- 2864 - ENCELADUS - Saturn’s moon? - Enceladus, named after one of the Giants in Greek mythology, has an icy surface that reflects 81 percent of the light falling on it. Saturn’s sixth-largest moon, Enceladus has a diameter of only 310 miles, and a mass less than 1/50,000 that of Earth.
--------------------------- 2864 - ENCELADUS - Saturn’s moon?
- When it comes to places to look for life, however, Enceladus is at the top of the list, and it’s right in our backyard.
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- English astronomer William Herschel discovered Enceladus in 1789, but it remained an enigma until the Cassini mission began orbiting Saturn in 2004. Prior to Cassini, Enceladus was a bit ignored. We didn’t know liquid water could exist that far out in the solar system, so why would anyone be that interested in another boring, dead ball of ice?
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- That all changed one year later, when Cassini’s magnetometer detected something strange in Saturn’s magnetic field near Enceladus. This suggested the moon was active. Subsequent passes by Enceladus revealed four massive fissures, dubbed “tiger stripes”, in a hot spot centered on the south pole. And emanating from those cracks was a massive plume of water vapor and ice grains.
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- Was it really an underground ocean, or more of a local southern sea? By verifying excess wobble over Enceladus’ orbital period, the imaging cameras confirmed that the icy crust is not connected to the world’s rocky core. This could only be possible if the crust is floating on a global, subsurface, liquid-water ocean.
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- Plumes spray water ice and vapor from many locations along the so-called “tiger stripes” crossing Enceladus’ south polar terrain. The four prominent fractures are about 84 miles long.
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- Mass spectrometers aboard the spacecraft analyzed the gas and grains during multiple flythroughs of the plume. These instruments, the Ion and Neutral Mass Spectrometer and Cosmic Dust Analyzer, found the plume contains mostly water, but also salts, ammonia, carbon dioxide, and small and large organic molecules.
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- These findings help us paint a picture of the world underneath the ice, possibly habitable ocean that’s slightly alkaline, with access to chemical energy in the water and geothermal energy at the rocky seafloor.
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- Enceladus possesses all three ingredients for life as we know it: water, chemistry, and energy. Water in the ocean. Chemistry in the simple and complex organics detected in the plume. These could be utilized to form the molecular machinery of life.
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- Energy takes a bit more explaining. It is likely that hydrothermal vents are present at the seafloor of Enceladus. We know this because of three lines of evidence. First, we detected methane in the plume, at higher concentrations than would exist if sourced from clathrates (water-ice cages at high pressure with methane trapped inside) or other reservoirs in the ice. Methane is a key product of hydrothermal systems.
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- We also discovered silica nanograins of a particular size and oxidation state traced to the ocean. These only could have formed where liquid water is touching rock at temperatures of at least 194 degrees Fahrenheit, in the range of hydrothermal vents like “white smokers” here on Earth.
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- The 2020 confirmation of molecular hydrogen in the plume strongly suggests interaction of liquid water with a rocky core. On Earth, hydrothermal vents at the base of the Mid-Atlantic Ridge host teeming ecosystems, living as far removed as one can imagine from photosynthesis. These habitats survive off of geothermal and chemical energy. A similar community might exist near a hydrothermal vent at the seafloor of Enceladus.
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- Assuming an energy-limited scenario, a good analog is Lake Vostok, a body of water in Antarctica that’s been covered with ice for the last 35 million years, we are probably looking at cell densities in the range of 100 to 1,000 cells per milliliter of ocean water. For reference, Earth’s oceans have about 1 million cells or more per milliliter.
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- We assume this life would use readily available building blocks, such as amino acids, which are abundant in carbonaceous chondrites and likely present all over the saturnian system, in numbers on par with Earth-based life.
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- This assumption is reasonable because life needs chemical complexity to carry out the reactions that keep cells functional. Then we are looking at concentrations of biomarkers on the order of less than 1 part per billion.
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- Of all the ice grains detected a fraction had a high concentration of organic molecules, called high mass organic cations (HMOC). While the instrument couldn’t specifically identify the structures of the HMOCs, a thorough analysis led to some educated guesses, such as aromatics (carbon-containing ringed structures) and oxygen- and nitrogen-bearing species. Within Enceladus’ ocean, there may be a complex organic soup of molecules.
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- On Earth we have something similar floating at the surface of our ocean. It’s a film called an “organic microlayer,” as it’s not very thick and is typically made up of organics from biological activity , bits of cells, and from other sources. Wave activity causes bubbles in this microlayer to burst, generating aerosols that are organic-rich and salt-poor.
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- A similar process may be happening on Enceladus. Organic molecules in the ocean may be concentrated at the ocean-ice boundary, and, just like on Earth, may force out the water and salts from this film. As the liquid surface at the base of the plume boils into vacuum, bubbles might burst and disperse the organic film, producing some grains that have a lot of organics inside, and little salt.
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- Aerosols on Earth boast organic molecules enriched hundreds to thousands of times over typical ocean concentrations. If we collect samples by flying through the plume or by landing on the surface, we may have a greater chance of detecting evidence of life on Enceladus, if it exists.
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- Cassini spacecraft spent more than 13 years studying Saturn, its rings, and its moons. It captured some 450,000 images and returned 635 gigabytes of science data. A sample return mission, with a round-trip time of 14 years, to get that sample or various climbing or melting robots to descend the 1.2 to 6.2 miles through the ice shell and reach the ocean itself.
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- The next mission to Enceladus will need a well-designed suite of instruments capable of searching for multiple, independent lines of evidence for life. Our understanding of life’s characteristics has advanced greatly since the Viking era, the last time NASA openly stated the search for life as the primary goal.
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- Back when the two Viking landers touched down on Mars in 1976, we knew only two of the three branches of life. Archaea, the third and most primitive branch of the tree of life, was discovered in 1977.
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- The Viking landers had three biological experiments designed to search for life in the martian regolith. One test result was positive, one was negative, and one was ambiguous. Since then, we have learned a great deal about how to design experiments such that an ambiguous result is much less likely.
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- A future mission to Enceladus might not target DNA, which is Earth-life-specific, but it might look for a molecule that could serve the same function for alien life: a large molecule with repeating subunits (akin to an alphabet) capable of storing information, such as the blueprints to build an alien cell. If such a molecule is detected, along with positive identification of multiple other biosignatures, a strong case could be made for the first detection in human history of life on another world.
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- Enceladus is not the only place that could host life. Europa has an even larger liquid water reservoir, and Titan’s ocean may entertain an unimaginably rich organic chemistry.
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- But Enceladus is the one place where researchers know for certain that they can access material from the ocean without the need to dig or drill (or even land). We can use technology available right now to test the hypothesis of whether life may be present somewhere else in the solar system.
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- The Cassini spacecraft left a legacy of discoveries behind when its 13-year-mission to Saturn ended in 2017. A 2038 mission, Orbiter and a Lander, “Orbilander“, would start by orbiting Enceladus for approximately 200 days, This is no simple task with Saturn’s giant gravity field next door.
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- Despite their brilliant appearance in Cassini images, Enceladus’ plumes are not very dense. Orbilander will be flying through something more like a cloud than a garden sprinkler. Particles from the plumes will funnel into science instruments at high speeds as the spacecraft zips along, requiring the team to devise ways to gently decelerate them so they aren’t pulverized.
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- Once a location is found, Orbilander would turn on its side and convert to a lander. It would descend using terrain-relative navigation similar to what OSIRIS-REx will use to capture a sample from asteroid Bennu, and what the Dragonfly mission to Saturn’s moon Titan will use to fly around the surface. Two nuclear power sources will keep Orbilander running surface for up to a year and a half.
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- Orbilander would rely on a complex suite of instruments to determine whether Enceladus’ water has a blend of chemicals conducive for life as we know it, and search for amino acids, lipids, and cells.
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- The instruments include mass spectrometers to weigh and analyze molecules, a seismometer, a microscope, and a DNA sequencer. For remote sensing, the spacecraft would have cameras, radar sounders, and a laser altimeter.
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- Orbilander has 6 different instruments that provide indications of several different biosignatures. Enceladus’ geysers offer a unique opportunity to sample the water without having to drill through the surface. Not only will Orbilander access the plumes by flying through them in orbit, it will also capture plume material falling back to the surface after the spacecraft lands.
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- Orbilander wouldn’t launch until 2038 and arrive at Enceladus until at least 2050. It’s just an opportunity waiting for us to take advantage of a better place in the solar system to do the search for life in the near-future.
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-------------------------------- Other reviews about Enceladus:
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- 1786 - Enceladus - Saturn’s moon. This review describes what we have learned from recent space probes. It’s a whole new world to spur the imagination. - Enceladus has groves in its surface that are measurably warmer than the surrounding terrain. These groves are venting huge clouds of water vapor and ice crystals. Enceladus is barely 500 kilometers across ( 311 miles ). It would fit inside the borders of Colorado.
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- 1714 - Saturn’s moon - Enceladus New discoveries suggest an underwater ocean that contains ammonia anti-freeze and exits plums or geysers through the thick icy crust on the surface. Check the data and see if you agree?
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- 1362 - Enceladus - Saturn’s moon Enceladus was first discovered in 1789. It is only 310 miles in diameter, 1/7th the diameter of the Moon. The surface of Enceladus is water ice. In 1980 Voyager 2 spacecraft photographed Enceladus’ craters, narrow valleys, groves and ridges. It found sharp edged canyons 120 miles long, 5 miles wide, and ½ mile deep. All later to be discovered as water ice.
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- 1166 - Saturn has 62 moons. The plums on Enceladus are called cryo-volcanoes. The temperature of the water is -136 F but still liquid because it contains ammonia which is a natural anti-freeze.
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- 1362 - The escape velocity is only 500 miles per hour. The average temperature is
-337F
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- 957 - Enceladus is 311 miles in diameter and would fit inside the state of Colorado. It is the second closest moon orbiting Saturn outside the rings by 148,000 miles
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- October 17, 2020 2864
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--------------------- --- Wednesday, October 21, 2020 ---------------------------
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