Would Earth look like a habitable planet from afar?

June 30, 2014 by Elizabeth Howell, Astrobio.net, Astrobio.net
A zoomed-in image of Earth from the perspective of NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS). The small dot below it is the moon. Credit: NASA Ames

Even when a distant world has the trademarks of habitability—it's Earth-sized, it's in the zone around its star where liquid water is possible—finding signs of life is tricky. The telescope technology of today falls short of being able to distinguish clues of life.

But readying the tools to find life now will help astronomers when telescopes get better in the next few decades. Sometimes, this requires looking at a planet that we already know has life—that would be Earth, the only confirmed one so far—and pretending that we are looking at it as a visiting extraterrestrial.

When viewing Earth from space, how could you tell that this planet is well-suited for life? Are there telltale signatures in the atmosphere or from our oceans? These are some of the questions that controllers of a sought to answer when it took a bit of a side mission. Instead of observing the Moon, NASA's Lunar Crater Observation and Sensing Satellite (LCROSS) briefly looked at Earth.

"The LCROSS spacecraft observed the Earth and made statements about ozone in Earth's atmosphere and also liquid water," said lead researcher Tyler Robinson, a postdoctoral researcher at the NASA Ames Research Center in Mountain View, Calif. "We also used it to validate a tool to simulate how a distant Earth would appear."

A paper on the research, "Detection of Ocean Glint and Ozone Absorption Using LCROSS Earth Observations," is available now on the pre-publishing site Arxiv and has been accepted in the Astrophysical Journal.

Searching for water

LCROSS, which was smashed into the Moon as planned in 2009, had a primary mission to look for the signature of lunar water. About a decade before, NASA's Lunar Prospector mission found hints of hydrogen in craters at the Moon's poles. The divots are permanently shadowed from the heat of the Sun.

LCROSS was to follow up on those observations, and it repaid the investment in spades. It tracked what happened after its spent Centaur rocket stage crashed into the crater Cabeus near the Moon's south pole, and found signs of hydrogen in spectroscopic measurements spanning infrared and ultraviolet light.

When LCROSS crashed into the itself, observations with NASA's Lunar Reconnaissance Orbiter and others revealed about 100 kilograms of water in the crater it punched in the regolith, which was about 20 meters (66 feet) across.

The spacecraft was indeed successful in finding (and helping other spacecraft find) water on the Moon. But could it also find water on our ocean-rich Earth at a distance? Scientists became curious about the prospect, especially after seeing that our oceans make a mirror-like reflection, called "glint," when a distant Earth appears as a crescent from the perspective of the Moon.

LCROSS did three observation sessions of Earth in 2009. Interestingly, the spacecraft was not originally tasked to look at Earth as an exoplanet. Instead, scientists were evaluating how accurately the spacecraft was pointing after launch, said co-author and NASA astrophysicist Kimberly Ennico-Smith. The data was later repurposed for the exoplanet modeling used in this research.

Three-quarters of Earth’s surface is ocean. Designing procedures to detect the water from afar could help scientists do the same thing for exoplanets. Credit: NASA

"You never know what else another pair of eyes looking at data can bring you," she wrote in an e-mail. "That's why having and maintaining archives is so important."

For example, finding hydroxyl—a type of water—on the Moon came from combining sets from India's Chandryaan-1 lunar spacecraft, and NASA's Cassini spacecraft on its way to Saturn. Both missions were using the Moon to calibrate their instruments; ocean examinations were not the main objective.

Seeing a glint

Looking at the repurposed data yielded a surprise. Not only did LCROSS see a glint, but it was a lot different than what researchers expected.

"The glint detection I found to be surprising for a couple of reasons," Robinson said. "The spacecraft observation of glint was in disagreement with some previous observations that were done from the ground."

Specifically, some researchers had tried to make predictions of the Earth's glint based on gazing at the Moon. When looking at the Moon outside of full phase, it's possible to see the Earth's light shining faintly off of it in a phenomenon called "Earthshine."

By comparing Earthshine data from a crescent-phase Earth with data from other phases, it's possible to get measurements of how significant glint is in observations of Earth's crescent sliver. These measurements predicted a much stronger glint than what Robinson's team saw using the LCROSS data.

What also surprised researchers was how different the glint appeared in different wavelengths of light. At some wavelengths, glint dominated Earth's appearance, while at other wavelengths, the glint effect was more muted, as it was masked by certain atmospheric phenomenon.

"Also, the Earth at crescent phase, thanks to the ocean, can be twice as bright. If it's something you look for in exoplanets, it can be a significant effect," added Robinson.

Artist’s conception of NASA’s James Webb Space Telescope, which is expected to launch in 2018. While this will be an able planet-hunting telescope, it will likely take a future generation of telescopes to seek oceans from afar. Credit: NASA

Designing future telescopes

If over the course of several orbits, a planet is observed as more reflective at crescent phases and less reflective at other phases, then can it be assumed that ocean glint is the cause? Robinson cautions that the answer is not that simple.

"There could be other explanations," he said. "Clouds have a tendency to reflect better at crescent phases than at other phases, and recent work has shown that, under some circumstances, the ice-covered polar regions can mimic certain glint effects."

But there could be other indications of habitability and life as well. One thing they noticed from a distance was ozone, which was not as much of a surprise to scientists but still a useful tool for observations. Ozone especially showed up in ultraviolet light, and it could be a "bio-indicator," or sign of life, on distant planets, Robinson said.

"Ozone is a key potential indicator of life, and it appears most strongly in ultraviolet observations of Earth," he said. "So, future telescopes could look to the ultraviolet as a place to more easily detect biosignature gases."

Artist’s conception of Kepler-69c, a rocky planet larger than Earth that orbits in what could be a habitable region of its star. Credit: NASA

Such a telescope, however, will be a couple of decades down the line. While NASA's James Webb Space Telescope will be an able planet-hunter, it will take the resolution of something like the cancelled NASA Terrestrial Planet Finder project to make better progress in searching these worlds, he said. There were a few different ideas for what it would look like, but one design had intended to combine four, 3.5-meter telescopes in space to look at parameters such as temperature and atmosphere, among others.

Another important aspect of the observations performed by LCROSS is that they become the basis for new telescope designs. NASA's work allows researchers to gather data on which designs would best pick out certain features of planets, such as the reflectivity or ozone that LCROSS observed.

"It's using current tools to predict and understand what future telescopes might one day see. By studying Earth now, you can ensure that we don't accidentally engineer the telescope of the future and find out we didn't build it strong enough," Robinson said.

Explore further: NASA launches LCROSS Lunar Impactor

More information: "Detection of Ocean Glint and Ozone Absorption Using LCROSS Earth Observations." Tyler D. Robinson, Kimberly Ennico, Victoria S. Meadows, William Sparks, D. Ben J. Bussey, Edward W. Schwieterman, Jonathan Breiner. arXiv:1405.4557 [astro-ph.EP]. DOI: 10.1088/0004-637X/787/2/171

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5 / 5 (3) Jun 30, 2014
This article is interesting because it illustrates the biggest challenge in finding and characterizing habitable exoplanets, and that is the fact that we have no sure-fire baseline from which to launch plausible evaluations. What is really needed is a deep space probe that can look at Earth from afar and use the same or similar tools to characterize Earth as are now being used to attempt characterization of exoplanets. Only then will we gain a baseline from a known habitable planet, which can then be used to evaluate potentially habitable exoplanets.
3 / 5 (3) Jun 30, 2014
Truly difficult to answer. If one looks at Mars from far away they would determine there is no life. Yet, there is robotic life. As a consequence, of some other life that created them. We have a long ways to go before we can see outside our own selves.
3.3 / 5 (4) Jun 30, 2014
Spectroscopy will tell the tale....at least as far as life as we know it.

Then we can finally get some more concrete data, and I can find out if I'm just a curmudgeonly pessimist or a realist :P
1 / 5 (4) Jun 30, 2014
By using spectroscopy they'll be looking for telltale signs of methane. The methane they will be looking for must be gaseous, not the liquid kind found on the outer planets & moons of Jupiter & beyond, and of course it must not be the dominant gas of the planet.
5 / 5 (5) Jun 30, 2014
Nice to know that Earth has life.

Now for habitat #2...

@Benni: "By using spectroscopy they'll be looking for telltale signs of methane."

They will look at many biosignature gases such as O2 and NO2. Methane is derived from many biotic and abiotic habitats. (Among the endpoints of the kerogen carbon recycling, say.)
1 / 5 (4) Jul 01, 2014
1) If you know the history of the Mission To Planet Earth, and why it failed politically, then you know that it will be vigorously opposed by the conservatives, and fail. Another try at a launch of OCO is being attempted tomorrow. This should be interesting.
1 / 5 (3) Jul 01, 2014
2) I noticed a phenomenon today while looking at the sun glinting off the waves of the Chesapeake Bay. I was wearing yellow filtering, polarized sunglasses, and I noticed that the sun's reflection off the waves near me and far from me was white, but the sun light reflected at intermediate angle was bright blue. This told me that the reflection at intermediate distance was at Brewster's Angle. The horizontally polarized light reflected off the water to my eyes. The vertically polarized light refracted into the water. The horizontally polarized light appeared to be intensely blue. Rotating the polarized sunglasses showed a golden colored light at all angles.

1 / 5 (3) Jul 01, 2014
The horizontally polarized light should have been blocked by the horizontally polarized filter of the sun glasses, and the blue should have been blocked by the yellow filter of the sun glasses. So where did the blue light come from? I can only think it likely that the blue sky provided the blue light, which was masked by the full spectrum reflection at the other angles. At the Brewster Angle, the direct reflection of the sun light was blocked and the blue light was apparent. I can't explain why it was intensely blue.

Looking at exo-planet evaluation, it would appear that using polarized light versus wavelength would pick out the Brewster Angle of the reflection. The Brewster Angle depends on the ratio of the index of refraction of the gas and water (fluid).
1 / 5 (3) Jul 01, 2014
While all of the light reflected off an exo-planet would appear as a point source, if the body of water (fluid) is bounded by land masses, then the polarized signals noted above, would turn on and off as the planet rotated. During the orbit about the star, the angle and timing should yield an estimate of the Brewster Angle and the the ratio of the index of refraction of the gas and water (fluid).

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