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.

Wake up and taste the water

Comets have made the news a number of times in recent months. These interplanetary travellers, which were once thought to herald doom, are now known to be among the most primitive objects in the solar system. And of course, comets periodically make visits from the far reaches of the solar system (regions known as the Oort Cloud and the Kuiper belt) to our more local neighbourhood.

Back in December, Comet ISON—the 'comet of the century'—made a much-watched and disappointing suicide plunge into the Sun. And just this week, the European Space Agency successfully 'woke up' its Rosetta spacecraft from its two-and-a-half-year hibernation.

Currently more than 400 million miles away from home, Rosetta is journeying towards the comet 67P/Churyumov-Gerasimenko. Once it arrives, it will first orbit, and then land on the comet's surface. Together, the orbiter and lander portions of the spacecraft are equipped with over 20 scientific instruments that will make important characterizations of the comet. Scientists working on the mission hope to find out if comets such as this, which contain complex organic molecules, may have played a role in seeding life on Earth.

Artist's impression of the European Space Agency's Rosetta spacecraft orbiting the comet 67P/Churyumov-Gerasimenko. Credit: ESA, C.Carreau / ATG medialab

There has also been a long-lived debate over whether or not comets contributed significantly to the delivery, early in its history, of Earth's vast water inventory.

Because water is such a vital ingredient for the sustenance of life here on Earth, we target our search for extraterrestrial life on places where water exists. Water therefore seems like a pretty obvious choice for an Earth-material to send as one of our interplanetary postcards. But how might an alien scientist be able to tell our Earth-water apart from any other foreign H₂O? We all know that water can vary drastically in its colour, salinity, taste, etc. So is there a characteristic signature of our water that portrays its Earthly provenance?

Earth: A water world. Credit: NASA

Lucky for us, chemistry has an answer. Hydrogen (H), like all elements is defined by the number of protons it contains. 'Normal' hydrogen contains just one proton in its nucleus. However, it is possible for a stable hydrogen atom to contain one or two neutrons in addition to the lone proton. The hydrogen isotope that has one proton and one neutron is known as deuterium (D). Heavy water is enriched in molecules that contain deuterium in place of the normal hydrogen.

Planetary scientists have shown that D/H ratios in water vary throughout the solar system. Measurements for a number of Oort Cloud comets reveal that they have D/H ratios which are more than twice the value for Earth's oceans, and are thus unlikely to have been the source of water on our planet.

Range of deuterium/hydrogen (D/H) ratios in solar system objects.
Credit: A. E. Saal et al. 2013, Science

Paul Hartogh and colleagues, however, showed in 2011 that a Jupiter-family comet (103P / Hartley 2), which probably originated from the Kuiper belt, has a D/H ratio that is much more consistent with that of Earth. This means that at least some of Earth's water may have been delivered by comets. Although in more recent work, Conel Alexander et al. argue that CI chondrites (the class of meteorite whose composition most closely resembles that of the Sun) were the principal source of terrestrial water.

And so the great water debate continues.  But no matter how, and from where, the water got here; get here it did. And we wouldn't be alive without it.