The polarized light
reflected from the leaf contains a footprint of the leaf's biopigments.
These biosignatures
can be detected with a polarization filter, shown here
as a pair of
sunglasses. Illustration: Svetlana Berdyugina
A new technique enables scientists to search for traces of
life on exoplanets in reflected light
(August 6, 2015) A
new approach to searching for life on other planets: An international team has
discovered that biopigments of plants, so-called biological photosynthetic
pigments, leave behind unique traces in the light they reflect. Prof. Dr.
Svetlana Berdyugina from the Institute of Physics of the University of Freiburg
and the Freiburg Kiepenheuer Institute for Solar Physics studied these
biosignatures together with researchers from the University of Hawaiʻi at
Mānoa, USA, and the University of Aarhus, Denmark, with the help of
polarization filters: If biopigments were present as a sign of life on a
planet, they would leave behind a detectable polarized signature in the
reflected light. The scientists have now published their findings in the
International Journal of Astrobiology.
Figure 1: A green
leaf absorbs almost all red, green and blue light (RGB), but it reflects
and transmits
infrared light (shown in grey). The reflected infrared light is only weakly
polarized
due to the reflection
of a healthy leaf, but the reflected RGB light is strongly polarized due to
biopigments.
Measuring the amount of polarized light at different colors reveals the
signature
of the leaf
biopigments. Green sand reflects and polarizes sunlight almost equally in all
wavelengths, which
distinguishes it from a leaf that is a similar color.
Similarly, yellow
plants are different from yellow sand, etc. (Credit: S. Berdyugina)
Photosynthetic pigments are plant substances that absorb and
reflect particular wavelengths of visible light, making them appear in color in
the reflected wave ranges. Biopigments are what gives plants, algae, bacteria,
and human skin and eyes their colorful appearance. Chlorophyll pigments in
plant leaves, for instance, absorb blue to red light but reflect a small part
of green in the visible spectrum and thus appear green. An exception is
infrared light: half of it is reflected and the other half passes through the
leaf. Carotenoids absorb blue and red light but reflect yellow light and are
thus typically red, orange, or yellow in color.