Biosignatures again and again...

The "biosignature" of K2-18b means that there is phosphine and some other gasses linked with lifeforms. That means that there are certain chemical compounds. Not that there are bacteria. To confirm that thing we should make interstellar spaceflight. And that means the question remains open. The seas on that large exoplanet can protect those hypothetical life forms against the radiation of the eruption of its star, the red dwarf that this planet orbits near the habitable zone. The planet is locked but there can be more comfortable in its oceans. 

That means there might not be any lifeforms on that exoplanet's drylands. If there are even drylands. The K2-18b is a giant water world. That means the entire planet can be covered by the ocean. So there might not even be drylands. 

Biosignature means a mark for the possibility of finding lifeforms. It doesn't mean the same as a techno signature. That means. There might be some kind of lifeforms on those planets, or their oceans. That doesn't mean the same as "there are lifeforms". But those lifeforms might not be more intelligent than bacteria. Intelligent lifeforms might not necessarily build Dyson's spheres. 

The point is that the world "might". This means there could be life on some exoplanet's oceans. The evidence is not clear. We need space trips to other solar systems. To confirm those biosignatures as biosignatures.  And those tips last even millenniums. The a possibility that there are some kind of alien bacteria or algae in the exoplanet's oceans. Is what we want. We can say that they are high or low. That depends on the point of view of how we handle those molecules. 

If we say that phosphine, oxygen, methane, and carbon dioxide are marks of life we must realize that those gasses form also in other reactions. And also Titan moon's atmosphere is mainly methane. When we think about cases where some kind of algae or bacteria living in the exoplanet's ocean and that water world orbits a red dwarf that star's eruptions can launch chemical reactions where oxygen connects with carbon and forms carbon dioxide. Or maybe that carbon dioxide formed a long time ago. 

Structures in meteorite Alan Hills, ALH84001. "Electron microscopy revealed chain structures resembling living organisms in meteorite fragment ALH84001" (Wikipedia, Allan Hills 84001)

But then to our solar system and Venus and Mars. Can some bacteria survive in Venus's atmosphere? Sometimes I suggested that Maxwell Montes could be a good place for some very tough bacteria. But the temperature on that mountain is 380 degrees Celsius. The reason for that high temperature is that Venus's atmosphere is very close to liquid. Winds near those highlands push hot carbon dioxide up those mountain's hills. And that destroys almost all bacteria. 

Another place where the bacteria can live is the sulfur acid clouds most up layers. Venus's atmosphere is different than Earth's. The extremely dense carbon dioxide allows bacteria to hover in its atmosphere. If we think that bacteria can cut carbon dioxide molecules into carbon and oxygen that allows the bacteria to form the oxygen swimming bag that allows them to hover in that planet's high atmosphere. The thing is that the oxygen is lighter than carbon dioxide and that makes bacteria easy to hover in that dense atmosphere. But that means that there might not be surface lifeforms on that planet. 

Asteroid Vesta: can it be the remnant of an ancient planet? "Vesta, once believed to be a mini-planet, may not have a core after all. New data suggest it’s either an incompletely formed world or a broken chunk from another forming planet. This surprising twist challenges everything scientists thought they knew about one of the solar system’s largest asteroids. Credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA" (ScitechDaily, Vesta Isn’t What We Thought – Could It Be a Chunk of a Lost Planet?)

The Mars rivers and water areas might be real. That means that there was water on Mars in its last period. Then the weak gravity and cosmic radiation broke those water molecules. And that can tell where the water has gone. Water and rivers don't mean that there was life. But Mars's violent past might crush those primitive organisms. There is so-called weak evidence that there were some kind of primitive organisms.  Part of this evidence is from the meteorite Alan Hills, or ALH84001. 

The problem is that this meteorite is polluted on Earth. Making sure that those structures formed before the meteorite came from space requires that Mars-rovers find a similar structure on Mars. But that requires that ALH84001 is from Mars. There is the possibility that the origin of that meteorite can be anywhere else. And maybe the meteorite's chemical structures should compare with the asteroid Vesta which could be remnants of an ancient planet. 

That space rock was found in Antarctica. Some structures seem the bacteria fossils. But those bacteria can also from Earth. Some researchers say that those fossils are not fossils they are some kind of structures in that stone, that formed when that meteorite traveled through the atmosphere. But things like the Perseverance rover collected evidence that there was water on Mars in its last period. 

So, if Mars lost its lithosphere what made that impact? Maybe we find the answer from a dwarf planet or asteroid Vesta. The new evidence tells us that. Maybe Vesta is some kind of remnant of an ancient planet. 

Some kind of impact blew up the Martian outer lithosphere. So, could the Vesta be the remnant of that impact? In that case, the planet Mars was far bigger than it is now. There was a theory that the asteroid belt or large part of it is the remnant of ancient planet collisions. The position of Mars could be different. But Mars's position today is more vulnerable than Earth's. 

There is a possibility that cosmic impact pushed Mars to the Asteroid belt. And then it pulled lots of asteroids on it. Maybe, those impacts formed a situation in the entire Mars shell turned into lava. 

But finally...

If we face the bacteria on other planets, can that change our way of thinking so much that we should talk about revolution? Bacteria will not communicate with us. That means we would be alone. Even if there were bacteria on another planet. We are alone in the universe until intelligent civilizations are found. And when or if we find another civilization we must be very careful in that situation. We don't know anything about them. And that makes those creatures a potential threat.  

https://astrobiology.nasa.gov/news/an-update-from-alh84001/

https://www.gadgets360.com/science/news/doubts-over-signs-of-alien-life-on-exoplanet-k2-18b-rise-as-new-analysis-questions-data-validity-8313564

https://www.nasa.gov/universe/exoplanets/webb-discovers-methane-carbon-dioxide-in-atmosphere-of-k2-18-b/

https://scitechdaily.com/martian-time-capsules-uncovering-clues-to-ancient-water-and-potential-life/

https://scitechdaily.com/nasas-curiosity-rover-discovers-surprise-clues-to-ancient-water-on-mars/

https://scitechdaily.com/nasas-perseverance-rover-reveals-mars-volcanic-past-and-hints-of-ancient-life/

https://scitechdaily.com/vesta-isnt-what-we-thought-could-it-be-a-chunk-of-a-lost-planet/

https://www.space.com/alien-life-search.html

https://www.space.com/space-exploration/search-for-life/doubts-mount-further-over-signs-of-alien-life-on-k2-18b-this-is-evidence-of-the-scientific-process-at-work

https://www.space.com/25325-fermi-paradox.html

https://en.wikipedia.org/wiki/Allan_Hills_84001

https://en.wikipedia.org/wiki/K2-18b

https://en.wikipedia.org/wiki/Mars

https://en.wikipedia.org/wiki/Maxwell_Montes