While we’re this early in the game of detecting life signs from distant planets, it makes sense to focus on surface habitability, which is why oceans are so interesting. Sure, we can imagine potential biospheres under the ice of a Europa or even an Enceladus, but given the state of our instrumentation and the distance of our target, going after the most likely catch makes sense, and that means looking for oceans. Significant work from the EPOXI mission has given us some of the parameters for studying a planet like ours using multi-wavelength photometry.
EPOXI, you’ll recall, is the extended mission of the Deep Impact spacecraft that drove an impactor into Comet Tempel 1 in 2005 and is now enroute to Comet Hartley 2. Its views of Earth are being used to help scientists prepare for studies of terrestrial worlds around other stars. Planets with large bodies of water should reflect light from their star differently than dry planets, and as the observed planet goes through its phases as seen from Earth, the changes in that reflectivity can be measured. EPOXI showed us that we can make useful observations at different points in the Earth’s rotation. We’ve also seen specular glints on Titan, and now the focus is on what else we can learn to help us exploit this phenomenon.
Tyler Robinson (University of Washington) is involved in the study of such glints to help find Earth’s twin somewhere among nearby stars. Robinson’s team has been using the NASA Astrobiology Institute’s Virtual Planetary Laboratory, which allows them to model the Earth as it would appear to a distant observer tracking the planet’s progress through an entire orbit. It turns out that in a variety of simulations the ‘glinting Earth’ can be as much as 100 percent brighter at crescent phases than when modeled without the glint effect, a result that may be observable with the James Webb Space Telescope. Robinson describes the glint colorfully to BBC News:
“The glint I’m talking about is pretty much the exact same thing when you talk about gorgeous sunsets over the ocean. With the sun low on horizon, sun beams come in and glance off the ocean surface which is acting like a mirror and you get these beautiful red sunsets.”
Image: Glinting sunlight off Lake Erie (not an EPOXI image). Source: Image Science and Analysis Laboratory/NASA JSC.
And now we know that the glint effect (‘specular reflection,’ to be precise) produces major changes in brightness. For all its powers, though, the JWST wouldn’t be able to spot a glinting planet without the use of an external occulter, a shield that blocks starlight to reveal much fainter planets. And the new work tells us what wavelengths are the most likely to produce results. Here the authors discuss them in the context of Rayleigh scattering, the scattering of light by particles smaller than the wavelength of light, which must be incorporated in the analysis:
At crescent phases, pathlengths through the atmosphere are relatively large and optical depths to Rayleigh scattering can be larger than unity even at longer wavelengths. This indicates that observations which aim to detect the brightness excess due to glint should be made at wavelengths in the near-infrared range. Earth’s brightness drops by over an order of magnitude between 1-2 μm, arguing that searches for glint should occur below 2 μm for higher signal-to-noise ratio (SNR) detections. Since glint is a broad feature in wavelength space (it is the reflected solar spectrum, modulated by Rayleigh scattering, liquid water absorption at the surface, and atmospheric absorption), photometry can be used to detect glint provided that strong absorption features are avoided.
All this is helpful information as we add the items we need for detecting habitability to our tool chest. We can take into account the fact that the size of a ‘glint spot’ compared to the illuminated portion of a disk is highest at crescent phases and add in the fact that the reflectivity of water increases at glancing illumination angles, but as the authors do, we also have to factor in how the glint effect can be duplicated by liquid and ice crystals in high clouds. New work following up on high clouds and their uses in detection will be presented in October at the Division of Planetary Sciences meeting in Pasadena, and I’ll have more on it then.
The paper is Robinson et al., “Detecting Oceans on Extrasolar Planets Using the Glint Effect,” Astrophysical Journal Letters 721 (2010), L67 (preprint).