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14 Sep, 25 tweets, 7 min read
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)
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,
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

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More from @CColose

30 Jun
This is a terrible article. There are some errors, but the main issue is to take the wildest claims from activists that never came from the clim sci community, and in fact have been called out ("12 years", "Amazon is lungs of planet"), and apologize that the activists were wrong.
Naturally, the nonsense that came out of the activists are not "the IPCC." This has been robust across multiple assessment reports.
This doesn't mean climate change isn't the biggest environmental problem. It almost certainly is. A 3 degree world is one never encountered by humans, and the scope of the problem and its solution set is extraordinarily difficult to communicate.
Read 4 tweets
31 Jan 19
Thread time (mildly technical) for some broader context in relation to #PolarVortex

Because it’s popular, let’s dig in, and also is this thing related to global warming? Let’s start with the basics. We’ll get to polar vortex toward the end.
It turns out when you take a “fluid,” like an atmosphere, or a big water tank in a lab, and subject it to temperature gradients (e.g., driven by differential solar heating) and then make the fluid rotate, interesting things happen.
Digression (a): In reality, when you take (under)graduate atmospheric science classes, you bond not over how you will become rich promoting global warming, but rather hours spent writing down equations that describe those interesting things.
Read 24 tweets
29 Jan 19
Intro to seasons:

The Earth is tilted bigly. Venus wishes it had our tilt. Lying Venus says it has a tilt of 180 degrees. They just say that because it spins backwards. Lying Venus. Venus doesn't have the guts to have seasons! I watched her on FOX news and agree she is hell.
Then in the summer we tilt and get more sunshine. A LOT MORE. I know all about sunshine, believe me, you wouldn't believe the amount of sunshine we get.
Don’t know how the poll numbers are so good, but it looks like the sunshine is up 1009%? The fake news won't report this.
Read 5 tweets
15 Dec 18
After a week of cool science at AGU, these give me headaches, but here's a short history of Venus.
Once upon a time, there was a planet, pretty comparable to Earth in size. It could of had a nice climate for a bit...depending on things like topography and rotation rate. A slow spin exposes its dayside to the sun for almost two months at a time, which sounds bad but...
...if you tell a General Circulation model about that rotation state, you can make the sun-lit part of the planet overcast for many months, shading it, so you can retain temperate conditions for some billions of years. It wouldn't work for Earth rotation. Depends on other things.
Read 12 tweets
7 Aug 18
Because it’s in the news, this is a thread on #HothouseEarth.
@bobkopp has one too

But I want to say some other things. First bit a summary, then my thoughts.
This cartoon in their paper is an anchor point for the thread, which the authors (Steffen et al.) present to frame their discussion of climate stability and tipping points
Stefan et al. envision Earth’s climate as resting in a stable state in the various valleys in the cartoon, or at least oscillate between but remain bounded by two states (e.g., glacial-interglacial limit cycles).
Read 24 tweets
19 Jun 18
Mini thread on climate without the Sun!!

So the other day I started a GCM run, and because I was silly, I effectively zeroed out the solar constant by accident. So I can turn a scrap run into a learning experience!
What would the climate do if the Sun went black today?

The model has made it a few decades without crashing (which it will at some point), but here is a plot of some results.
Things get cold quick, but it actually takes many years to get things *really cold." The oceans have a lot of heat capacity, and a lot of energy is being suddenly mined out of the oceans. It takes about 10 years for sea ice to cover the whole planet.
Read 7 tweets

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