Climate collapse, Fermi paradox 

I wonder if it's possible that an artificial satellite orbiting Venus would have remained invisible to this day. If it has a small enough radar cross-section and stable orbit...

I wish I would know more astronomy.

/cc @anne

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@uint8_t They have bounced some pretty serious radar off of Venus - like, transmitted with the 300m Arecibo dish. But it's a long way away, and artificial satellites can be small. How long they could last, well, many of our satellites will come down soon but that's because we put them up as low as wee can get away with. The ones that are high up are going to be there a very long time.

@uint8_t Venus is actually a pretty comfortable place, a reasonable temperature, reasonably clear sky, just a bit acidic, at the one-atmosphere altitude. The ground is just a long way below that. But breathable air is a lifting gas on Venus, so there's this idea that you could build floating cities at the 1 atmosphere depth on Venus.

@uint8_t It certainly is possible for runaway global warming to occur, and it is thought to have happened to Venus - the oceans would have boiled, and the water vapour would have added to the greenhouse effect until it escaped to interplanetary space. At that point the result is really irreversible. It seems like maybe we can't make it happen here:
royalsocietypublishing.org/doi

@uint8_t TIL: Venus has 100 times more deuterium mixed into its hydrogen than Earth. This is strong evidence for an ocean that boiled away, and it was measured by one of our probes before they died.

@kragen @anne Deuterium is an isotope of hydrogen. Heavy water (D2O) has a slightly higher boiling point, lower vapor pressure, and lower mean speed, so if you got an ocean with a H2O-D2O mixture, the concentration of D2O increases as you boil the water away. Our oceans have a higher D2O concentration than our rivers.

@kragen @anne @uint8_t because it is heavier, it's less distant from oxygen in a water molecule, and has a stronger bond.

Also any molecule it is on, it tends to make it heavier than the identical one with regular hydrogen, so it typically ends up lower in the atmosphere.

@jasper @anne @uint8_t Do we mean 100 times more deuterium in Venus's atmospheric hydrogen than in Earth's hydrogen, most of which is, I think, in its oceans? Is there a big difference between the deuterium fraction in Earth's atmosphere and in its oceans?

@kragen @jasper @uint8_t So the article describing the measurement is: ncbi.nlm.nih.gov/m/pubmed/1778
Terrestrial spectroscopy cast some doubt on the result but Venus Express seems to confirm it:
sci.esa.int/venus-express/5406

The deuterium enrichment is thought to have occurred because hydrogen is lost more easily to space; fortunately the Earth doesn't lose a lot of hydrogen because it's mostly in water and the "cold trap" keeps that too low for photodissociation.

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