Light Off Distant Oceans

by Paul Gilster on September 17, 2010

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).

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{ 8 comments }

Carl Keller September 17, 2010 at 10:46

Percival Lowell had written an account of high albedo seen at one session: http://en.wikisource.org/wiki/Page:Mars_-_Lowell.djvu/110

“…I saw suddenly two points like stars flash out in the midst of the polar cap. Dazzlingly bright upon the duller white background of the snow, these stars shone for a few moments and then slowly disappeared. The seeing at the time was very good. It is at once evident what the other-world apparitions were,—not the fabled signal-lights of Martian folk, but the glint of ice-slopes flashing for a moment earthward as the rotation of the planet turned the slope to the proper angle; just as, in sailing by some glass-windowed house near set of sun, you shall for a moment or two catch a dazzling glint of glory from its panes, which then vanishes as it came. But though no intelligence lay behind the action of these lights, they were none the less startling for being Nature’s own flash-lights across one hundred millions of miles of space. It had taken them nine minutes to make the journey; nine minutes before they reached Earth they had ceased to be on Mars…”

Some of his conclusions have been relegated to fancy, but this may be valid.

Dale September 17, 2010 at 11:22

Practical application of this I think will be very complex. But I agree it is another great tool in our on going search for a planet in the habitable zone of another solar system.

Dale September 17, 2010 at 11:25

Are there any systems they are targeting to try and apply this?

Paul Gilster September 17, 2010 at 13:13

Dale, it’s too early for targets, but I would guess that Margaret Turnbull’s list of potentially habitable planets would be a good place to start:

http://www.centauri-dreams.org/?p=548

Ideally, the ‘glint’ method would reveal targets for deeper astrobiological studies, including (some day) spectroscopic analysis of atmospheres, etc.

andy September 18, 2010 at 16:16

Well of the known exoplanets, only Gliese 581 d stands a chance of having liquid water oceans, and even if it does they would be beneath an atmosphere far thicker and denser than that of Titan. I’d guess detecting glints would be quite difficult in this case.

Wonder how well it would work for Earthlike exomoons, would the contamination of the light by a giant planet prevent the detection of glints?

spaceman September 20, 2010 at 8:30

What a clever idea– makes me wonder if and when we detect this effect what creatures might lie beneath the glint.

Andy– indeed, it would seem like the detection of this effect would be hampered in the case of the exomoon by the glow of the giant planet.

Eniac September 22, 2010 at 20:52

If glints can be detected, they will not only provide proof of existence of oceans, when observed over longer times they will also permit mapping the extent and shape of those oceans. The “glint spot” scans over the planet as it spins and rotates around its sun. It will blink on over ocean and off over land, and with sufficient observation time we should be able to assemble a fairly high resolution map of a significant part of the planet’s surface.

ljk May 9, 2012 at 17:19

Searching for Exoplanet Oceans More Challenging Than First Thought

by Jon Voisey on May 8, 2012

As astronomers continue to discover more exoplanets, the focus has slowly shifted from what sizes such planets are, to what they’re made of. First attempts have been made at determining atmospheric composition but one of the most desirable finds wouldn’t be the gasses in the atmosphere, but the detection of liquid water which is a key ingredient for the formation of life as we know it. While this is a monumental challenge, various methods have been proposed, but a new study suggests that these methods may be overly optimistic.

One of the most promising methods was proposed in 2008 and considered the reflective properties of water oceans. In particular when the angle between a light source (a parent star) and an observer is small, the light is not reflected well and ends up being scattered into the ocean. However, if the angle is large, the light is reflected.

This effect can be easily seen during sunset over the ocean when the angle is nearly 180° and the ocean waves are tipped with bright reflections and is known as specular reflection. This effect is illustrated in orbit around our own planet above and such effects were used on Saturn’s moon Titan to reveal the presence of lakes.

Translating this to exoplanets, this would imply that planets with oceans should reflect more light during their crescent phases than their gibbous phase. Thus, they proposed, we might detect oceans on extrasolar planets by the “glint” on their oceans. Even better, light reflecting off a smoother surface like water tends to be more polarized than it might be otherwise.

Full article here:

http://www.universetoday.com/95065/searching-for-exoplanet-oceans-more-challenging-than-first-thought/

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