While the rovers seem to get most of the attention, they’re just one part of a suite of instruments we’re using to understand the history and geology of Mars. We have an orbiting telescope pointed down toward its surface and an orbiting atmospheric observatory trying to help us understand why Mars is so sparse. And, for nearly a year, we have had a seismograph, weather observatory, and magnetic sensor parked at Mars’ equator.
The InSight mission (from the bacronym “Interior exploration using Seismic Investigations, Geodesy, and Heat Transport”) is a stationary lander and contains a suite of instruments designed to give us a clear picture of Mars’ workings. It landed toward the end of 2018 and has had instruments in operation since early last year. Now, in a large series of papers, the teams behind the lander’s hardware have analyzed the first data to come back from InSight, which includes the first recordings of marsquakes, along with some details on the local magnetic field.
At the equator
InSight landed at a region of Mars called Elysium Planitia, a region sandwiched between the southern highlands and the second largest volcano on the planet, Elysium. Billions of years ago, that volcano left large rock deposits that spread across parts of Elysium Planitia. But to the east, there’s additional volcanic terrain that appears to have formed as little as 10 million years ago and terrain that’s associated with the flow of liquid water.
InSight is cradled in the middle of all this interesting history, in the bottom of a small crater that is old enough to have largely decayed. The lander has a variety of instruments to help us understand the geology of the site, including a magnetometer to read the planet’s magnetic field, a probe that will allow us to track the diffusion of heat in the upper layers of Mars’ soil, and the most sensitive seismograph sent to another planet. While there were seismographs sent to Mars on the Viking landers, they weren’t sensitive enough to pick up anything other than noise from changing temperatures on the Viking landers themselves.
InSight also has a weather station that may not seem directly relevant to the geology, but it turns out to have been rather important. A significant fraction of the seismic noise picked up by the other instrument was due to wind disturbing the surface of Mars or shifting the InSight lander itself. And the local magnetic field was also influenced by events in the atmosphere, which we may ultimately be able to correlate with things we can detect using the meteorological instruments.
Mars is thought to have once had a magnetic dynamo at its core, much like Earth’s. But it appears to have shut down very early in the Red Planet’s history, roughly four billion years ago. But any rock deposits that formed prior to that would have had magnetic materials that were aligned with the planetary magnetic field. These would preserve the original field’s orientation but be substantially weaker. Orbiters have detected this residual magnetic field, but InSight is giving us the opportunity to study a local field for the first time.
And, as it turns out, that has been extremely informative. The field strength at the lander’s location is an order of magnitude stronger than that predicted to be found there based on the data taken from orbit. Given that the orbital data has a resolution of about 150 kilometers, that means there are magnetic features on Mars that are smaller than that—and InSight happens to have landed in the middle of one. The field strength at the site is about 2 microTesla; for context, the Earth’s is usually at least 10 times that.
This means that, somewhere beneath the lander, there are rock deposits that are old enough to have formed back when Mars’ core was still producing a magnetic field. Based on the seismic data, these rocks have to be at least 200 meters deep and may be as much as 10km down.
InSight also detected daily variations in the magnetic field, induced by the activities of the solar wind interacting with Mars’ magnetic field. These include a series of pulses that occurred around local midnight, which may be the product of Mars’ magnetotail, which trails off the side of the planet away from the Sun. Another set occurred in late afternoon and may represent variations in the altitude at which Mars’ magnetic field pushes back against the solar wind. Layered on top of these are daily variations in the strength of the magnetic field and a 26-day cycle associated with the rotation of the Sun. With more time on the surface, scientists expect that an annual pattern (based on a Mars year, not an Earth year) will gradually emerge.
The quakes of Mars
But the real star of the InSight show at this point is the first seismographic data from Mars, including clear evidence of about 175 marsquakes. As mentioned above, there’s a background of noise caused by the local winds pushing against the rocks and the lander itself. These followed daily patterns (the instrument was extremely quiet at night) and included a sandstorm moving over the landing site and could easily be correlated with the readings from the weather instruments on InSight. The marsquakes stood out from this noise as distinct events, and over 20 of them reached a moment magnitude of three or higher. None were above magnitude four, however.
The researchers could divide the quakes into two types of events based on whether they were dominated by high- or low-frequency waves. The high-frequency wave events were smaller in magnitude and entirely limited to the crust. These were probably caused by relatively local events and so don’t tell us much about Mars’ structure as a whole.
But the low-frequency events reached the landing site by traveling through Mars’ interior and thus carry information about what’s underneath the Red Planet. In general, the shape of these events is similar to that of moonquakes. They seem to fade with distance while traveling through the mantle before reaching the local crust. That crust slows down the waves dramatically compared to the Earth’s crust, suggesting that the upper 10km or so is highly altered, with lots of local fractures and unconsolidated material. The layer of regolith—unconsolidated dust and sand—at the landing site was thin enough to have a minimal effect on the seismic waves.
While it’s difficult to figure out where the quakes originate, given that we only have one instrument on Mars, difference in the timing of different waves can provide some hints as to the source. While there are large uncertainties, some of the largest quakes detected appear to come from the nearby Cerberus Fossae region, the site of relatively recent volcanic activity. Which raises the prospect that this area is seismically active.
The researchers are continuing to examine other possible reasons for the quakes, such as orbital or thermal effects due to the changes in the planet’s tilt over the seasons. But, given the frequency and magnitude of these events, these effects would have to be rather large to produce all of Mars’ seismic activity.
All the instruments
Part of the answer to the question of seasonal warming may come from InSight itself. Missing from this series of papers is data from an instrument designed to measure heat flow through the Martian surface. Anything driven by seasonal warming will have to make its presence felt by warming the surface, so we should have a decent idea of the energy available to drive internal events based on that data.
Which emphasizes something that’s made extremely clear by all of the papers describing these results: we’re really benefitting from having a lot of hardware on Mars. Some of that should be credited to the people who put InSight together and realized that knowing the local weather was going to be important to understanding the seismic noise seen by the other instruments on the lander. But other key information came from orbital spacecraft like MAVEN and the Mars Reconnaissance Orbiter, which have monitored the atmosphere, magnetic fields, and surface geology of the site long before InSight landed there.
Without the information provided by this other hardware, it would be much, much harder to interpret the data we’re now receiving from Mars’ surface.
Source: Nature Geoscience, 2020. DOI: 10.1038/s41561-020-0544-y and papers referenced there. (About DOIs).
* This article was automatically syndicated and expanded from Ars Technica.
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