Friday, May 3, 2024

4452 - MARS & VENUS - were they water worlds

 

-    4452  -   MARS &  VENUS -  were they water worlds?   -    Evidence of ancient lake sediments at the base of Mars' Jezero crater offer new hope for finding traces of life in samples collected by NASA's Perseverance rover.  Perseverance touched down on Feb. 18, 2021 inside the Red Planet's 28-mile-wide Jezero Crater, which is believed to have once hosted a large lake and river delta.

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-----------------------------  4450    -   MARS &  VENUS -  were they water worlds

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-    The rover has been scouring the crater in search of signs of past life and collecting and caching dozens of samples along the way for a possible future return to Earth.   Using the rover's Radar Imager for Mars' Subsurface Experiment (RIMFAX) instrument revealed new clues about how sediment layers formed over time on the crater floor.

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-    As Perseverance rover travels across the surface of Mars, the RIMFAX instrument sends radar waves downward at 4-inch intervals and measures pulses reflected from depths of about 65.6 feet below the surface to create a subsurface profile of the crater floor.

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-    The RIMFAX data showed evidence of sediment deposited by water that once filled the crater. It's possible that microbial life could have lived in the crater at this time and, if such life existed on Mars, sediment samples from this area would contain signs of their remains.

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-    Two distinct periods of deposition occurred, creating layers of sediments on the crater floor that appear regular and horizontal, much like strata layers seen on Earth. Fluctuations in the lake's water levels caused some of the sediment deposits to form an enormous delta, which Perseverance traversed between May and December 2022.

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-    The radar measurements also show an uneven crater floor below the delta, which is likely due to erosion before sediments were first deposited. After, as the lake dried up over time, the sediment layers in the crater were eroded, forming the geologic features visible on the Martian surface today.

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-   The changes we see preserved in the rock record are driven by large-scale changes in the Martian environment.

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-    Why also want to learn why Venus died on the other side of the Earth closer to the Sun?  Venus is only slightly smaller than the Earth, and so has enjoyed billions of years of a warm heart. But for this planet, sometimes called Earth’s sister, that heat has betrayed it. That planet is now wrapped in suffocating layers of a poisonous atmosphere made of carbon dioxide and sulfuric acid.

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-    The pressures on the surface reach almost 100 times the air pressure at Earth’s sea level. The average temperatures are over 700 degrees Fahrenheit, more than hot enough to melt lead, while the deepest valleys see records of over 900 degrees.

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-    If Venus is indeed Earth’s sister, she’s a twisted one. Like Mars, we suspect that Venus also once hosted a thinner, balmier atmosphere and a surface replete with liquid water oceans.

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-    The reasoning here is a little more tenuous than for Mars where we can literally see the evidence for water before our very eyes, but the thinking is that both Venus and Earth formed in a roughly similar fashion, in roughly the same orbits with roughly the same material. Thus we should have been born with roughly the same amount of water.

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-   Like Earth, most of that water would have been chemically bound up in rock, buried deep in the mantle. But some of it may have leeched to the surface or been delivered by hosts of water-rich comets shortly after formation, building up a supply on the surface, once again stabilized by a thick atmosphere.

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-    What doomed Venus was not any fault of its own, but our own treacherous Sun. As stars age they gradually brighten. Day by day it’s imperceptible, but over the course of millions of years it completely changes the character of a star. Billions of years ago our Sun’s habitable zone was shifted inwards compared to where it rests now, but with increased brightness comes increased heat, and  that habitable zone steadily creeps outwards over time.

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-   Did Venus ever host life? I doubt we’ll ever know, given the excruciating temperatures on the surface that make exploration nearly impossible. But it’s likely that it had water and a rich atmosphere – the basic ingredients were there. But if life did gain a foothold it did not last long. As our Sun aged, Venus got warmer and warmer. On a warmer planet, more water exists as vapor in the atmosphere than as liquid on the surface.

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-    At first the changes were small, with nothing more than a higher dew point to mark the inexorable path to destruction. But at some point in the past Venus reached a tipping point. With too much water vapor, the atmosphere of Venus became too good at trapping the heat radiating from the surface. That radiation could not penetrate the haze and make into space, but instead was ensnared within the atmosphere itself, heating it up.

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-   Venus entered a feedback loop, dumping more heat into the atmosphere, which boiled the oceans into more vapor, which increased the temperatures, and so on. First the shallow lakes and streams were gone, then came the deeper oceans, until every scrap of water was blowing in the winds of the atmosphere.

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-    With its proximity to the ever-brightening Sun, the water vapor did not last long. Solar radiation pummeled it, disassociating its chemical bonds and sending the oxygen and hydrogen flying away, joining a grim procession beyond our solar system.

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-   If Venus had plate tectonics like the Earth, then this is where that process came to end. With no water to act a lubricant, the great slow grinding of the plates seized up, locking the crust in place. This constant churning acts as a natural sink for carbon: the carbon dioxide binds to rocks which get pulled deep into the mantle, preventing too much carbon from building up in the atmosphere.

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-   But without the cleansing effect of plate tectonics, carbon dioxide levels rose to dangerous heights, its own ability to absorb radiation from the surface choking off any remaining hope for rescuing the planet. Eventually the atmosphere would pile upon itself until it reached its present swollen size.

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-    As our Sun aged, Venus strangled itself.  Venus is not alone in sharing that fate, for the Sun has not yet reached its final days. It continues to brighten, bringing more warmth to the solar system day by day, its habitable zone steadily inching outwards with every passing year.

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-    At some point, approximately 500 million years from now, Venus will not be alone, The Earth’s oceans will boil, our continents will halt their ancient motion, and we will finally be twins with our sister: dead, lifeless, and strangling on our own bloated atmosphere.

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-    Hycean planets may be able to host life even though they’re outside what scientists consider the regular habitable zone. Their thick atmospheres can trap enough heat to keep the oceans warm even though they’re not close to their stars.

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-   The word hycean is a portmanteau of ‘hydrogen’ and ‘ocean’ and it describes worlds with surface oceans and thick hydrogen-rich atmospheres. Scientists think that they may be common around red dwarfs and that they could be habitable, although any life that exists on a hycean world would be aquatic.

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-    Because they contain so much water, scientists think they’re larger than comparable non-hycean planets. Their larger size makes them easier targets for atmospheric study by the JWST. Though hycean worlds are largely hypothetical now, the JWST is heralding a new era in planetary science and may be able to show that they do exist.

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-   The telescope’s ability to characterize exoplanet atmospheres could be the key to confirming their existence. Using transmission spectroscopy, the space telescope can watch as starlight travels through their atmospheres, revealing the presence of certain important chemicals and even biosignatures.

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-    The exoplanet “TOI-270 d” could be a hycean world.  The JWST has ushered in a new era in atmospheric characterizations of temperate low-mass exoplanets with recent detections of carbon-bearing molecules in the candidate Hycean world “K2-18 b”.

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-     TOI-270 is an M-dwarf (red dwarf) star about 73 light-years away. Red dwarfs are known to sometimes flare violently, ruling out habitability on nearby planets. However, they describe TOI-270 as a quiet star. It hosts three sub-Neptune planets, and the pair of outermost planets, TOI-270 c and d, are both candidate hycean worlds. TOI-270 d is considered the strongest candidate.

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-    TOI-270 d is about 4.2 Earth masses and measures about 2.1 Earth radii. It takes just over 11 Earth days to complete an orbit, a fact that aids atmospheric study. The Hubble Space Telescope looked at TOI-270 d recently, and its observations suggested a hydrogen-rich atmosphere with some evidence of H2O. Those results warranted further examination with the more powerful JWST.

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-   Though scientists still haven’t proven that hycean worlds exist, they know something about their atmospheric chemistry. On an ocean world with a thick, hydrogen-rich atmosphere, scientists expect to find strong signatures of CH4 (methane) and CO2 and no evidence of NH3 (ammonia.) This is what the JWST found at K2-18b, though there is still uncertainty if that exoplanet is a hycean world.

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-    Every planet is different, but each type should have things in common.  For Hycean worlds, the presence of an ocean below a thin H2-rich atmosphere may be inferred by an enhancement of CO2, H2O, and/or CH4, together with a depletion of NH3.

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-    Since TOI-270 d is a candidate hycean world, its spectroscopy should be similar to other hycean candidates like K2-18b.   Therefore, for the Hycean candidate TOI-270 d, observations of these key carbon-, nitrogen-, and oxygen- (CNO) bearing molecules are required to assess whether or not it is a Hycean world.

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-   In October of 2023, the JWST observed TOI-270 b and d during two transits. The observations amounted to a total exposure time of 5.3 hours.  This rare event when the planets pass in front of their star allows for transmission spectroscopy of both planets.

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-   Our atmospheric retrieval results support the inference of an H2-rich atmosphere on TOI-270 d and provide valuable insights into the abundances of dominant CNO molecules. Furthermore, the abundances are similar to what the JWST found on K2-18 b, another suspected hycean world.

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-   But when it comes to water, the results are less certain.  We found only tentative evidence of H2O, with the detection significance and abundance estimates varying. The detection and abundance of H2O were more strongly dependent on what method the researchers used to analyze the data.

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-    The appearance of CS2 (carbon disulphide) in TOI-270 d’s atmosphere is intriguing. It’s considered a detectable biomarker in hycean world atmospheres, as well as in hydrogen-rich atmospheres of rocky worlds, although the direct sources could also be volcanic or photochemical.

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-    The atmospheric spectrum also contains hints of C2H6 (ethane.) Ethane can be a byproduct of photochemical reactions involving methane and other gases, including biogenic ones. Its presence is another indication that methane is present. The researchers also point out that the abundances of ethane and carbon disulphide are well above theoretical predictions.

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-    All the researchers can conclude is that TOI-720 d is a candidate hycean world. But while the previous HST observations that hinted at its status showed the presence of H2O in an H2-rich atmosphere, the JWST observations provide more depth. The JWST’s more robust detections of CH4 and CO2, along with its non-detection of NH3, makes it an even stronger hycean world candidate.

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-   The planet stands out as a promising Hycean candidate, consistent with its initial predictions as a world with the potential for habitable oceans beneath an H2-rich atmosphere.

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May 2, 2024                MARS &  VENUS -  were they water worlds?                     4452

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