While the rovers seem to get the most attention, they’re just one part of a suite of tools we use to understand Mars’ history and geology. We have an orbiting telescope that faces the surface and an orbiting observatory that is trying to help us understand why Mars is so scarce. And for almost a year, we’ve parked a seismograph, weather observatory, and magnetic sensor on Mars’ equator.
The InSight mission (from the acronym “Interior Exploration Using Seismic Investigations, Geodesy, and Heat Transport”) is a stationary lander and contains a series of instruments designed to give us a clear picture of the workings of Mars. It landed towards the end of 2018 and has had instruments in use since early last year. Now, in a large series of papers, the teams behind the lander’s hardware have analyzed the first data returned from InSight, including the first images of Marsquakes, along with some details about the local magnetic field.
on the equator
InSight landed on an area of Mars called Elysium Planiti, a region sandwiched between the southern highlands and the second largest volcano in the world, Elysium. Billions of years ago, that volcano left behind large rock deposits that spread over parts of Elysium Planitia. But to the east, there is additional volcanic terrain that appears to have formed only 10 million years ago and terrain associated with the flow of liquid water.
InSight sits in the midst of all this interesting history, at the bottom of a small crater old enough to be largely destroyed. 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 allows us to measure the diffusion of heat in the upper layers of the Martian soil. and sent the most sensitive seismograph to another planet. Although seismographs were sent to Mars on the Viking landers, they were not sensitive enough to pick up anything other than the sound of changing temperatures on the Viking landers themselves.
InSight also has a weather station that may not seem directly relevant to geology, but it turns out to be quite important. A significant portion of the seismic sound picked up by the other instrument was due to wind disrupting Mars’ surface or displacing the InSight lander itself. And the local magnetic field was also affected by events in the atmosphere, which we can eventually correlate with things we can detect using the meteorological instruments.
It is thought that Mars once had a magnetic dynamo at its core, much like Earth’s. But it seems to have shut down very early in the history of the Red Planet, about four billion years ago. But any rock deposits that formed before that would have had magnetic materials aligned with the planetary magnetic field. These would retain the orientation of the original field, but be significantly weaker. Orbiters have detected this residual magnetic field, but InSight gives us the opportunity to study a local field for the first time.
And, as it turns out, that was extremely informative. The field strength at the lander’s location is an order of magnitude stronger than predicted from the orbital data. 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 just happened to land in the middle of one. The field strength at the site is about 2 microTesla; for context, that of the Earth is usually at least 10 times larger.
This means that somewhere under the lander there are rock deposits old enough to have formed when the core of Mars was still producing a magnetic field. Based on the seismic data, these rocks must be at least 200 meters deep and can be up to 10 km deep.
InSight also detected diurnal variations in the magnetic field caused by the activities of the solar wind interacting with Mars’ magnetic field. These include a series of pulses that happened around midnight, which may be the product of Mars’ magnetotail, which runs away from the sun on the planet’s side. Another set occurred in the late afternoon and may represent variations in the height at which Mars’ magnetic field pushes back against the solar wind. On top of that are diurnal variations in the strength of the magnetic field and a 26-day cycle related to the sun’s rotation. With more time on the surface, scientists expect an annual pattern (based on a Martian year, not an Earth year) to gradually emerge.
The earthquakes of Mars
But the real star of the InSight show right now is the first seismographic data from Mars, including clear evidence of about 175 Marsquakes. As mentioned above, there is a background of noise caused by the local wind 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 measurements from the weather instruments on InSight. The marsquakes distinguished themselves from this sound as separate events, and more than 20 of them reached a moment magnitude of three or more. However, none of them exceeded magnitude four.
The researchers were able to 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 confined entirely to the crust. These were probably caused by relatively local events and so don’t tell us much about the structure of Mars as a whole.
But the low-frequency events made it to the landing site by traveling through the interior of Mars to hold information about what lies beneath the Red Planet. In general, the shape of these events is similar to that of moonquakes. They appear to fade with distance as they travel through the mantle before reaching the local crust. That crust dramatically slows the waves compared to Earth’s crust, suggesting that the top 10km or so is heavily modified, with lots of localized fractures and unconsolidated material. The layer of regolith – unconsolidated dust and sand – at the landing site was thin enough to have minimal effect on the seismic waves.
While it’s difficult to figure out where the earthquakes are coming from, since we only have one instrument on Mars, the difference in the timing of different waves can provide some hints about their source. While there are significant uncertainties, some of the largest quakes detected appear to originate from the nearby Cerberus Fossae region, site of relatively recent volcanic activity. Which raises the prospect of this area being seismically active.
The researchers continue to investigate other possible reasons for the earthquakes, such as orbital or thermal effects due to the changes in the planet’s inclination over the seasons. But given the frequency and magnitude of these events, these effects would have to be quite large to produce all of Mars’ seismicity.
Part of the answer to the question of seasonal warming may come from InSight itself. This set of papers lacks data from an instrument designed to measure heat flow through the Martian surface. Anything powered by seasonal warming will have to make its presence felt by heating the surface, so we should have a good idea of the energy available to power internal events based on that data.
That highlights something that is made extremely clear by all the articles describing these results: we really benefit from having a lot of hardware on Mars. Some of that must be attributed to the people who put together InSight and realized that knowledge of the local weather would be important in understanding the seismic sound perceived by the other instruments on the lander. But other important information came from orbital spacecraft like MAVEN and the Mars Reconnaissance Orbiter, which tracked the site’s atmosphere, magnetic fields and surface geology long before InSight landed there.
Without the information provided by this other hardware, it would be much, much more difficult to interpret the data we are now receiving from the surface of Mars.
Natural Geosciences, 2020. DOI: 10.1038/s41561-020-0544-y and documents referenced there. (About DOIs).