Good morning Earthshine

When I first mentioned the overall idea for this blog to a friend and colleague, he immediately found an interesting way to slightly reframe the question. Instead of considering how alien planetary geologists might recognize rock specimens representative of Earth, he wondered how Earth might look to an alien astronomer observing us with a faraway telescope.

This question is actually a pretty obvious one, especially given the popular pursuit of extrasolar planets in current astronomical research. The first confirmed detection of an 'exoplanet'—a planet that orbits a star other than our own Sun—was not made until 1992, but this field of research has now, almost literally, exploded. More than 1800 exoplanets have since been discovered, and this has largely been possible because of NASA's Kepler mission. The aim for this space observatory was to discover Earth-like exoplanets that are located in, or near, the 'habitable zone' of their parent star. This habitable—or Goldilocks—zone is the region around a star where planets (with sufficient atmospheric pressure) can support liquid water at their surface.

The habitable zones—where liquid water can exist on the surface of a planet—of different size and temperature stars. Credit: NASA Kepler Mission

This is all part of humankind's everlasting desire to discover life—potentially sentient—elsewhere in the universe. As our home continues to be the sole 'datapoint' for life, it is natural that we use Earth-like planets as a base for our search. And it is the special feature of liquid water at the surface that makes our planet so hospitable. The modern-day search for extraterrestrial life is therefore often focused on the hunt for this precious H₂O. A discovery of a planet hosting water at its surface within another star's habitable zone, however, does not equal the discovery of life elsewhere in the cosmos.

To be more certain of a water-hosting planet's potential to harbor life, an additional telltale detection is required. The spectra from the observed exoplanets must include signs of life—biosignatures—along with the sign of water. But what would an astronomical biosignature look like, and would we even recognize such a signal from an exoplanet? To begin to answer this question, we first need to understand how Earth's atmosphere looks from afar and which of its properties hint at the rich biosphere that lies beneath. This information can then be used as a reliable baseline with which to compare exoplanet detections.

 

Spectra for Venus, Earth, and Mars illustrate Earth's unique biosignatures. All three planets have a strong atmospheric absorption caused by carbon dioxide (CO₂), but only Earth's atmosphere has signals due to water (H₂O) and ozone (O₃) that can be representative of life. Credit: Mark Elowitz

As detailed in a 1993 study by Carl Sagan and colleagues, observations of Earth's atmosphere from space—in this case from the Galileo spacecraft—reveal several biosignatures. These include abundances of molecular oxygen and methane that are far from chemical equilibrium, as well as a sharp increase in albedo at wavelengths longer than 700 nm, which is caused by vegetation. It is also known that as light passes through Earth's atmosphere it can be polarized due to scattering by aersols and cloud particles, and reflected at variable amounts by oceans and land. In a more recent paper, Michael Sterzik et al. use a technique known as spectropolarimetry to make a detailed analysis of Earth's atmospheric properties.

Instead of using space-based measurements of Earth, Sterzik and co-workers made observations of 'Earthshine' by pointing their telescopes at the Moon. This rather romantic sounding light originates from the Sun before being reflected by the Earth onto the Moon, and then back to Earth again. It is the reason you can sometimes to see the 'dark' part of a non-full Moon.

Earthshine illuminates the 'dark' portion of the Moon. Credit: Will Gater

Sterzik et al. used a technique known as spectropolarimetry (a combination of spectroscopy and photopolarimetry) to conduct a detailed investigation of Earth's atmosphere. This methodology is better than standard spectroscopy for characterizing exoplanet atmospheres. The Earthshine observations could be used to determine the fractional contribution of cloud and ocean contributions within the reflecting surface, and were sensitive to relatively small areas of vegetation.

It is measurements such as these, using the Moon as a handy mirror, that can be used as a benchmark for diagnosing the atmospheric composition and surfaces of potential life-bearing exoplanets. They also serve as a clue to what an alien astronomer might see when they glance in our direction. I hope they too can recognize how special our Earth is.