- 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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- Venus and Earth
----------------------------- 4450
- MARS & VENUS -
were they water worlds
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- The changes we see preserved in the rock
record are driven by large-scale changes in the Martian environment.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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”.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
-
May 2, 2024 MARS & VENUS -
were they water worlds? 4452
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--------------------- --- Friday, May 3, 2024
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