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I said I'd do a thread about my new Space Science Reviews paper, so here it is! This is part of a collection of papers arising from a meeting at the International Space Science Institute in Bern in 2024: a workshop to better understand rocky extrasolar planets. A 🧵:

The basic idea is that we're finding more and more exoplanets, including ones that—on the basis of radius and mass—are surely rocky. But that's usually all we have, even though it's tempting to call them "super-Earths" or to otherwise attribute (intentionally or not) certain properties to them.

So, until/if we start imaging the surfaces of these worlds, what might we learn about them? Well, we argue that the Solar System's rocky worlds provide a useful template to start from. And that's the gist of the paper! So we start with Earth, Venus, Mars, Mercury, and the Moon.

A big part of the paper (and my career) is comparative planetology, the comparing of one world with another. So in this paper, we talk about how and when the crusts of these worlds developed. Those of Mars, Mercury, and the Moon are ancient; some of Earth is very recent. And Venus?? who tf knows

We take a deep dive into Earth in particular—since it's the world we understand best (even if there's LOTS about Earth we still *don't* understand). Here's a plot showing how we think Earth's bimodal crust grew through time; continental crust wasn't always there, but oceanic crust probably was.

A big part of the paper is the distribution and formational history of the major geological elements of Solar System worlds. So we included these maps, which show things like volcanoes, tectonic systems, etc. As our baseline case, this is modern Earth.

Next is Venus; we talk about its geological history and phenomena as we understand them (which isn't all that well). Here's a global map of Venus' geology; it's important to note that these are schematic maps, designed to be compared with each other. tl;dr: lots of volcanoes, no tectonic plates

Next is Mars*, a substantially smaller world than either Earth or Venus. Mars still has giant volcanoes, rifts, ice caps, and huge impact basins (especially in the southern uplands), but again no sign of plate tectonics. It was too small to replenish the atmosphere lost to space. *spits on floor

We treat Mercury and the Moon together, not so much because they're all that alike but because they're both *small* rocky planets. Lots of volcanism—for a while. But all major geological activity stopped a loooonnnnng time ago on these worlds, because they're small. Anyway, here are their maps.

Finally, we turn to Io, Jupiter's crazy volcanic moon. Left to its own devices, Io would be inert. But it's subject to terrific tidal heating around Jupiter, and so serves as a useful template for understanding tidally heated rocky exoplanets. Io is so active, it has no impact craters! Map:

We then talk about other kinds of worlds we *don't* see in the Solar System: super-Mercuries, super-Ganymedes, super-Earths, or bodies more enriched in radiogenics than our own. But on the basis of what we see in the Solar System, we argue that rocky planetary outcomes are *variations on a theme*.

And, we posit, we should expect that underlying theme to be familiar, even if the particular notes and rhythms are different, even markedly so. The Solar System's rocky worlds tell us this, graphically shown in this figure. And we should heed this lesson!

This paper took a long time, in part because it's probably the most technically broad paper I've led so far. We cover Hadean Earth, exoplanet detection methods, comparative planetology, and the putative geology of rocky exoplanets. I had awesome co-authors, and I hope you read—and enjoy—this paper!