Thread by @CColose

Thread on Venus—a bit on phosphine detection, but for broader context

Venus is a very hot (~900 Fahrenheit) planet with a thick atmosphere (~90 x the pressure on Earth, like being a kilometer deep into the ocean).

But it may have been the first habitable planet.

The starting point for habitability discussions—be it on ancient Mars or distant exoplanets— is usually having a climate in which liquid water is stable in the liquid form at the surface. That doesn’t need to be the only starting point, but it is compelled by Earth requirements.

Planet formation models don’t suggest Earth &Venus should have started with vastly different water amounts, while the Pioneer Venus mass spectrometer measured very high Deuterium-to-Hydrogen ratios(~150x terrestrial water, 1), often interpreted as an ancient ocean that was lost.

So what happened to this ocean if it ever existed? Why is Venus so hellish now? There’s a lot of uncertainty, but some storylines have emerged. So here’s a quick present and past Venus tour.

Venus’ high temperature today isn’t caused by its proximity to the Sun. In fact, a bare rock at the Venus-Sun distance would only be a bit hotter than Earth’s tropics. A bare rock that was as bright as Venus would actually be colder than Earth’s average temperature.

The brightness is caused by reflection of sunlight from thick sulfuric acid clouds, cooling the planet— but even without these clouds, Venus would be very reflective due to Rayleigh scattering by CO2 (this term was talked about a few days ago when California’s skies were orange).

Instead, CO2’s greenhouse effect (absorption of infrared) keeps Venus hot. Radiation is a very bad way for Venus to lose heat, so its atmosphere convects deeper than on Earth. The reported phosphine detection would be up at 10^3 to 10^2 mb on this plot (2), at mild temperatures.

As much as humans are trying to put a wrench on this, weathering and geologic processes keep CO2 on Earth to ppm levels, rather than 90 bars of it. So Venus is kept warm by just a trickle of sunlight and the strong “blanket” of a greenhouse effect. (ref 3)

However, this atmosphere is probably new-ish in Venus’ history; it may have been much thinner long ago. On Earth, almost all the carbon is in rock. With liquid water, the formation of carbonates balance long-term CO2 outgassing & removes atmospheric CO2 efficiently.

So the CO2-dense atmosphere on Venus would come after the loss of water. There’s some different ideas for how Venus lost its water. A popular one is an ancient runaway greenhouse, a sort of rapid transition to a super-hot climate where liquid water is unstable.

The idea is that Venus was close enough to the Sun such that it absorbed more sunlight than it could shed infrared energy (there are limits on outgoing energy in moist atmospheres). An alternative is a “moist greenhouse,” a slower transition w a “warm” surface & wet stratosphere.

In a moist greenhouse, if it gets warm enough for a rather wet stratosphere, the breakup of H2O and loss of hydrogen to space gradually results in ocean loss over time…it takes about ~3 g/kg mixing ratio to lose an Earth ocean over Earth’s ago (something we are well under).

These older calculations were fairly simple & treated the planet in 1-D. Recently, another factor has come to the forefront: Venus’ rotation rate. Venus currently rotates very slow (~once in 243 Earth days), which is actually longer than its year! (225 days to orbit the Sun)!

It also rotates backwards relative to its orbit. The rotation evolution of Venus is highly uncertain, but it is nearly tidally locked with a slow rotation rate set by gravitational and thermal tides (tides also tend to damp the obliquity to near zero or, if you want, 180 degrees)

See also this thread

https://twitter.com/CColose/status/1263528190619852800?s=20

At very slow rotations, 3-D modeling suggestions thick water cloud decks form near the substellar point, increasing the reflectivity of the planet and stabilizing the climate, expanding the “inner edge” of habitability beyond those 1-D estimates (ref 4).

A recent paper (5) explored the evolution of Venus if very slowly rotating, showing it has no problem remaining temperate even near present. This plot moves forward in geologic time w simulations at 16-243x Earth rotation w different assumptions about atmosphere, soil type, etc

If this works, then it’s possible something else other than proximity to a gradually brightening Sun “triggered” the collapse of Venus’ habitability, perhaps outgassing after large igneous provinces or a rather recent global resurfacing event hundreds of millions of years ago.

Whatever happened, Venus is left now with a thick CO2 atmosphere with hyperacidic sulfuric acid clouds (likely supplied by volcanic activity) and only a small bit of water that is all in the vapor form.

That is context from which the new paper on phosphine detection paper is laid. PH3 is produced only in extremely small amounts on Earth from biological activity, and has a very short lifetime in Venus’ upper atmosphere, destroyed by photolysis-induced chemistry.

On Earth, some metabolically active micro-organisms exist in the upper atmosphere (6),inside cloud droplets but some free-floating in the atmosphere, but not permanently (there is transfer from the surface).

I’m not sure how long the lifecycle would be; I imagine Venusian organisms would need to spend most of their life in cloud droplets and jump between them! Anyway, there has been a lot of skepticism of the detection, much of which is technical, see. E.g,

https://twitter.com/chrislintott/status/1305521729662529536?s=20

Note that 1) the phosphine might not be there, it could be due to interference w other spectral features, or 2) chemically produced by abiotic means. But maybe it will help with the case that we should actually go back to Venus!

/end

some refs
1) agupubs.onlinelibrary.wiley.com/doi/full/10.10…
2) agupubs.onlinelibrary.wiley.com/doi/full/10.10…
3) rmets.onlinelibrary.wiley.com/doi/full/10.10…
4) iopscience.iop.org/article/10.108…
5) pubs.giss.nasa.gov/abs/wa02800e.h…
6) nature.com/articles/s4139…
& phosphine paper: nature.com/articles/s4155…

One more thread

https://twitter.com/physicsj/status/1305541269435817984