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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 lunar spacecraft 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.”
When LCROSS crashed into the moon 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.
“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.
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.”
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.
The model has been extensively validated from the ultraviolet to the thermal infrared, and is in excellent agreement with data from NASA’s Atmospheric Infrared Sounder (Aqua/AIRS), and temporally- and spectrally-resolved observations of Earth from NASA’s Deep Impact flyby spacecraft (a VPL collaboration with the NASA EPOXI mission) (Robinson et al., 2011). The model has also been validated as a function of phase using Earthshine and EPOXI data (Robinson et al., 2010).
Products from the Earth model can be used to explore the detectability of signs of habitability and life for the Earth, to validate the retrieval techniques developed in Task E, and to expand our 3-D spectral visualization capability to planets other than the Earth. We are currently working on upgrades to the Earth model that will further increase its versatility.
Highlights of scientific applications of the VPL 3-D Spectral Earth model are described below.
Detecting Alien Oceans Using Glint
In Robinson et al. (2010), we used the Earth model to simulate Earth's appearance in reflected light over a year, including the realistic evolution of cloud, snow, and sea ice cover. We used this VPL-generated dataset (which is publicly available on this website) to investigate the detectability of "glint", the mirror-like reflection of sunlight off a body of water, in the Earth’s disk-integrated brightness. Including the possibly confusing effects of realistic forward scattering clouds, our models of the Earth's phase-dependent brightness show that the crescent-phase Earth is as much as 100% brighter than an identical non-glinting Earth at some near-infrared wavelengths. Such an excess in brightness may be detectable by NASA's James Webb Space Telescope if it were to fly with an external occulter.
The VPL 3-D Spectral Earth model has also been used to explore the detectability of a moon around an Earth-like exoplanet as a function of wavelength and observed phase (i.e. whether the exoEarth and moon are observed at full, or near crescent phase) (Robinson, 2011). The models showed that the contribution of the exomoon to the exoEarth spectrum is very strongly phase dependent, and more likely to be detectable in the exoEarth’s carbon dioxide absorption bands.
The E-ELT will search for extrasolar planets — planets orbiting other stars. This will include not only the discovery of planets down to Earth-like masses through indirect measurements of the wobbling motion of stars perturbed by the planets that orbit them, but also the direct imaging of larger planets and possibly even the characterisation of their atmospheres. The telescope will attempt to image Earthlike exoplanets, which may be possible.
“While the current state of the technique cannot detect earthlike planets around stars like the Sun, with Keck it should soon be possible to study the atmospheres of the so-called ‘super-Earth’ planets being discovered around nearby low-mass stars, many of which do not transit,” said Caltech professor of cosmochemistry and planetary sciences Geoffrey Blake. “Future telescopes such as the James Webb Space Telescope and the Thirty Meter Telescope (TMT) will enable us to examine much cooler planets that are more distant from their host stars and where liquid water is more likely to exist.”
.....the James Webb Space Telescope, now scheduled for launch in 2018, has a chance of examining the atmospheres of a handful of these bodies. So might a new generation of extremely large ground-based telescopes, with mirrors of 30 meters or more, that have recently been proposed.
Some of the exoplanets these telescopes will attempt to study have a rare alignment. Like the more distant exoplanets identified by Kepler, they regularly pass in front of, or transit, their parent stars as seen by the detectors. During a transit, starlight filters through an exoplanet's atmosphere, with each chemical constituent leaving its own imprint on the light. The signal is extremely faint but planets in the habitable zone of M stars make frequent transits, enabling astronomers to accumulate individual observations to make a stronger detection. "The habitable zone of M stars are the first places that we can look for bio-signatures," Seager says.
originally posted by: LightYearsAhead
a reply to: JadeStar
It is wrong to think how civilization of which way of thinking, development, technology you don't know, would react, discover and look for planets. First thing, they would find a proper way to travel,
I am seeing a pattern of technology development from using antigravity - using the magnetosphere and such means of flying. The whole technology that exists in public today is ridiculously primitive to achieve anything of that kind.
Also there may be some shortcuts in space that humanity hasn't discovered and such to be used to shorten distances - a real scientist would be open minded for such undiscovered things existing in space like many other.
originally posted by: mirageman
Thanks for another fascinating thread JS (away from the World Cup ) . You have a good grasp of how to explain this all to someone who has no formal qualifications in this sort of thing. Duly S&F.
Both the Geneva team and my team will use the next few years to develop innovative instruments with the goal of reaching 10 centimeter-per-second precision – a factor of ten gain over current precision. The Geneva team is designing a high-resolution instrument, ESPRESSO, for the 8-meter telescopes at Paranal in Chile. My team is designing EXPRES for the Discovery Channel Telescope. As the acronyms imply, we are both aiming for the extreme precision needed to robustly detect Earth-mass planets orbiting at habitable-zone distances.