As is often said by those with a flair for the dramatic: it begins.
NASA’s InSight robot, just a handful of months into its exciting geological exploratory mission on Mars, has made a literally groundbreaking discovery. As announced by NASA earlier today, it has likely heard a genuine quake on Mars for the first time. Much like scientists on terra firma, such quakes will let InSight peer through the planet’s subterranean realm, allowing them to visualise its somewhat enigmatic internal structure.
The signal, picked up by the lander’s incredibly sensitive and precise Seismic Experiment for Interior Structure (SEIS) instrument, was recorded on the lander’s 128th Martian day, on April 6 of this year. Plenty of rumblings had already been detected by the lander, but these were being generated by the wind as it lightly shook the rust-hued surface.
This time, however, it’s almost certainly the real deal, as its waveform – the shape of the signal – much more closely resembles a genuine quake, based on terrestrial and lunar data. This makes it the first time a quake has been detected on another planet other than our own.
It was a fairly weak signal, and right now there’s not enough information to say why this is; it’s perhaps because the signal came from far away, or that it was of a very low magnitude, or a bit of both.
Mark Panning, a planetary seismologist at NASA’s Jet Propulsion Laboratory, told me on Twitter that “it’s hard to know for sure” why the signal is so weak. They can’t yet properly identify individual components of the seismic signal, and researchers “don’t yet know Mars well enough to understand the characteristics of the waveforms.” It’s likely to be a result of multiple factors, though.
In any case, this monumental detection “means that we’ve now entered a new era of planetary geophysics,” says Paul Byrne, a planetary scientist at North Carolina State University not involved with the work. “With this finding by InSight, we have or are now listening to the interiors of three of the five major rocky bodies in the inner solar system” – meaning Earth, the Moon and now Mars.
This is just the start of a prolonged and no doubt revelatory mission. As time goes by, more marsquakes will be spied by InSight. “Hopefully, before very long, we’ll have a much better understanding not only of the interior of the Red Planet,” and of other rocky planets in general, says Byrne, within and outside of our cosmic backwater.
As the name suggests, earthquakes are unique to Earth, but that doesn’t mean that tectonic activity – the movement of faults, in this case – doesn’t happen elsewhere. Until now, though, scientists couldn’t say for sure that so-called marsquakes were still taking place on the Red Planet.
Unlike Earth, Mars is a geologically quiescent world. Its small size meant that plenty of its internal heat – generated by radioactive decay and by leftover embers from its ancient, fiery formation – escaped into space long ago, and the engine that drives so much on our own pale blue dot, from many of its earthquakes and volcanoes, from mountain building to ocean basin carving, would have shut down long ago. It’s not clear that Mars ever had plate tectonics, which didn’t stop it from creating the largest volcano in the solar system, but it’s certainly not operating today.
That doesn’t mean that Mars doesn’t still have quakes, though. Plate tectonics may be responsible for plenty of geological activity on Earth, but large chunks of rock will still move and slide around under the influence of gravity, long-term erosion, pre-existing structural weaknesses, the heat-driven expansion and shrinkage of surficial geology, or perhaps even slithers of heat still seeping out from its cold heart.
At least, that’s the idea. The reason InSight is so exciting – and unparalleled in space mission history – is that it is the first fully-fledged geological robot to explore another world. Among other things, it will be taking the subsurface temperature of Mars, letting us know how much of its internal heat still remains.
Then there’s SEIS, which will listen to the pulse of our planetary neighbour. The detection of marsquakes will give us an understanding of how seismically active the Red Planet still is, eons after it gave up the geological ghost (for the most part). At the same time, the manner in which those seismic waves propagate through its rocky innards will betray to InSight’s science team just what those innards are like.
Scientists have a vague idea of what we might find already. After all, we have a fairly good understanding of what lies beneath our feet thanks to the earthquakes that constantly rock our world, and it’s suspected that Mars isn’t going to be hugely different in this respect. The way Mars orbits the Sun also hints as to what it’s made of. The only way to know for sure, though, is to do some geological experiments on Mars itself, which is why InSight’s mission is so revolutionary – as is its suite of scientific instruments.
SEIS is undoubtedly a work of scientific art. Hiding inside a stable dome, it is shielded from the effects of wind – although it can still detect these sorts of rumbles on the Martian surface – and even surface temperature fluctuations. Unlike previous seismometers, it is not attached to the frame of the lander itself; on previous missions, this awkward placement has filled the signals with noise, mainly recording gusts of wind as they shook the lander or rover itself.
With that in mind, InSight – hiding out in Elysium Planitia near Mars’ equator – is uniquely positioned to detect even the faintest of marsquakes. Now, finally, it’s pretty much a certainty that marsquakes exist.
As scientists pour over the data to shore up their analysis of it, InSight will be listening out for more marsquakes. In fact, there is a chance it might have already detected several more. Three more possible seismic signals had been picked up on March 14, April 10 and April 11, but their waveforms were not obviously seismic in nature.
SEIS is essentially comprised of two types of detectors: a high-frequency sensor, designed in France, and a low-frequency sensor, designed in the UK. One of the architects behind the high-frequency component, Philippe Lognonné, a professor at the Institut de Physique du Globe de Paris, explained that these three were only picked up on this part of the instrument. This was because they were extremely weak, and it’s possible that something else other than a tremblor could have generated them – a meteorite impact, perhaps.
Even if these turn out to be false positives, InSight will very likely pick up on more genuine marsquakes as time goes by, which will be required to uncover the Red Planet’s well-hidden geological secrets. Right now, scientists can’t say much about Mars based off a single, (pretty much) confirmed tremblor, and that particular quake is too weak to reveal much about the planet’s interior structure, says Byrne.
The April 6 shaking, however, has already hinted that marsquakes aren’t exactly like earthquakes. Lori Glaze, the Planetary Science Division director at NASA’s headquarters, noted in a press release that “its size and longer duration fit the profile of moonquakes detected on the lunar surface during the Apollo missions.”
As explained by NASA, lunar quakes come in all shapes and sizes: some are caused by Earth’s gravitational pull tugging on the Moon; some are caused by the frequent change in temperature on the Moon’s surface, which causes it to contract and shrink; some are perhaps caused by the collapse of ancient rocky structures, like unstable crater walls; some are triggered by meteorite impacts. It’s likely some of these mechanisms also apply to Mars, and perhaps some others that are unique to that planet.
Lunar quakes also last for ages – hours, compared to Earth’s seconds-to-minutes-long quakes – because the Moon, a frozen orb of volcanic material, permits the continual transmission of seismic waves without much to weaken them. If marsquakes do turn out to be more like moonquakes, then that already hints that its innards are structured in such a way that fails to readily absorb seismic energy. One possibility is that, unlike Earth, the subsurface doesn’t contain an abundance of energy-absorbing water.
It has been said that lunar tremblors make the Moon “ring like a bell.” When Earth shakes, it’s more like a muted piano. In time, we’ll find out what instrument Mars represents in this solar system-wide orchestra too.