Abstract
Sagan et al. (1993) used the Galileo space probe data and first principles to find evidence of life on Earth. Here we ask whether Sagan et al. (1993) could also have detected whether life on Earth had three-dimensional structure, based on the Galileo space probe data. We reanalyse the data from this probe to see if structured vegetation could have been detected in regions with abundant photosynthetic pigments through the anisotropy of reflected shortwave radiation. We compare changing brightness of the Amazon forest (a region where Sagan et al. (1993) noted a red edge in the reflectance spectrum, indicative of photosynthesis) as the planet rotates to a common model of reflectance anisotropy and found measured increase of surface reflectance of 0.019 ± 0.003 versus a 0.007 predicted from only anisotropic effects. We hypothesize the difference was due to minor cloud contamination. However, the Galileo dataset had only a small change in phase angle (sun-satellite position) which reduced the observed anisotropy signal and we demonstrate that theoretically if the probe had a variable phase angle between 0–20°, there would have been a much larger predicted change in surface reflectance of 0.1 and under such a scenario three-dimensional vegetation structure on Earth could possibly have been detected. These results suggest that anisotropic effects may be useful to help determine whether exoplanets have three-dimensional vegetation structure in the future, but that further comparisons between empirical and theoretical results are first necessary.
Highlights
Carl Sagan and others published a seminal paper in Nature using the Galileo space probe data to see if signs of life could be detected remotely on Earth to serve as a type of “control” experiment for the new field of astrobiology [1]
Do the photosynthetic pigments seen by Sagan et al (1993) have three dimensional structure, or are they structurally more similar to “green slime”? We view the reflectance for this region in the near infrared (NIR) images as the region moves from a solar zenith angle of 16.6 ̊ to one doi:10.1371/journal.pone.0167188.g002
Does this change in reflectance match what we would expect based on directional anisotropic effects from a changing solar and view zenith angle? Based on the parameters of the reflectance anisotropy model and the geometry listed in Table 1, we calculate the reflectance anisotropy in the NIR for the red box in Fig 2A and 2B (Fig 3A and 3B)
Summary
Carl Sagan and others published a seminal paper in Nature using the Galileo space probe data to see if signs of life could be detected remotely on Earth to serve as a type of “control” experiment for the new field of astrobiology [1]. We reanalyse the original Galileo space probe data (http:// pdsimg.jpl.nasa.gov/data/galileo/galileo_orbiter/) to test whether they missed a big potential life stage, that is, the existence of vegetation with three-dimensional structure on Earth [15]. Using the instruments of the space probe, Sagan et al (1993) found evidence on Earth for an oxygen atmosphere, an ozone layer, atmospheric methane, photosynthetic pigments, and radio signals indicative of intelligent life on Earth. An oxygen atmosphere and photosynthetic pigments can be signs of single-cellular life and are not sufficient evidence of abundant vegetation with three-dimensional structure. The patch of photosynthetic pigments observed by Sagan could have been “green slime,” since according to isotopic evidence, single cellular photosynthetic organisms were likely present on land from 1.2 billion years ago [16]. We would want a technique to distinguish between multicellular and intelligent (technological) life, since the ~500 million year period since the Cambrian explosion gave rise to “endless forms most beautiful and most wonderful” [17]
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