A glint from Earth

By Phil Plait | January 7, 2010 7:34 am

Back in 2005, NASA’s Deep Impact probe slammed a hunk of copper into the comet Tempel 1 to determine what was under its surface, as well as to see what happens in a hypervelocity collision.

The copper block vaporized in the high-energy impact, but the spacecraft lived on. It’s now on an extended mission called EPOXI, and one part of that is EPOCh: Extrasolar Planet Observations and Characterization, designed to look back at the Earth and see what a habitable and inhabited planet looks like from a distance. The idea is to see what we can observe about our own world that can be used to look at worlds orbiting other stars.

EPOXI_glint

One hope was that the spacecraft would see sun glints; flashes of light from standing water on Earth (much like Cassini did with Titan). And see them it did! Check out the video below. It’s short, so you can watch it a few times; there are several glints, and I labeled the region of Earth where they occur.

That video was put together by Don Lindler (my old boss back in the STIS/Hubble days) using images from Deep Impact. It shows a full rotation of the Earth as seen above the north pole, taken when Deep Impact was still 18 million kilometers (11 million miles) from home (that’s 75 times the distance to the Moon!). Another video, from a different part of Deep Impact’s travels, shows the view as seen from the south. Due to the geometry of the Sun, Earth, and spacecraft, the glints all appear on the same place on the Earth’s face, though the location on the Earth’s surface changes as it rotates.

These images were taken in the infrared, where the contrast between land and water is highest. There may come a day when our spacecraft observe other planets orbiting other stars, and glints like these may be the tell-tale signs of liquid on their surface. In those cases, the planet may not be more than an unresolved dot, but the sudden increase in brightness may be the giveaway we’re looking for.

There’s been speculation lately that some extrasolar planets may be water worlds. We can’t know for sure just yet, but EPOXI may be showing us one way we might be able to find out.

Related post: HOLY FRAK! Moon transits Earth!

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (25)

  1. StevoR

    Awesome! Thanks for that BA – & your ex-boss. Great clip. :-)

    Sure looks a lot like visible light to me – near infra-red?

    Was the size of the Earth constant throughout? I’d have thought a spaceprobe moving that quickly would see the Earth swell and shrink as it grew nearer & further?

    How far is it in Astronomical Units …. & how does that compare to spotting something light years away – at eleven or so for the nearest exoplanet* we know of today?

    —-

    * Epsilon Eridani b at a distance of 10.5 light yeras unless one has been discovered closer that I haven’t heard about & assuming Gatewood’s old claim that Lalande 21185 (8.3 ly, 4th closest star system) has an exoplanet still remains unconfirmed.

  2. Kevin F.

    Why would the lakes glint and the oceans not? Because the lakes have a flatter surface than the oceans?

    March 27-28-29 was Friday-Saturday-Sunday in case you’re wondering what you were doing at the time. :)

  3. Michel

    So that´s what bosses are making. Little videos…
    Anyway.
    If they ever make another Star Trek series, there will be a dialog like:
    “Sir, I just spotted a glint!”
    “Set a course to the habitable planet!”

  4. Pi-needles

    Will we say in the future?

    I knew that lifeform/planet back when it was just a glint in the astronomer’s eyes? ;-)

  5. Matt Tarditti

    “In those cases, the planet may not be more than an unresolved dot, but the sudden increase in brightness may be the giveaway we’re looking for.”

    BA: How sensitive are these instruments, anyways? I mean, if that tiny glint on the surface of a comparably massive planet (earth >> glint), how much of a percentage increase are we talking, and is that really enough for these instruments to pick up at such a great distance?

  6. I would think you’d get a constant specular highlight (glint) from the oceans, but it didn’t look like that. Any idea why? Maybe the oceans were never in the right spot?

  7. Paul M

    I see I’m not the only one to wonder why the oceans did not glint. After a couple viewings, the seems to be a brief ocean glint at around 11 seconds, just before the “California” title disappears. (fullscreen HQ helps).

  8. kevbo

    Couldn’t that actually be Gilligan and the castaways reflecting an SOS signal? (those poor people…)

  9. It does seem strange that on a planet that’s 70% water, a space probe would get only brief glints indicating that there was any water at all. I mean, you can see the oceans in the video, and then there’s the occasional flash where the probe ‘finds’ water. There must be something we’re not understand about this. Phil?

  10. Sahil P

    A more detailed story (http://www.physorg.com/news181935324.html) makes a small mention of the flatness requirement. Maybe the ocean is just too wavy to glint?

  11. T.E.L.

    StevoR Says:

    “Sure looks a lot like visible light to me – near infra-red?”

    The pics are color combos of independent images in blue, green and near-IR filters.

  12. Torbjörn Larsson, OM

    That is so awesome to see! It makes Earth so much more like a familiar object despite it’s size.

    I wonder if one can also see the elliptic polarization that liquid or ice reflection should give?

    when it was just a glint in the astronomer’s eyes

    That was awesome too. LOL!

    @ Kathy:

    You likely need little cloud cover when light goes in and out. What is the combined likelihood of that?

    Also, maybe glinting is brought out by polarization (by way of specular reflection), i.e. it may enhance contrast. Incoming light would be polarized in cloud scattering as well, and maybe not all is randomly. (Randomly should in effect average to circular precisely as the sun). Of course, in light of my ignorance here I’m only specular-izing at this point. :-)

  13. Maybe the ocean is glinting, it’s just a “continuous glint.”

    Oceans are large, continuous bodies of water, while lakes are small, land-locked puddles. The light reflecting off the former gets bright and stays bright for a long time. The latter is surrounded by much-darker rocks, so it tends to wink on-and-off very quickly. While the brightness curve of an ocean could be mistaken for a geologic feature from a long distance, I can’t think of any rock formation that could duplicate the winks of a lake.

  14. Tom

    A number of years ago I was involved in the launch of a geosync com satellite and during the transfer orbit we got a “glitch” in our earth sensor data we had never seen before. After some head scratching someone got the Sun Earth Vehicle geometry figured out and we decided it was possible that we got a glint from the sun off the ocean. It did repeat on other orbits and since it seemed to be working fine, we convinced management to not worry and marvel at the discovery.

  15. ScienceTim

    Hey! What do ya know? That’s my work! I didn’t think we had publicly released the videos yet…

    The glint is definitely not ever-present. If you look closely, you’ll see that the strongest glint is over the Aegean Sea, which is certainly not a lake, but has different wave characteristics than open ocean, I expect. I think you would need a real physical oceanographer to provide a completely thought-out explanation, but I would guess it’s a combination of effects requiring direct illumination of the ocean surface, surface roughness requirements, and wave amplitude.

    If you look VERY closely, you will see that there are glints both from the ocean surface and from lakes. The lakes are much smaller, so a smaller physical region contributes to the observed glint.

    Specular reflection has significant polarization effects, of course, but EPOXI has no polarimeter. Have to save that project for another mission…

    The color range is 450±50 nm (blue channel), 550±50 nm (green channel), and 850±50 nm (red channel) = deepest red or nearest-infrared. The critical issue in choosing the color mapping is that the 850 nm channel is just on the high-reflectivity side (longer-wavelength side) of the vegetation red edge feature, so plant life shows up as rather strongly red.

    The spacecraft was about 0.18 AU from Earth during these observations, distance barely changing, so the Earth does not noticeably vary in angular size.

    Visit our mission web site at http://epoxi.umd.edu/

  16. StevoR

    @ 12. T.E.L. Says:

    StevoR Says: “Sure looks a lot like visible light to me – near infra-red?”
    The pics are color combos of independent images in blue, green and near-IR filters.

    Thanks. :-)

    Thanks too ScienceTim (16) for a great video & your extra info there. :-)

  17. coolstar

    Phil once again misses the point in favor of pretty pictures (images we didn’t really need EPOXI to get). The more important work is what we can learn when the EPOXI data is collapsed to an unresolved single pixel (go to their web page to see some of this). This is the type of data we’ll have long before we have images with the resolution in the above time-lapse (in fact, NO mission even in the planning stages will give us data anywhere NEAR that good).

  18. ScienceTim

    Let me speak up for Phil in defense against coolstar. The reason for doing the measurements is, indeed, to simulate observations of exoplanets as point sources, but that does not invalidate other ways to appreciate the data. The advantage of using spatially-resolved imagery to simulate the observation of Earth as a point source is that we can verify our deductions about the relative contribution of different phenomena, such as the sun glint. We can directly extract the sun glint from our images (work in progress) and thereby estimate its significance on a world with different properties — more water surface, less water surface, etc. We can construct very realistic models of the Earth from various vantage points, of course, but the EPOXI data (pretty pictures and all) permit us to evaluate those models and determine how much is actually true, and how much just seemed like it would be true. To do that, spatially resolved imagery (pretty pictures) are an advantage to interpretation.

    No spacecraft have been flown with the intention of imaging Earth in the way that we have done, capturing the whole disc from a distant vantage point. Why would you? If your intention is to learn about Earth, you can do better from relatively nearby, looking at patches of Earth. *Our* intention was to learn more about radiative transfer modeling, which has been used to convert high-quality Earth data into simulations of the distant perspective. EPOXI lets us test those models. We have learned important things already (publication under review).

    Other spacecraft *have* imaged Earth before, but it was not technically a serious component of the mission. Truth is, pictures of Earth from some other spacecraft (like HiRISE on MRO) actually are prettier (better resolution) than the EPOXI pictures, but they have not expended the resources to do what we have done: collected high sample rate through full rotations, with seasonal variation (we have observed on 5 separate occasions), with a generic instrument. Most instruments are highly-targeted on specific attributes — Mars cameras, for instance, have filters selected to emphasize geologic features. Because our spacecraft was designed to observe goodness-knew-what from the Deep Impact mission, it has a very non-specialized wavelength coverage. Perfect for modeling an observation that we don’t fully know yet how to do.

  19. ScienceTim: thanks, and of course you are right and coolstar is wrong. But don’t sweat it; after repeated warnings he came here and threw insults around, so I’m henceforth marking his comments as spam.

  20. oqwhdoahd

    Misleading headlines deflate the value of all headlines!

  21. Messier Tidy Upper

    @ ScienceTim : Thanks! :-)

    Visited your website & look forward to hearing more – great to have the people who are actually capturing these uimages & making these clips & flying these probes commenting here! Well done. :-)

    I’ve only got a couple of very minor gripes – why not leave the name as Deep Impact rather than risk confusion with the EPOXI name change & how about having a facebook page too like the WISE people have? ;-)

    @ 14. HJ Hornbeck Says:

    Maybe the ocean is glinting, it’s just a “continuous glint.”Oceans are large, continuous bodies of water, while lakes are small, land-locked puddles. The light reflecting off the former gets bright and stays bright for a long time. The latter is surrounded by much-darker rocks, so it tends to wink on-and-off very quickly. While the brightness curve of an ocean could be mistaken for a geologic feature from a long distance, I can’t think of any rock formation that could duplicate the winks of a lake.

    Halite (rock salt) & gypsum spring to mind as does sand … Sometimes a dry salt lake / salt pan or an expanse of flat sandy ground or a sheet of ice or snow could be confused at least by my eyes. (There was a dry creekbed along somewhere where I used to walk – I always liked imagining it as ice or salt rather than sand & it looked like it coudl have been those too …)

    Not sure how this works at infra-red wavelengths or if EPOXI have already considered this -expect they have – but interested to see if they can tell the difference between equally white surfaces of ice, salt* and sand?

    ——

    * “Salt” here meaning a range of evapourite deposits such as halite, gypsum, etc …

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