Thread by @ThePlanetaryGuy

Psst.

Interested in learning more about #Venus—and why @NASA's decision to go back there with the #VERITAS and #DAVINCI+ missions is such a big flippin' deal?

Read on.

(a 🧵 on comparative planetology)

It all comes down to one simple question:

Why is Earth's sibling not its twin?

(Credit to @efrankplanetary for that line.)

Venus and Earth are the same(ish) size, age, and composition (in roughly the ~same proportions), orbiting the same star.

So, uh... what's going on?

Here's where we come to our two different stories for Venus—and why figuring out which one is correct is *critical* to understanding not only our own world, but Earth-size worlds everywhere.

Scenario 1: Venus was *always* fucked.

In Scenario 1, Venus started life with a magma ocean, and likely a thick, CO₂ atmosphere.

But it was close to the Sun, and couldn't lose heat as fast as it built up.

So Venus, pretty much from the start, entered a "runaway greenhouse effect".

Game over, thanks for playing.

But in Scenario 2... Venus might have been just like Earth.

*If* Venus survived its early childhood, recent models suggest that it might have had oceans, and maybe even plate tectonics.

It's *possible* that, for >3 billion years, there were two blue marbles in the Solar System.

In Scenario 2, Venus enters a runaway greenhouse, but not because of its proximity to the Sun—but from a self-inflicted cause: volcanoes.

Those same models say that several MASSIVE eruptions could have dumped so much CO₂ into the air that the climate simply could not cope.

The temperature rises, the oceans start to evaporate more and more, which dumps more and more water vapor—itself a greenhouse gas—into the atmosphere, which causes the temperature to rise—

You get the picture.

A once Earth-like world starts to die.

And, eventually, that once-blue world loses its surface water entirely, plate tectonics (which requires water) shuts down, but volcanic activity continues... pouring out lava and continuing to thicken the atmosphere with SO₂ and CO₂.

And we end up with the Venus we see today.

One of these models is wrong.

But we don't know which one.

We don't know if what happened to Venus could happen to Earth.

And how the hell can we make sense of Earth-size exoplanets, if we don't know which scenario's the right one?

And that's where #VERITAS and #DAVINCI+ come in.

NASA has not sent a dedicated mission to study Venus since the Magellan radar mission in 1989.

That mission ended in 1994. And we have come up with a LOT of questions since then.

So, these missions being picked is a HUGE deal because it's been 32 years since NASA sent a mission to Venus.

It's *also* a huge deal because, though we hoped for 2 selections from the 4 shortlisted missions, we thought we'd get VERITAS *or* DAVINCI+.

BUT HOLY FUCK WE GOT BOTH.

VERITAS will carry a radar to obtain image and topographic data for Venus that we BADLY need, at resolutions comparable to what we have for Mars.

I cannot begin to emphasise just how important those data will be.

Like, we'll be using those data for *decades* more.

VERITAS will *also* carry an instrument capable of measuring—for the first time from orbit—the chemical composition of rocks across the ENTIRE planet.

Folks, we don't really know what Venus' surface is made of.

VERITAS will help tell us.

"BUT PAUL",

I hear some goddamn Mars sympathiser ask,

"WhY aRe We SeNdInG tWo MiSsIoNs To VeNuS dOn'T bE gReEdY"

The second mission isn't the same. Where VERITAS maps the surface, DAVINCI+ takes a deep dive *into* the atmosphere...

DAVINCI+ (which also has an orbiter, btw) will drop a probe into the hellsphere—and that probe will measure the atmosphere's composition during an hour-long descent to its death.

The measurements it'll take?

They'll tell us whether Venus ever really did have oceans or not.

And there's something extra cool about DAVINCI+...

The probe isn't designed to survive the landing, but it'll come down in one of Venus' enigmatic "tesserae"—an ancient terrain type on the planet about which we have no real goddamn idea.

And it'll take pics the whole way down.

DAVINCI+ will tell us what the atmosphere's made of, and from that the planet's climate history, and will give us our first real glimpse at Venus' ancient terrain.

VERITAS will show us the entire surface as never before, and help us figure out if it's geologically active today.

I want to be clear.

These two missions won't "solve" Venus. No two, or three, or even four missions will do that.

But having both VERITAS and DAVINCI+ exploring different aspects of Venus this decade?

It's going to REVOLUTIONIZE how we understand planets... including our own.

I've skipped a LOT of details and nuance—but I'll be talking about these missions for years to come.

Just know this: in the 1980s, we thought we understood Mars. We were wrong.

We still don't really understand Venus.

But we're going to put that right.

And it'll blow you away.

Oh, one more thing.

Next week, @ESA will announce the mission they're picking for their *own* competition.

One of them is @EnVisionVenus—a Venus mission.

It might get picked.

We might get a fleet.

Stay tuned.