  For the time being, Earth remains the best and only example of a habitable
(and inhabited) world. Therefore, it is important to explore and understand the
full range of spectral signatures and variability of Earth in order to inform
the design of future instruments and missions, and understand their diagnostic
power as well as potential limitations. In this work we use Earth observation
data collected by the MODIS instrument aboard the Aqua satellite. The complete
data set comprises 15 years of thermal emission observations in the 3.66-14.40
microns range for five different locations on Earth (Amazon Rainforest,
Antarctica, Arctic, Indian Ocean and the Sahara Desert). We then determine flux
levels and variations as a function of wavelength and surface type (i.e.
climate zone and surface thermal properties) and investigate whether periodic
signals indicating Earth's tilted rotation axis can be detected. Our findings
suggest that (1) viewing geometry plays an important role when thermal emission
data is analyzed as Earth's spectrum varies by a factor of three and more
depending on the dominant surface type underneath; (2) typically strong
absorption bands from carbon dioxide (15 microns) and ozone (9.65 microns) are
significantly less pronounced and partially absent in data from the polar
regions implying that estimating correct abundance levels for these molecules
might be challenging in these cases; (3) the time-resolved thermal emission
spectrum encodes information about seasons/planetary obliquity, but the
significance depends on the viewing geometry and spectral band considered
