Infrared spectra of three terrestrial planets in our solar system.

Finding Planets Finding Life

Biosignatures and Planetary Properties to be Investigated by the TPF Mission

The Terrestrial Planet Finder Interferometer (TPF-I) is designed to directly detect terrestrial exoplanets around nearby stars and to measure their spectra. These spectra will be analyzed to establish the presence and composition of their atmospheres, investigate their capability to sustain life as we know it (habitability), and to search for signs of life. TPF-I also has the capacity to investigate the physical properties and composition of a broader diversity of planets, to understand the formation of planets and search for the presence of potential biosignature compounds. The range of characteristics of planets is likely to exceed our experience with the planets and satellites in our own Solar System, and Earth-like planets orbiting stars of different spectral type might also evolve differently.

Biomarkers are detectable species whose presence at significant abundance requires a biological origin. They are the chemical ingredients necessary for biosynthesis (e.g., oxygen and methane) or are products of biosynthesis (e.g., complex organic molecules, but also oxygen and methane). Our search for signs of life is based on the assumption that extraterrestrial life shares fundamental characteristics with life on Earth, in that it requires liquid water as a solvent and has a carbon-based chemistry. Therefore we assume that extraterrestrial life is similar to life on Earth in its use of the same input and output gases, that it exists out of thermodynamic equilibrium, and that it has analogs to bacteria, plants, and animals on Earth.

Candidate biomarkers that might be detected by a low-resolution TPF instrument include oxygen, ozone, and methane. There are good biogeochemical and thermodynamic reasons for believing that these gases should be ubiquitous byproducts of carbon-based biochemistry, even if the details of alien biochemistry are significantly different than the biochemistry on Earth. Production of oxygen by photosynthesis allows terrestrial plants and photosynthetic bacteria (cyanobacteria) to use abundant water as the electron donor to reduce carbon dioxide, instead of having to rely on scarce supplies of hydrogen gas and hydrogen sulphide. Oxygen and nitrous oxide are two very promising bio-indicators. Oxygen is a chemically reactive gas. Reduced gases and oxygen have to be produced concurrently to produce quantities large enough to be detectable in disk-averaged spectra of terrestrial planet atmospheres, as they react rapidly with each other. Nitrous oxide is a biomarker in the Earth's atmosphere, being produced in abundance by life but only in trace amounts by natural processes. Although a relatively weak feature in the Earth's spectrum, it may be more pronounced in extrasolar terrestrial planet atmospheres of different composition or host star spectral type. Currently, efforts are ongoing to explore the plausible range of habitable planets and to improve our understanding of the detectable ways in which life modifies a planet on a global scale.

Biomarker Signatures in the Mid Infrared

In the mid-IR, the classical signature of biological activity is the combined detection of the 9.6 µm ozone band, the 15 µm carbon dioxide band and the 6.3 µm water band or its rotational band that extends from 12 µm out into the microwave region. Because oxygen is a chemically reactive gas, it follows that reduced gases and oxygen have to be produced concurrently to be detectable in the atmosphere, as they react rapidly with each other. The oxygen and ozone absorption features in the visible and thermal infrared respectively could indicate the presence of photosynthetic biological activity on Earth anytime during the past 50% of the age of the solar system. In the Earth's atmosphere, the 9.6 µm ozone band is a poor quantitative indicator of the oxygen amount, but an excellent qualitative indicator for the existence of even traces of oxygen. The ozone 9.6 µm band is a very nonlinear indicator of oxygen for two reasons. First, for the present atmosphere, low resolution spectra of this band show little change with the ozone abundance because it is strongly saturated. Second, the apparent depth of this band remains nearly constant as oxygen increases from 0.01 times the present atmosphere level of oxygen (PAL) to 1 PAL. The primary reason for this is that the stratospheric ozone increases that accompanied the oxygen buildup lead to additional UV heating of the stratosphere. At these higher temperatures, the stratospheric emission from this band partially masked the absorption of upwelling thermal radiation from the surface.

Methane is not readily identified using low resolution spectroscopy for present-day Earth, but the methane feature at 7.66 µm in the IR is easily detectable at higher abundances. When observed together with molecular oxygen, abundant methane can indicate biological processes. Depending on the degree of oxidation of a planet's crust and upper mantle, non-biological mechanisms can also produce large amounts of methane under certain circumstances.

Nitrous oxide is a also a candidate biomarker because it is produced in abundance by life but only in trace amounts by natural processes. There are no nitrous oxide features in the visible and three weak nitrous oxide features in the thermal infrared at 7.75 µm and 8.52 µm, and 16.89 µm. For present-day Earth one needs a resolution of 23, 23 and 44 respectively to detect nitrous oxide. Spectral features of nitrous oxide also become more apparent in atmospheres with less water vapor. Methane and nitrous oxide have features nearly overlapping in the 7 µm region, and additionally both lie in the red wing of the 6 µm water band. Thus nitrous oxide is unlikely to become a prime target for the first generation of space-based missions searching for exoplanets, but it is an excellent target for follow up missions. There are other molecules that could, under some circumstances, act as excellent biomarkers, e.g., the manufactured chloro-fluorocarbons in our current atmosphere in the thermal infrared waveband, but their abundances are too low to be spectroscopically observed at low resolution.

Other biogenic trace gases might also produce detectable biosignatures. Currently identified potential candidates include volatile methylated and sulfur compounds. It is known that these compounds are produced by microbes, and preliminary estimates of their lifetimes and detectability in Earth-like atmospheres around stars of different spectral type have been made. However, it is not yet fully understood how stable (or detectable) these compounds would be in atmospheres of different composition and for stars of different spectral type and incident UV flux. These uncertainties, however, could be addressed by further modeling studies.

Detection of Water

If water is present on a planet it should be detectable in the 6.3 µm band or its rotational band that extends from 12 µm out into the microwave region. Both water features are difficult to interpret and quantify for an extrasolar planet atmosphere. A spectrometer that spans the wavelength range from 5 to 20 microns could be capable of detecting both water features. The equivalent width for the two features is 1.66 and 0.66 respectively for a current Earth model atmosphere with the average 60% cloud coverage.

However, the feature at 6.3 microns may be so strong, that it would be relatively insensitive to the abundance of atmospheric water. Methane and nitrous oxide have features nearly overlapping in the 7 µm region, and additionally both lie in the red wing of the 6.3 µm water band. The 6.3 µm water feature could act instead as a "true/false" indicator of the presence of even very small amounts of water if the atmosphere is an Earth-analog. The broad rotational band, extending between 12-200 µm , has little spectral structure, which makes it difficult to discriminate its absorption from other factors affecting the planetary spectrum, such as the temperature of the emitting layer, which could also result in reduced flux in this wavelength region. The instrument design has to take the noise levels at both wavelength ends into account as well as the photons emitted from the planet. Preliminary studies indicate that it will be equally challenging to detect the water line at both features