"As it turns out, Earth’s delicate color can be closely mimicked by hypothetical exoplanet types that are completely uninhabitable. A broader portion of Earth’s overall spectrum, however, does display a subtle signature only attributable, insofar as we know, to life. Seeking this signature from pale blue worlds in stars’ habitable zones with future telescopes could be a powerful tool for identifying worlds deserving of intense further scrutiny. “One important takeaway is that color should be used with caution because we found it’s relatively easy to make lifeless planets that are pale blue in color,” said lead author Joshua Krissansen-Totton, ..... “With that said, I was very excited to find that Earth’s spectrum has an intriguing signature that is biogenic, unique and potentially quite useful.”
Nitrogen, the overwhelming component of Earth’s atmosphere, is transparent. An atmosphere that is clear to visible light preferentially scatters short wavelength, bluer light, as opposed to longer wavelength, redder light. As a result of this so-called Rayleigh scattering, blue light appears to come from all directions, and presto: the sky looks blue, as do the oceans. The blue light scatters into space as well, and thus to external observers. Earth’s white clouds boost reflectivity at all wavelengths, diluting the intensity of the Rayleigh scattering. Some red light also mixes into Earth’s color palette as it’s reflected by the continental land masses. That redness bends Earth’s overall color viewed from afar more toward the light blue than, say, the rich azure of Neptune, whose atmosphere’s high methane levels absorb red and reflect blue. Meanwhile, the oxygen in our air
chemically reacts with many types of molecules that would otherwise form an opaque haze. Such atmospheric haze is evident in the visages of Venus, Jupiter, Saturn, and Saturn’s moon Titan.
Earth’s free oxygen would not exist were it not for our planet’s teeming plant and microbial life, which constantly replenish it through the process of photosynthesis. This information in hand, the University of Washington researchers then compiled the spectra of various real and theoretical exoplanets.
“Several types of worlds, the analysis showed, could trick astronomers into thinking they were perhaps habitable by aping Earth’s special hue. The ruse could even be pulled off by planets similar to Mars, whose surface is a rusty red and plainly visible through its currently tenuous, carbon dioxide-dominated atmosphere. With a more substantial atmosphere, though, the Red Planet would look from a great distance like a Pale Blue Planet.”
On the matter of a certain spectral signature, though, no other world type displayed it except Earth. This telltale signature assumes a U-shape as it stretches from the near-ultraviolet (beyond visible light) through the visible and then into near-infrared wavelengths. Its up-down-up shape derives from the very features that make Earth a pale blue dot in the first place: Rayleigh scattering in the bluer portion of the spectrum; some absorption in the greener portion because of the ozone in our atmosphere, which like the oxygen we breathe would not exist without life; and finally red light, again owing to continental reflectance, but also from the Earth’s widespread vegetation that reflects strongly in infrared. Krissansen-Totton is pleased that this signature could separate Earthly pretenders from the genuine article of a potentially inhabited world." Astrobiology
Nitrogen, the overwhelming component of Earth’s atmosphere, is transparent. An atmosphere that is clear to visible light preferentially scatters short wavelength, bluer light, as opposed to longer wavelength, redder light. As a result of this so-called Rayleigh scattering, blue light appears to come from all directions, and presto: the sky looks blue, as do the oceans. The blue light scatters into space as well, and thus to external observers. Earth’s white clouds boost reflectivity at all wavelengths, diluting the intensity of the Rayleigh scattering. Some red light also mixes into Earth’s color palette as it’s reflected by the continental land masses. That redness bends Earth’s overall color viewed from afar more toward the light blue than, say, the rich azure of Neptune, whose atmosphere’s high methane levels absorb red and reflect blue. Meanwhile, the oxygen in our air
Earth’s free oxygen would not exist were it not for our planet’s teeming plant and microbial life, which constantly replenish it through the process of photosynthesis. This information in hand, the University of Washington researchers then compiled the spectra of various real and theoretical exoplanets.
“Several types of worlds, the analysis showed, could trick astronomers into thinking they were perhaps habitable by aping Earth’s special hue. The ruse could even be pulled off by planets similar to Mars, whose surface is a rusty red and plainly visible through its currently tenuous, carbon dioxide-dominated atmosphere. With a more substantial atmosphere, though, the Red Planet would look from a great distance like a Pale Blue Planet.”
On the matter of a certain spectral signature, though, no other world type displayed it except Earth. This telltale signature assumes a U-shape as it stretches from the near-ultraviolet (beyond visible light) through the visible and then into near-infrared wavelengths. Its up-down-up shape derives from the very features that make Earth a pale blue dot in the first place: Rayleigh scattering in the bluer portion of the spectrum; some absorption in the greener portion because of the ozone in our atmosphere, which like the oxygen we breathe would not exist without life; and finally red light, again owing to continental reflectance, but also from the Earth’s widespread vegetation that reflects strongly in infrared. Krissansen-Totton is pleased that this signature could separate Earthly pretenders from the genuine article of a potentially inhabited world." Astrobiology
.....fringe of the borders a ribband of blue:
And it shall be unto you for a fringe, that ye may look upon it,
and remember all the commandments of the LORD, and do them;
Numbers 15:38,39
