Knowledge comes from differences, from variations and gradients. Studying ten copies of the same system won't generally tell you very much more than studying a single one.
Studying ten different systems of the same kind will tell you all kinds of things.
Geology suffers from this limitation more than many fields, because we've only got one planet that is easy to study. We have some information about the Moon from seismometers left behind by the Apollo missions in the 1970s, and recently have started to get some data from Mars.
The InSight probe isn't a rover like Curiosity or Perseverance, and has had a lower public profile, but has slowly been accumulating data on Martian geology--there is a debate over the word, since "geo" means "Earth" and Mars is not Earth, but having a different word for studying geology on every planet would be silly--for the past year.
There are a number of instruments, one of which was intended to measure heat flow out of the planet by driving a probe up to five metres into the Martian soil Unfortunately, that particular instrument didn't work. The digging system was designed to hammer its way into the soil, but depended on friction from the soil to operate properly. The Martian soil in the area has properties that fall well outside the model parameters used to design the instrument: it is friable and clumps easily, so rather that providing friction for the digging element to work against, it creates a void around it.
This isn't so much a failure as an education: with robotic probes not everything can be anticipated, and the most we can hope for is we learn enough from the things that don't work very well to find things that work better next time.
Despite that issue with a single instrument, the InSight probe has produced a lot of data on Martian seismology, some of which can be used to construct a model or two of the planet's interior, which lies at the extremes of what was considered plausible going into the investigation.
This is reassuring in a couple of ways: it's always nice to learn new things, and when expensive and difficult experiments do nothing but "confirm what we knew all along", as every single research effort ever funded by organizations like Greenpeace do, one wonders if they haven't been rigged somehow to generate politically anodyne results rather than actual knowledge about the world, as we know that the world almost never looks anything like what we expect: every time we look more closely, if we do so honestly and with humility, we find something new.
In the case of Mars there was a general feeling that since it is a smaller planet than Earth it would likely have a thicker crust and smaller core. Mars is about half of Earth's radius (3400 km vs 6400 km) and volume scales as the cube of the radius and surface area as the square, so the ratio of the two scales as the radius. On that basis you'd expect that the Mars would be losing heat a good deal faster than Earth, so //if// they have similar composition you'd expect the outer crust of solid rock to be thicker and the hot, liquid, inner core to be smaller.
That's a naive model, though, and like many naive models, it didn't survive contact with the data, which show the crust to be 24-70 km thick, as opposed to 15-20 km thick on Earth. So the crust is thicker, but not enormously so.
The liquid core on the other hand was expected to be relatively small, but turns out to be almost 2000 km in radius. This implies that the liquid is less iron-rich than Earth's liquid core, and models of planetary formation will be constrained by that fact.
Collecting these data took years, which makes it difficult to imagine doing anything similar on Venus, where the longest-lived probes survive for just hours. The surface of Venus is hot enough to melt lead and the atmospheric pressure is like being 100 m down in the ocean. There is, however, a band in the upper atmosphere where the temperature and pressure would be comfortable to a naked human, assuming they were able to breath carbon dioxide and their skin could withstand sulphuric acid rain.
If only it were possible to do seismology from a balloon... which may actually be the case.
This is one of those crazy ideas that turns out to maybe work when you bother to get past the fact that it's a crazy idea and try it, probably doing some pretty careful calculations first.
The idea is disarmingly simple: as anyone who has been in one knows, earthquakes make noise. Lots and lots of noise. How far away could that noise be heard by sufficiently sensitive instruments, particularly ones designed to detect the low-frequency ("infrasound") motion of the surface that earthquakes induce?
It turns out the answer is "quite far", to the extent that a "microbarometer", which measures tiny, low-frequency pressure changes, was able to detect a real earthquake with sufficient fidelity that the data could be used to infer the parameters of the event:
Our comprehensive analysis of seismo-acoustic phenomenology demonstrates that seismic activity is detectable from a high-altitude platform on Earth, and that Rayleigh wave-induced infrasound can be used to constrain subsurface velocities, paving the way for the detection and characterization of such signals on Venus.
After the Ridgecrest quake in 2019 a team of researchers at Caltech rapidly constructed four balloon-borne seismic systems and deployed them quickly enough to catch the aftershocks. The original quake involved two events of magnitude 6.4 and 7, separated by several days. The aftershocks lasted for weeks, with upwards of ten thousand of them above magnitude 1.5.
Only one aftershock was clearly detected, and that during the ascent phase of their first attempt, which involved two balloons that rose to about 20 km and drifted for 12 hours, although the electronics gave up after 5 hours due to temperature effects high in the atmosphere.
Even so, this very first attempt, cobbled together in haste to take advantage of an immediate opportunity, did produce a positive result, and that bodes well for the future.
Alien Earthquakes
I agree with what Hilary said. Love reading your blog posts which have a lovely balance of technical depth and lay understandability for us science-interested folks to feel like we’re learning something interesting.
Another fascinating piece on a topic that was virtually unknown to me. Congratulations on making esoteric topics accessible!