# Detecting 3D vegetation structure with the Galileo space probe: Can a distant probe detect vegetation structure on Earth?

**Authors:** Christopher E. Doughty, Adam Wolf  
**arXiv:** [1608.08125](https://arxiv.org/abs/1608.08125)  
**Date:** 2016-08-29  
**Summary page:** https://www.summarizepaper.com/en/paper/1608.08125v1/

## Key points

- Sagan et al. used Galileo space probe data and scientific principles to identify signs of life on Earth
- This study investigates whether the Galileo probe could have detected the three-dimensional structure of life on Earth
- Researchers reanalyze the probe's data to determine if structured vegetation could be identified through reflected shortwave radiation
- Changes in brightness observed in the Amazon forest are compared with a model of reflectance anisotropy
- A measured increase in surface reflectance is found, suggesting structured vegetation presence
- Cloud contamination is hypothesized as a factor contributing to the difference between observed and predicted values
- The small change in phase angle reduced the observed anisotropy signal, but a larger change would have allowed for better detection of three-dimensional vegetation structure
- Anisotropic effects may serve as indicators for determining three-dimensional vegetation structures on exoplanets in future studies
- Further comparisons between empirical and theoretical results are needed before drawing definitive conclusions about their effectiveness as indicators for extraterrestrial life detection efforts

## Summary

In their study, Sagan et al. (1993) utilized data from the Galileo space probe and fundamental scientific principles to identify signs of life on Earth. Building upon this research, the authors of this paper investigate whether the Galileo probe could have also detected the three-dimensional structure of life on Earth. Specifically, they reanalyze the data collected by the probe to determine if structured vegetation, characterized by abundant photosynthetic pigments, could have been identified through the anisotropy of reflected shortwave radiation. To assess this possibility, the researchers compare changes in brightness observed in the Amazon forest (a region previously noted by Sagan et al. for its red edge reflectance spectrum indicative of photosynthesis) as the planet rotates with a common model of reflectance anisotropy. They find that there is a measured increase in surface reflectance of 0.019, whereas only a 0.007 increase is predicted based solely on anisotropic effects. The difference between these values is hypothesized to be due to minor cloud contamination. However, it is important to note that the Galileo dataset had a small change in phase angle (sun-satellite position), which reduced the observed anisotropy signal. To demonstrate this effect theoretically, the researchers propose that if the probe had a variable phase angle ranging from 0-20 degrees, a much larger predicted change in surface reflectance of 0.06 would have been expected. Under such conditions, it would have been possible to detect three-dimensional vegetation structure on Earth with greater accuracy than before. These findings suggest that anisotropic effects may serve as valuable indicators for determining whether exoplanets possess three-dimensional vegetation structures in future studies and provide insight into potential methods for detecting and characterizing life beyond our planet based on analysis of reflected radiation and anisotropic effects. However, further comparisons between empirical and theoretical results are necessary before drawing definitive conclusions about their effectiveness as indicators for extraterrestrial life detection efforts. Overall, this study expands upon previous research conducted by Sagan et al., providing new evidence regarding potential methods for detecting life beyond our planet using reflected radiation and anisotropic effects analysis techniques.