- 3603 - EARTHSHINE - detects life? Astronomers are trying to detect life on Earth. Actually, it is just for practice. What astronomers really want to do is to detect life on other planets or moons. On Exoplanets that exist in other solar systems outside our own. Astronomers have already discovered over 5,000 planets outside our Solar System.
--------------------- 3603 - EARTHSHINE - detects life?
- Earthshine detecting life on Earth? The difficulty in measuring reflected light off a distant planet in another solar system is that the star in that system shines 10,000,000 to 1,000,000,000 times brighter than the light reflecting off the planet.
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- Somehow astronomers have to subtract the starlight and see the dot of light that is left. Any image that we get will be reflected light over an entire hemisphere of that planet. If we were out their looking back at the Earth we would at best see a pale blue dot. What can astronomers possible learn from that?
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- To find out astronomers are practicing on the Earth’s reflection off the Moon. Astronomers are measuring the light reflected off a hemisphere of Earth, re-reflected off the dark side of a crescent Moon, and back to Earth. This reflected light is called, “Earthshine”.
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- “Earthshine” is two reflections so astronomers have to somehow subtract the reflection that comes from the Moon. To get this astronomers measure the light spectrum from the crescent side of the Moon. This has the light spectrum of sunlight and the Moon’s reflection. Subtract that spectrum from the Earth’s reflected light from the dark part of the crescent Moon and we should have the light reflection due to the Earth alone.
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- When we study this Earthshine spectrum we can see the variation of the light caused by the Earth’s rotation and Earth’s weather variation. The Earthshine varies about 10% during Earth’s rotation.
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- Large scale patterns repeat themselves consistently enough to determine the Earth’s rotation rate. Observations from California show Earthshine brighten noticeably when Asia’s huge land mass rises.
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- Sunlight contains a broad electromagnetic spectrum, but when it reflects off the Earth and Earth’s atmosphere it is mostly in the visible light and the near-infrared part of the spectrum. This is because Solar radiation must penetrate Earth’s atmosphere, reflect off the ground and back into space. The atmosphere absorbs the mid-infrared and prevents it from reaching the ground. Ultraviolet is also absorbed in the upper atmosphere by the Ozone Layer.
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- Our atmosphere is made up of several layers. The Troposphere extends up to about 6 miles and contains 75% of the mass of Earth’s atmosphere. Extending above that to 31 miles is the Stratosphere. At the top of the Stratosphere between 25 and 31 miles is the Ozone layer. Further up to 53 miles is the Mesosphere and to 248 miles the Exosphere where the gas is so rare molecules loose contact with each other.
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- A significant reflection occurs in the Ozone Layer and below. At 480 nanometers, the wavelength of blue light, there is significant scattering of blue light by the small particles in the atmosphere. This is called “ Raleigh Scattering” and it accounts for the fact that the sky is blue.
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- Red light has a wavelength of 750 nanometers. Due to the size of the particles Blue light is much more scattered than Red light. Smoke appears blue. The sky appears blue. The setting Sun appears reddish because the light travels though a considerable thickness of atmosphere and the light you see is robbed of much of the blue end of the spectrum.
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- The Moon does not have an atmosphere so it radiates heat, mostly at the mid-infrared wavelengths. Unfortunately, the Moon reflection at these mid-infrared wavelengths even from its dark side completely swamps out Earth’s reflection. However, astronomers can still learn a lot at the other frequencies.
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- Looking at the Earthshine from 480 nanometers to 940 nanometers wavelength and measuring the percentage reflected: The average reflection is around 16%:
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------------------- 480 nanometers - 18% - this is the Raleigh Scattering of blue light.
------------------- 600 nanometers - 14.5% - Ozone (O3) absorption
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------------------- 680 nanometers - 12% - Oxygen (O2) absorption
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------------------- 740 nanometers - 13% - Water Vapor (HO2) absorption
------------------- 750 nanometers - 17% - Vegetation reflection
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------------------- 760 nanometers - 5% - Oxygen (O2) very strong absorption
------------------- 825 nanometers - 12% - Water Vapor (HO2) absorption
------------------- 910 nanometers - 11.5% - Water Vapor (HO2) absorption
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- Oxygen, Ozone, and Water Vapor in our atmosphere create several strong absorption lines in the spectrum. These molecules absorb photons at these particular wavelengths. When we find these absorption lines and identify these molecules it suggest biological activity exists.
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- Oxygen (O2) - reacts so quickly with other gases that very little could be present in the atmosphere unless it was being replenished continuously. The Oxygen in our atmosphere is replenished through vegetation photosynthesis and photosynthetic bacteria. Therefore, O2 is an indicator of life.
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- Methane (CH4) - in the atmosphere is another strong indicator of life replenishing Oxygen because CH4 strongly reacts with Oxygen (O2) and removes it from the atmosphere. If there is Oxygen with it then new Oxygen was recently supplied.
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- Ozone (O3) - is created by solar ultraviolet light breaking apart molecular Oxygen. The Oxygen (O2) atoms then recombine to form Ozone (O3). Ozone indicates life because it requires the presence of molecular Oxygen.
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- Water Vapor (H2O) - indicates a reservoir of surface water to replenish it in the atmosphere. H2O water vapor is also broken apart by solar ultraviolet light. Hydrogen (H2) escapes into space and Oxygen(O2) recombines with other molecules in the atmosphere or in the surface rocks. Water Vapor has to be continuously replenished in order to be there.
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- Chlorophyll - is indicated by the reflection lines at 750 nanometers wavelength. Vegetation reflects nearly 50% of incoming light at near infrared frequency. Chlorophyll is the pigments in plants that collect sunlight and cause photosynthesis. Photosynthesis entraps sunlight energy in the form of sugar molecules that creates the carbohydrates in the food we eat.
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------------------ 6 Carbon Dioxide atoms (CO2) + 6 Water Molecules (H2O) + Sunlight = C6 H12 O6 Glucose + 6 Oxygen (O2)
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- If your eyes could see a little way into the infrared, green vegetation would look extremely bright red. Astronomers finding this red-edge signature in Earthshine at 750 nm would say there is life there for sure.
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- The red-edge reflection in the Earthshine spectrum is less than 50% because the clouds absorb it and only 1/3 or the Earth is land that has vegetation. Earthshine is looking at the average across the entire hemisphere.
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- In 1996 the Mars Surveyor spacecraft looked back at Earth. It saw this pale blue dot 4,000,000,000 miles in the distance. Instruments on board measured the light reflection spectrum from 400 nanometers to 25,000 nanometers wavelength. Strong absorption lines could still be detected:
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- 9,000 nanometers - Ozone (O3)
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- 14,000 to 16,000 nanometers - Carbon Dioxide (CO2)
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- Venus, Mars, and Earth have strong CO2 spectrum absorption lines indicating they are all terrestrial planets. The gas giant planets Jupiter, Saturn, Uranus, and Neptune do not have a CO2 absorption in their reflected spectrum.
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- Carbon Dioxide makes up only 0.03% of our atmosphere but land plants are entirely dependent on this source for life. We in turn are dependent on plants for life. Could this balance of life exist on another planet?
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- Astronomers are getting newer and better telescopes. Soon they will be able to study Exoplanet’s reflected light spectrums. Practicing on Earthshine to see if they can detect life on Earth is good preparation for this day to come. However, extracting these details from lightyears away is no easy challenge.
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- 2022 , We have over 5,000 exoplanet to work on.
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June 17, 2022 EARTHSHINE - detects life? 637 3603
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--------------------- --- Friday, June 17, 2022 ---------------------------
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