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New paper alert! tl;dr: the seafloor of Europa is probably tectonically inert, meaning little to no active fracturing that could expose fresh rock to seawater. Without such water–rock reactions the prospect for there being life within Europa just took a big hit. A thread:

Our paper, "Little to no active faulting likely at Europa’s seafloor today", was just published today in Nature Communications! This version of the paper was submitted early last year and accepted in November, but the genesis of this paper started a VERY long time ago (relatively speaking) (2/n)

The original science question my co-author Christian and I asked was simply: What does the seafloor of Europa actually *look* like? This question came about because, although we can see Europa's icy exterior pretty well, we know virtually nothing of its interior. (3/n)

So we ran calculations widely used for rocks on Earth to figure out how strong the ocean floor might be, remembering that most of Europa is rock! Why? Because fractures are a GREAT way of exposing fresh rock to the ocean, driving chemical reactions for life. No fractures? No fresh rock. (4/n)

WAAAAIT A MINUTE I hear you say Water makes rock weak. And these rocks are *surely* saturated. You're right, they are! In fact, we *intentionally* consider rocks that are wet, have pores filled with water, and are in fact already fractured. We WANT fractures to slip and expose fresh rock! (6/n)

We then calculated what *stresses* these weak, fractured seafloor rocks could be subject to that would allow those fractures to move to expose fresh rock. Turns out tides don't do it. Interior cooling won't do it. And mantle convection won't do it. No plate tectonics within Europa! (7/n)

What we found is that, despite having pretty low gravity, the presence of Europa's overlying ocean and ice keep its seafloor rocks—*even when those rocks are already weakened*— strong enough that no obvious (to us) source of stress can move existing fractures, much less make new ones. (8/n)

Why is the difference so small between tension and compression? 2/3*density*g*z vs 2*, I assume for Byerlees law

Read the methods! At the seafloor itself, rock strength comes only from cohesion. Otherwise it's pretty much Byerlee's law, but note the axes. The strengths diverge pretty quickly, even under a very low g. Moreover, we report both absolute and differential fault strength values.