Before the InSight Lander arrived on Mars, scientists could only estimate what the planet’s internal structure might be. Its size, mass, and moment of inertia were their main clues. Meteorites, orbiters, and in-situ sampling by rovers provided other clues.
But when InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) arrived on Mars in November 2018 and deployed its seismometer, better data started streaming in.
One of the most compelling questions about Mars is “what happened to its magnetosphere?” Earth’s magnetosphere keeps our planet habitable and has for billions of years. Our planet’s rotating, convective outer core is the dynamo responsible for the magnetosphere. Evidence shows that Mars used to be habitable, so it must have had a magnetosphere and a liquid rotating core. Magnetization in Martian rocks tells us that. But what happened to it?
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To answer that, we need better data on Mars’ interior, and NASA and the DLR sent InSight there to get us the data. Its three primary instruments are SEIS (Seismic Experiment for Interior Structure,) HP3 (Heat Flow and Physical Properties Package,) and RISE (Rotation and Interior Structure Experiment.) The most ambitious of the three instruments was HP3, which needed to burrow into the ground to gather its data. Sadly, HP3 failed, but the other instruments are still working.
SEIS plays a significant role in a new study examining marsquakes. It’s a seismometer, and it measures marsquakes, meteorite impacts, and other internal activity by monitoring seismic waves.
The new study is “Repetitive marsquakes in Martian upper mantle,” The lead author is Weijia Sun, from the Institute of Geology and Geophysics at the Chinese Academy of Sciences. The co-author is Professor Hrvoje Tkalcic from the ANU Research School of Earth Sciences. The study is in the journal Nature Communications.
Most of what we know about Mars’ internal structure comes from physical parameters provided by astronomical and orbital measurements. NASA’s Viking Landers carried seismometers, but they were largely ineffective. They weren’t deployed directly on the ground. Instead, they remained on the lander decks, and the Martian wind degraded their data. Viking 1’s seismometer failed, but Viking 2 did detect one marsquake, and that data limited Mars’ seismicity to levels much lower than the Earth’s.
InSight’s SEIS instrument is a huge improvement on the Viking Landers’ seismometers, but it has its limitations. Marsquakes have relatively small magnitudes compared to Earthquakes, so their seismic waves can be scattered or lost in the noise. This leads to uncertainties, especially when it comes to their exact locations. And since InSight’s SEIS instrument is the only recording station, it’s difficult to pin down each quake’s physical cause. That makes it hard to determine the nature of Mars’ deep interior, and it’s challenging to come to any inferences or conclusions on the activity in Mars’ mantle.
The researchers wanted to dig deeper into the SEIS data in this study. They thought there might be more marsquakes hidden in the data, which both automated algorithms and manual searches missed. “Therefore,” they write, “there is the need to complement the existing searches with dedicated searches for potential, smaller marsquakes buried in noisy waveforms.”
Interest in marsquakes is focused on Mars’ Cerberus Fossae region. Cerberus Fossae is a pair of almost parallel faults on Mars over 1,000 km (620 miles) long. They’re geologically young and formed only several million years ago—probably less than 20 million years ago. InSight has detected four strong, clear quakes originating in the region. The authors of this study used their seismic characteristics to help them filter through InSight data looking for more quakes.
The researchers found 47 more marsquakes in existing InSight data that scientist hadn’t identified before. The marsquakes were under the Cerberus Fossae region.
More than 90% of the 47 new marsquakes are associated with two previously-known quakes. These two quakes are especially important in Martian seismic studies because they’re high-quality detections. They’re the strongest and clearest marsquakes yet detected, and two of them—called S0173a and S0235b— were especially valuable detections, showing clear onsets and polarities.
The 47 marsquakes were repetitive, and all occurred in the Cerberus Fossae area. According to Professor Hrvoje Tkalcic, the quakes suggest that Mars is more seismically active than thought. “We found that these marsquakes repeatedly occurred at all times of the Martian day, whereas marsquakes detected and reported by NASA in the past appeared to have occurred only during the dead of night when the planet is quieter,” Professor Tkalcic said. This is an important point because many marsquakes seem to be caused by the tidal modulation of Mars’ moon, Phobos. They’re very slight and easily hidden in ambient noise.
But marsquakes that occur at all times of the day can have other causes, and as far as Tkalcic and Sun are concerned, the reason must be the convection of molten material in the upper mantle under Cerberus Fossae. The material is sandwiched between the crust and the core. This goes against previous studies showing that tectonic activity is causing the marsquakes under Cerberus Fossae.
“Therefore, we can assume that the movement of molten rock in the Martian mantle is the trigger for these 47 newly detected marsquakes beneath the Cerberus Fossae region,” Tkalcic said.
These quakes are enough for Tkalcic to conclude that Cerberus Fossae is a highly seismically active region and that the Martian mantle is mobile. That conclusion is essential to our understanding of Mars.
“The number of marsquakes recorded during both the day and night after the InSight landing suggests that Mars’ interior is in motion and that the Martian seismicity is continuous and long-term,” the authors conclude. “The frequency and magnitudes of the newly observed marsquakes indicate that the Martian mantle might be more dynamic than anticipated…”
Their conclusion isn’t rock-solid, and the researchers acknowledge that. But when they compare the seismic activity on Mars with similar activity on Earth, the conclusion almost reaches itself. “Although we cannot rule out the tectonic causes, the repetitive nature of marsquakes has its equivalence in repetitive tremors in magma transfer systems on Earth,” they write.
It all boils down to convection. On Earth, convection currents in the molten iron in Earth’s outer core help create our planet’s magnetosphere. The convection moves the molten iron around and creates electrical currents, which generate the magnetosphere. When it comes to Mars, scientists have wondered if its interior had cooled and prevented convection and electrical currents. This may have occurred billions of years ago, ending Mars’ possible period of habitability.
But if the mantle is still active, that complicates the picture.
“Knowing that the Martian mantle is still active is crucial to our understanding of how Mars evolved as a planet,” Tkalcic said. “It can help us answer fundamental questions about the solar system and the state of Mars’ core, mantle and the evolution of its currently-lacking magnetic field.”
“The marsquakes indirectly help us understand whether convection is occurring inside of the planet’s interior, and if this convection is happening, which it looks like it is based on our findings, then there must be another mechanism at play that is preventing a magnetic field from developing on Mars,” Tkalcic said.
This study isn’t the only indication that Mars still has some mantle mobility and probably a liquid core. But the details are still obscured. Professor Tkalcic told Universe Today that “However, we don’t know how thick that liquid part of its core might be – i.e. we don’t know yet for sure if Mars harbours a solid inner core in its centre.” In other words, we don’t know “… how much of its liquid core has been solidified so far or consumed by the solid phase (which grows from the Martian centre outwards). If the liquid layer is too thin, it won’t generate/sustain a sizeable dynamo.”
That’s the case on Mars, where what’s left of the liquid core can’t generate much of a magnetic field. Most of Mar’s magnetism is remnant crustal magnetism. But the main mystery remains: what happened to the core? All indications are that Mars had a larger liquid core in the past, and that it generated a magnetosphere that protected the planet’s atmosphere, keeping the planet warm, wet, and maybe habitable, for a long period of time. Perhaps long enough for simple life to flourish.
The answer might lie in the makeup of the core, and the influence of light elements like sulphur and hydrogen.
“There is an alternative explanation, which is a hypothesis at this stage that needs to be scrutinised,” Professor Tkalcic told Universe Today. “If during the differentiation some light elements like hydrogen and sulphur remained in the Martian core in some unfortunate ratio, they could have formed an immiscible solution with iron at those temperatures and pressures.” Immiscible means that the mixture is not homogeneous and the elements aren’t dissolved together.
Immiscible solutions tend to separate, and in a planet’s core, that means some of the material crystallizes and the solution becomes stratified. And that’s bad for convection.
“Thus, instead of sustaining convection, the liquid would preferentially separate to the Fe-H and Fe-S volumes, and this stratification would basically stop convection, which would have had catastrophic consequences for Mars, its atmosphere and potential life on its surface,” Professor Tkalcic told Universe Today.
Clearly the same thing didn’t happen on Earth. But why? Does it come down to size, and Earth’s core is simply taking longer to cool and remaining convective? “Luckily, this did not happen on Earth, where the pressure/temperature conditions were different in its centre due to its size, but also because the differentiation process was likely different,” Tkalcic told UT.
Or was Earth lucky in some way? No two planetary history’s follow the same path.
According to Tkalcic, Earth’s larger size might have played a role, and a titanic collision early in Earth’s history might have also played a role.
“For example, if a sizeable body like Theia collided with the Earth to eventually give birth to the Moon, this could have stirred the internal structure of the proto-earth and the differentiation consequently had two or multiple stages,” Professor Tkalcic told Universe Today. “All this could have resulted in the outer core conditions that sustained geodynamo until its present day.”
Complicating the whole picture is the possibility that Mars may have suffered its own giant impact that affected its core. There’s some evidence that may have happened. Mars bears the scars of at least five giant impacts. The largest and most ancient one is the Borealis basin. It’s almost 10,000 km (6,000 miles) wide and covers most of Mars’ northern hemisphere. Could that have affected Mars’ liquid core?
Some research suggests that it could have. A 2013 study said, “A giant impact occurring within the first 500 Myr of martian history may have … influenced the initiation or cessation of early and short-lived core dynamo.” So giant impacts could either influence a core’s longevity, or limit it.
“All life on Earth is possible because of the Earth’s magnetic field and its ability to shield us from cosmic radiation, so without a magnetic field, life as we know it simply wouldn’t be possible,” Tkalcic said.
Whatever the exact cause, Mars is dead and Earth is alive. But humanity has cast its restless eyes on Mars, and we’re driven to understand the planet. There’s something awesome and compelling about having a neighbour that may have harboured life in the past but then died. There are so many fascinating and unanswered questions. And if we intend to somehow live on Mars in the distant future, for reasons we might not clearly see in the present, understanding Mars’ history could be critical.
“Therefore, understanding Mars’ magnetic field, how it evolved, and at which stage of the planet’s history it stopped is obviously important for future missions and is critical if scientists one day hope to establish human life on Mars,” Professor Tkalcic said.
Leading Image Credit: S Cottaar, P Koelemeijer, J. Winterbourne and NASA.
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