Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Detectability of Earth's Biosignatures Across Time   Over the past two decades, enormous advances in the detection of exoplanets
have taken place. Currently, we have discovered hundreds of earth-sized
planets, several of them within the habitable zone of their star. In the coming
years, the efforts will concentrate in the characterization of these planets
and their atmospheres to try to detect the presence of biosignatures. However,
even if we discovered a second Earth, it is very unlikely that it would present
a stage of evolution similar to the present-day Earth. Our planet has been far
from static since its formation about 4.5 Ga ago; on the contrary, during this
time, it has undergone multiple changes in it's atmospheric composition, it's
temperature structure, it's continental distribution, and even changes in the
forms of life that inhabit it. All these changes have affected the global
properties of Earth as seen from an astronomical distance. Thus, it is of
interest not only to characterize the observables of the Earth as it is today,
but also at different epochs. Here we review the detectability of the Earth's
globally-averaged properties over time. This includes atmospheric composition
and biosignatures, and surface properties that can be interpreted as sings of
habitability (bioclues). The resulting picture is that truly unambiguous
biosignatures are only detectable for about 1/4 of the Earth's history. The
rest of the time we rely on detectable bioclues that can only establish an
statistical likelihood for the presence of life on a given planet.

10
The categories of this paper are: q-bio
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The categories of this paper are: gr-qc
16
The categories of this paper are: stat
12
The categories of this paper are: nucl-th
26
The categories of this paper are: bayes-an
11
The categories of this paper are: hep-lat
1
The categories of this paper are: cs
20
The categories of this paper are: acc-phys
14
The categories of this paper are: math-ph
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The categories of this paper are: nlin
3
The categories of this paper are: physics
13
The categories of this paper are: quant-ph
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The categories of this paper are: supr-con
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The categories of this paper are: nucl-ex
0
The categories of this paper are: hep-ph
18
The categories of this paper are: econ