microns) in disk-averaged spectra at atmospheric concentrations as small as 10-2 present atmospheric levels (PAL) and O3 can be detected at O2 concentrations as low as 10-3 PAL at millimeter imaging radiometer (MIR) wavelengths; 2) Mid-Proterozoic Earth-like atmospheres show strong signatures from both CH4 and O3, and it is potentially easier to detect life remotely in a mid-Proterozoic-type atmosphere than a modern Earth; and 3) The critical threshold for atmospheric oxygen to provide a planetary ultraviolet (UV) shield for host stars of different spectral types is typically ~10-2 PAL. We also discovered that planets with high-O2 atmospheres orbiting K2V and F2V stars were better protected from surface UV radiation than Earth, but O2 levels below ~10-2 PAL were insufficient to provide adequate surface protection. (Segura et al., Astrobiology, 2003, vol. 3, pp. 689-708).

Our second, ongoing, experiment is focused on detectability of characteristics for planets orbiting M dwarf stars, which are typically highly variable. As a first step, the observed time-averaged spectra of two M stars and a quiescent, modeled M star were used (Figure 1). The planets modeled in this case contained 1PAL Earth-like atmospheres. Our results show that methane could be an excellent biosignature, along with ozone (Figures 2 and 3). We are currently developing a time-dependent climate-photochemical model to determine how the flux variability of M stars would affect the lifetime and detectability of planetary biosignatures. We also show that CH4 is potentially observable along with O2 (or O3). The CH4 abundance is relatively high in these systems because the enhanced ozone shield at near-UV wavelengths between 200-300 nm produces reduced tropospheric OH densities. Hence, methane is long-lived even in O2-rich atmospheres of M-star planets.

{{ 1 }}

{{ 2 }}

{{ 3 }}