I'm trying to make carbon-based alien lifeforms which live in a
Venus-like world, where:
The temperature is 650−750 K and the pressure is 0.9−1.1 MPa Sulfuric
acid is abundant Because of point 2, I choose sulfuric acid as the
biosolvent. One literature suggests that certain organic molecules are
stable in it, so a carbon-and-sulfuric-acid-based-lifeform should be
possible.
The paper you link to states that there is significant chemical diversity possible in concentrated sulfuric acid solvent at relatively low temperatures, which is made possible by the use of a wider variety of bond types than exist in biological compounds on Earth. Many of the familiar molecules used by Earth life are not stable in concentrated H2SO4, and have to replaced using the alternative bonds that H2SO4 solvation allows, although some can be transferred over.
The problem is the point 1. As I know it, complex organic molecules
can't survive Venus-like temperature, although I don't know at what
temperature they start becoming unstable (I'd like to know it too).
They start becoming unstable around 50 degrees Celcius--note that pasteurization and sous vide cooking can be done at a mere 63C. DNA is not completely stable to start with, but it becomes insufficiently stable support life somewhere around 150C. Of course, you wouldn't use DNA in sulfuric acid anyway... but industrial hydrocarbon cracking can be done starting around 450C. So, carbon-based life at 650K (377C) seems... implausible. Especially in a highly reactive solvent like H2SO4. Note that the paper you linked to focuses on the possibilities for life at higher altitudes, and lower temperatures. Also note that, at an altitude of 55km, where the temperature is only around 25C and comfortable for humans, the mean stability of complex molecules and diversity of possible chemistry is already rapidly dropping off--and the hotter you get, the worse it is.
Meanwhile, the stabler substitutes for organics molecule would be
silicones in this condition. From most sources I've read, such
silicon-based biomolecules are usually paired with sulfuric acid as
the biosolvent.
Yup. If you want high temperatures, you need a wider variety of bond types, and a larger proportion of silicon (as well as other elements) to achieve equivalent complexity. However, we're not talking about purely silicone (siloxane) chemistry here--even if you transition fully to siloxane polymer backbones as opposed to carbon backbones, you'll still have carbon-centered functional groups. Fundamentally, this isn't silicon chemistry--it's organosilicon chemistry.
However, I doubt it will work, because as far as I know, polar
solvents like sulfuric acid can't dissolve silicones.
Depends on the silicone. Industrial silicone rubbers are extremely monotonous polymers, and are designed specifically to not be reactive. So, yeah, sulfuric acid won't dissolve polydimethylsiloxane. But that's OK--you need some insoluble molecules, and appropriately-designed siloxanes with thiophillic functional groups can absolutely be dissolved.
Additionally, per the paper you linked, if you keep the temperatures below 100C, even alkanes and triglycerides are stable and insoluble in H2SO4--so you can plausibly have macromolecular structures like cell membranes which are still entirely carbon-based, and reasonably call your aliens "carbon-based", even if silicon is a common heteroatom--like nitrogen is for us.
Also, any oxidation of biomolecules using oxygen compounds like SO₃
will produce solid and even insoluble SiO₂, so any respiration will
have to produce gaseous silicon compound, which have to involve
halogens like chlorine. But halogens are just too rare.
No it won't. Gaseous waste products are not ideal, even on Earth; what you really want is something liquid, or liquid-soluble. Note that we could get more energy from metabolizing proteins by releasing gaseous nitrogen, but we don't--we have considerable biochemical machinery dedicated to disposing of it as a liquid-soluble solid waste (urea). Similar strategies could be used for disposing of waste silicon, whether it is a major structural and energy-storage atom like carbon, or merely a common heteroatom like nitrogen. However, regarding halogens being rare... no, they aren't. They just aren't very accessible to terrestrial biochemistry, as they have very high bond energies and tend to be locked up in minerals. But fluorine is more abundant in Earth's crust than carbon is--and chlorine is an essential nutrient, and a component of sea salt! And despite their relative inaccessibility, there are a few natural organofluorine and organochlorine compounds already in our biosphere, and since chloride and fluoride salts are unstable in sulfuric acid, decomposing to produce hydrochloric and hydrofluoric acid (which are also present in the atmosphere of Venus), chlorine and fluorine will be much more readily bioavailable in such an environment. So, there's probably no need to excrete silicon in tetrahalogenated form, but organisms in sulfuric acid solvent certainly could.
So, yeah--you're absolutely right. Your big problem is point 1! Bring down the temperature, and life looks much more plausible.
There is, of course, still the issue of abiogenesis. Your world may be "habitable, but uninhabited", if there is a biochemistry that could function there, but no way for it to naturally develop. And indeed, the abiotic processes that produce prebiotic chemicals in terrestrial environments don't operate in sulfuric acid! But, on the other hand, other processes that produce chemicals suited to that environment might. And Venus itself isn't really a counter-example, because it doesn't actually have liquid sulfuric acid in contact with the surface.
Now, can life actually exist in sulfuric acid? We have no idea! As the paper you referenced says, there is much, much more work to be done. But, we can't rule it out. So, dial down the temperature, and you'll be within the realm of reasonable suspension of disbelief.
