[Blank Space (OP)]

Reasoning for such a question:

  When reading about the Mariana Trench I began to question it's appearance and the animals that lived there. Arthropods, and the fact that Chemosynthesis is commonly used as a source of nutrition. Looking at the arthropod section specifically I questioned whether it was actually possible for modern surface arthropods to exist as a descendant of said prehistoric arthropods. The appearance of the Mariana Trench is that of a wound that is beginning to close up, this unrelated similarity provoked the initial idea that maybe at one point it was open and the ocean expanded significantly or just a few thousand meters deeper than recorded today. Looking at Chemosynthesis the idea that life existing solely on chemical reactions introduced and stimulated by bacterium around thermal vents proposed an interesting possibility.

Theory

  Can deep sea life have come from deeper within the planet itself. Using my analysis of the appearance of the Mariana Trench, chemosynthesis as a source of food, and the existence of deep sea arthropods, the connection of the first deep sea organisms coming from within the planet begins to gain traction but is still unproven and lacking in a large variety of supporting evidence. Nonetheless the theory that there is a potential sub marine ecosystem sitting below the deepest point discovered on earth is one that could very well be supported by what is know of organisms alive today.

  If arthropods have the capability of surviving in dense oxygen environments and having grown based on past studies of land based fossils to larger sizes than recorded of surface arthropods today then deep sea arthropods being similar in that they both breathe oxygen, one just receiving it in a more direct way. Under the Mariana Trench posing a potential higher density of thermal pockets resulting in chemosynthesis happening on a larger scale and potentially without the inclusion of an ocean, all of this concludes to a theory that life could very well be more expansive, larger in size, and having sustained the oldest living fossils on record.

Sub Theory

  Based mainly on imagination, I wonder if it is possible for spiders to grow to be around 5 meters in length in an ecosystem like I described above. If so then there could very well be caverns with similarly sized non spider based arthropods.

[evan_au]

Can life exist beyond the deepest point in the ocean?

Over the past few years, scientists have been astonished by the number of microorganisms living in deep rocks - even in deep gold mines.
- Presumably this would include rocks under the Mariana trench.
- This is a place where ocean floor plunges beneath the Mariana tectonic plate, potentially taking living microbes deep under the ocean floor.
See: https://en.wikipedia.org/wiki/Mariana_Trench

The appearance of the Mariana Trench is that of a wound that is beginning to close up, this unrelated similarity provoked the initial idea that maybe at one point it was open and the ocean expanded significantly

The Mariana trench is part of the Pacific "ring of fire", characterized by subducting ocean floor, volcanoes and earthquakes.

I understand from this week's Naked Scientists podcast that the Pacific Ocean has been showing this behavior since the breakup of Pangaea, which is put at around 300 million years ago.

If you want to see ocean floor that is spreading and opening up, look at the mid-ocean ridge in the Atlantic ocean. At it's northern end is Iceland, which is effectively splitting in half due to sea-floor spreading.
Listen: https://www.thenakedscientists.com/articles/interviews/ground-moving-beneath-our-feet
See: https://en.wikipedia.org/wiki/Pangaea
https://en.wikipedia.org/wiki/Mid-Atlantic_Ridge

[Blank Space (OP)]

Over the past few years, scientists have been astonished by the number of microorganisms living in deep rocks - even in deep gold mines.
- Presumably this would include rocks under the Mariana trench.
- This is a place where ocean floor plunges beneath the Mariana tectonic plate, potentially taking living microbes deep under the ocean floor.

  Yes tectonic plates shift due to high gravitational pressure and that can result in microbes being dragged under. However is it more likely that there is nothing but earth beneath the shifting plate's or that there are several hundred to thousands of meters consisting of empty space spread about where some could have it's own catalyst event resulting in autotrophic organisms to evolve, given that autotrophic organisms have been seen in unexpected environments it would be possible for life to evolve utilizing chemosynthesis or even another kind of system that produces food, possibly even without living in a deep sea environment. Even more so than that, could anyone of the said pockets possess evolved macro organisms, not just micro organisms.

  The prospect that such a existence requires too many random ventures in order to make happen in the first place is only truly fueled by the assumption that the first living organisms existed on the surface and was fueled by a foreign object smashing into the planet. I simple propose the idea that not only is it possible for life to have evolved from deep within the planet but that upon evolving it became a potential catalyst for surface life to evolve, in addition to marine organisms as well of course.

  A moon having some kind of collision with earth could have easily resulted in new condition and resources to emerge resulting in the first surface based organisms. However what I proposed in my initial analysis was more along the lines of life existing deep within the earth and the moon merely opened a path for such organisms to spread towards the surface. Assuming of course the organisms in question had not already evolved to a macro scale and simply traveled to the surface with a passage way that is now closed.

If you want to see ocean floor that is spreading and opening up, look at the mid-ocean ridge in the Atlantic ocean. At it's northern end is Iceland, which is effectively splitting in half due to sea-floor spreading.

  Humanity lacks the technological advancement to definitely say that the ocean floor is not constantly opening and closing across many hundreds of millions of years simply because it lacks the general understanding of what does exist within the planet. Ultimate and finalized decisions on such a prospect lack in evidence simply because only knowledge of the surface is understood and the development and existence of any lifeforms beyond the surface cannot and should not be assumed based of what has been discovered on the surface.

[evan_au]

However is it more likely that there is nothing but earth beneath the shifting plate's or that there are several hundred to thousands of meters consisting of empty space spread about

This seems to be asserting two things which appear opposites.

Having large empty spaces beneath the deepest part of the ocean seems unlikely to me - it would collapse (fill with rock) or at least fill with water.

the assumption that the first living organisms existed on the surface and was fueled by a foreign object smashing into the planet

This sounds like the Theia hypothesis.

However, it is thought that this impact of a Mars-sized object with the early Earth would have released so much energy that it would have melted the surface of the Earth, effectively sterilizing any life that was there previously.

See: https://en.wikipedia.org/wiki/Origin_of_the_Moon

[Blank Space (OP)]

This seems to be asserting two things which appear opposites.

Having large empty spaces beneath the deepest part of the ocean seems unlikely to me - it would collapse (fill with rock) or at least fill with water.

  They are not opposites as cavern and deep sea trenches exist. A completely smooth inner planet is just as unlikely as a completely smooth surface area. A cavern may exist at a hundred meter's in size, a maximum may very yet be one thousand meter's in size.

  As to the comment about them collapsing that can't be assumed because of several factors.

 - Earths gravity comes from the core of the planet, therefore if an object has less of earths' gravity applied to it the further away you get then a object by extension would have more gravity applied to it the closer to the core you get.

 - The rock have a higher density and are able to sustain significantly higher surface tension and pressure before a crack emerges. Even more so than that if a highly dense rock is damaged it is more likely to have a smaller chunk break away rather then a full collapse.

 - Given than foundational support is a significant factor as to why thing's built high would collapse in the first place solely based on gravity then being so deep underground offers the position that surface cavern system simply do not have. The rock surrounding said cavern is just as dense as the rock making up the cavern because they are the same rock just in different layers.

  Seeing as how structures tend to hold together much better when surrounded with support it can be assumed cavern systems would react in a similar way. Though I will admit details and any definitive statement is only left to speculation simply because the research hasn't been done on anything really past the crust of the planet.

However, it is thought that this impact of a Mars-sized object with the early Earth would have released so much energy that it would have melted the surface of the Earth, effectively sterilizing any life that was there previously.

Only on the surface of the planet, however what I said was it could have opened a pathway. That was literal, it could have fractured a cavern system of split the ocean bed enough for life to come from it, that could have then closed over hundreds of millions of years due to gravitational pressure. Said organisms could have simply just evolved to live in their new environments slowing moving up more and more until they became what is know as the first land based organisms.

[evan_au]

Earths gravity comes from the core of the planet. If an object has less of earths' gravity applied to it the further away you get then a object by extension would have more gravity applied to it the closer to the core you get.

Actually, Earth's gravity comes from all of the mass of the Earth - including the piece of dirt you are standing on.
- If you are away from the Earth (eg an orbiting satellite, or the Moon), you can treat all of Earth's gravity as if it came from the center of the Earth: the closest piece of dirt balances the most distant piece of dirt, so on average, it seems to come from the center of the Earth.
- If you are within the Earth (eg in a deep mine, or at the bottom of the Mariana trench), you can still treat all of Earth's gravity as if it came from the center of the Earth, but you only count the mass that is closer to the core than you are.The mass that is farther away than you cancels out: the closest piece of surface dirt it is pulling you up, and the most distant piece of surface dirt is pulling you down; the total effect is cancellation. This means that gravity gets weaker as you approach the center of the Earth.

This was all worked out by Isaac Newton, about 300 years ago. It is summarized in his "Shell Theorem".

See https://en.wikipedia.org/wiki/Gravity_of_Earth#Depth
And specifically this graph: https://en.wikipedia.org/wiki/File:EarthGravityPREM.svg
 - The Dark Blue line is the actual variation of gravity from the center of the Earth out to 14,000 km

[evan_au]

[Theia] could have fractured a cavern system or split the ocean bed enough for life to come from it

The experience of researchers bringing up life from the deep sea is that it usually expires from the pressure change before it reaches the surface.

If you are talking about life escaping from underground caverns deep below the sea floor, the pressures there would be far higher than in the deep ocean. Any hypothetical life there would die if the pressure changed quickly.

And the temperatures become quite extreme even 15km below the deep ocean floor. Any hypothetical life living in the Earth's upper mantle would freeze solid before it reached the sea floor or Earth's surface. 

See: https://en.wikipedia.org/wiki/Geothermal_gradient