Thread by @ThePlanetaryGuy

OK, buckle up, kiddos—here's a 🧵 on our new Geology paper about the enigmatic tesserae on #Venus, and what our new results mean for our understanding of the Hell Planet

Here we go!

https://twitter.com/ThePlanetaryGuy/status/1306630436836212741

First off: what *are* the tesserae (sing. "tessera")?

In short: dunno! Ha.

The longer answer is that they're very heavily tectonically deformed rocks, and where they're found they're the oldest things around: everything else is on top of them.

Here's what they look like:

Lots of work has focused on the tesserae since they were discovered in the 1980s (and named by Russian scientists for parquet flooring!).

We've found lines in several tessera units that curve in such a way that they seem to follow topography. And that's a bit... unexpected.

In the figure above, you can see bright lines on this radar image that curve (marked by gold arrows).

Here's another view, with the lines sketched out in teal (centre) and shown with topography (right).

Radar-dark smooth stuff is shown by blue arrows. (We'll come back to this.)

So why are these lines of interest?

Well, they look *a lot* like the lines you get in the landscape when (near-)horizontal lines are exposed by erosion.

Check out this image of the limestone Burren in Co. Clare, Ireland...

...or this one, showing exposed layers of stacks of lava in the East African Rift:

So, it sure seems that the curving lines we're seeing in the tesserae look a lot like layers of some rock type that's been exposed by erosion, at least in part.

That's consistent with them being stacks of lava, say; compare these lines with those of lava traps in Siberia (top):

But what's doing the erosion?

Probably wind—but perhaps not wind on its.

We *do* know there's wind on Venus, for several reasons, including the fact that we saw particles move over the course of an hour after the Venera 13 lander touched down in 1982:

That said, wind probably isn't enough to make the ridges and valleys that characterize the tesserae.

But you know what could? Folding.

And there are shapes in the tesserae that look a bit like eroded folds; they're marked in these images with purple arrows:

The example in the lower half of that figure is from Earth, from the Sulaiman Mountains in Pakistan; this is an oblique view from Google Earth, and the distinctive lenticular (eye-shaped) patterns here result from the folding and then erosion of layered rocks:

So what can we say about the tesserae, or at least those parts that show these curving lines that follow topography?

We think that these rocks must be layered (top), and are then, at some point, folded, for some reason (middle). *Then* those folded layers are eroded (bottom):

And remember that radar-dark stuff? One of the reasons something is dark on radar imagery is because it's smooth (at the wavelength of the radar beam).

Maybe that dark stuff isn't lava, as is often assumed, but the stuff eroded off the tesserae ridges!

One thing I haven't talked about yet is what the rocks that make up the tesserae actually *are*... and that's because we don't know.

Maybe they're layers of lava, like our example from Siberia! That would make sense—Venus is COVERED in lava. They might even look like this...

There's one other possibility: maybe these layered tesserae are sedimentary—sandstone, say, or even evaporites.

So what? They're super common rocks!

Yes, but they can't form on Venus *today*. Which means, if they are sedimentary... they're from an older climate period.

The point is, although we now know more about these ancient and weird rocks, we still don't know some fairly fundamental things, like what they're made of.

But we *could* figure that out, if we got close to or even landed on the tesserae. For which, friends, we need a mission!

For now, here's the press release from @NCState:

news.ncsu.edu/2020/09/venus-…

And here's the paper, from @geosociety!

tinyurl.com/VenusTesserae