Task 5: The Inhabited Planet Model
Three major questions confront the biology task:(1)
what biospheres are most likely to develop on a
terrestrial planet in the habitable zone? (2) how
stable are these geo-biological systems on the stellar
(gigayear) time scale? (3) What is the predicted
impact of biology on the spectral signature expected
from a geologically similar but abiotic planet? To
address these questions we must build a biological
model capable of predicting the effects of prolonged
interaction between the biological, geological, and
atmospheric components of a terrestrial planet. Life
can affect the planet by mediating the planetary
cycling of volatiles between atmosphere, land, water,
crust, mantle, and core. From the other direction, the
geological, hydrological, and atmospheric composition
of the planet provides significant selection pressures
on the evolution of life. This dynamic interaction
between living organisms and their geochemical
surroundings will most likely not be in thermodynamic
equilibrium.
The production of reliable biological inputs for our
simulations requires the development of three tools:
(1) a mathematical formalism for a predictive model
capable of handling a nonlinear dynamic interaction
between the biological, geological, and atmospheric
components; (2) experimental data sets capable of
constraining the theoretical model; (3) a methodology
for predicting the long term stability of a
bio-geological system across the lifespan of the
parent star.
Q. What thermodynamic disequilibrium atmospheric
biosignatures might have existed on early Earth, that
is, an Earth not dominated by oxygenic photosynthesis?
What are the limits of detection for such markers?
Task 5 Highlights to Date:
Progress this year includes the completion of life module components, including microbial mats and Archean ecosystem models, and initial improvements to existing Earth land surface models to predict vegetation albedos. Our Archean ecosystem modeling indicates that the Archean biosphere was capable of generating methane surface fluxes comparable to those of modern Earth, which would have been large enough to keep the early Earth warm. These results emphasize that life can affect the climate and atmosphere of extrasolar planets in ways that can be detected by instruments such as TPF, even in the absence of oxygenic photosynthesis.
Research highlights this year include completion of the initial survey of the genetic diversity of microbes, which show considerable diversity, despite the extreme lack of nutrients and high pH (>11.5).
Task 5 Results
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