The life of the Mars InSight lander came to an end last year as its solar panels were covered with dust and its power supply slowly dwindled away. After four years of research and data collection, NASA officially declared the end of the mission in December 2022.
Insight leaves an incredible legacy though, having provided the best-ever look at the interior of Mars and being the first time a seismometer has been used on another planet. To learn about how a single little lander was able to look through an entire planet, we spoke to two leading scientists on the InSight team, Catherine Johnson of the University of British Columbia and Mark Panning of NASA’s Jet Propulsion Laboratory.
Why study the interior of Mars?
Learning about the interior of a planet like Mars isn’t just an abstract curiosity. Indeed, to understand topics from a planet’s atmosphere to its history, we need to understand its interior. “If you want to understand how Mars has evolved — Why did it have a different atmosphere in the past from today? Why did it have different amounts of volcanism in the past from more geologically recently? — you have to understand the interior structure,” Johnson said.
“Each mission results in a big leapfrog forward in our understanding.”
But it’s hard enough to analyze samples from Mars’s surface using rovers — how can you possibly tell what the interior of a planet located hundreds of millions of miles away is like? Fortunately, we have an idea of how to do this because we have practice from studying Earth. We learn about Earth’s interior structure by looking at how seismic waves travel through the planet, and we can do the same thing on Mars.
That was the justification for including a seismometer instrument on the InSight lander, which was the first seismometer ever landed on another planet. And its results have been, if you’ll excuse the pun, out of this world.
Shaken by quakes
The Earth has tectonic plates that shift and move over millions of years, causing earthquakes when they rub together. Mars doesn’t have plate tectonics today, but it is shaken by similar quakes called marsquakes. That means the right instrument can study those quakes and the way they bounce around the planet’s interior to learn more about its structure.
To detect tiny shakes from the surface, you don’t want a rover that moves around. Instead, you want something that stays in exactly the same place for months or years, which is why InSight is a stationary lander. It’s also located in a very quiet region.
When it comes to choosing a landing spot for seismic measurements, “You basically want somewhere that’s as boring as possible,” Johnson said. “and as quiet as possible, because you’re making these incredibly sensitive measurements.”
To allow for environmental differences day to day, InSight also measures factors like temperature, pressure, and wind speed using weather sensors, so these factors can be subtracted from the seismic data. As a bonus, this means InSight is also a tiny little Martian weather station, and has provided years of data on the weather in the Elysium Planitia region where it is located.
How to measure a marsquake
Seismometers are a fairly basic piece of equipment, and given how much experience we have using them on Earth, adjusting them for Mars is conceptually simple. The designers needed to adjust for the different level of gravity, and the instrument needed to be extremely sensitive to pick up small shakes. But that was the easy part.
The hard part was how to get by using only one of them. When you were at school, you might have learned that measuring an earthquake requires three stations so you can triangulate its origin. But there was only going to be one lander on Mars, and it was going to have to collect all the required data, all on its own.
“There are ways to locate earthquakes with a single station,” Panning said, but it isn’t typically done this way. So figuring out how effective this approach would be on Mars was a big part of getting the mission accepted. “We spent a lot of time talking about how well we could locate events with just a single station.”
It wasn’t easy to convince people that InSight would be able to detect useful information all on its own, especially considering that a seismometer had never been placed on another planet. But in its time since landing on Mars in 2018, it was able to record hundreds of seismic events. “History has borne us out,” on that front, Panning said. “We’ve been able to locate a lot of quakes.”
Understanding seismic waves
To understand how InSight works, you need to understand seismic waves. There are two types of seismic waves, called P and S. As waves pass through the planet, they can move in different ways: With P waves, the material moves backward and forward in the same direction that the wave is moving. With S waves, the material moves side to side compared to the direction of the wave.
Think about a slinky. You can push waves along the length of the slinky, which would be equivalent to a P wave, or you can wobble the slinky from side to side, which would be the equivalent of an S wave.
Once you can locate the source of a quake, you can use this information to learn about the planet’s interior.
InSight could detect both of these types of waves and use them to detect the source of a marsquake. To find out how far away a quake originated, you can look at the time at which the seismic waves arrived at the lander, as the two types of waves travel at different speeds. The separation in time between the arrival of the P waves and the S waves gives you the distance.
Working out the location of the source is a bit more complicated. The process uses a property called the polarization of the seismic waves, which refers to the direction of motion within the wave. “So if a P wave is coming in from the east, for example, its particle motions are going to be moving in the east-west direction. They’re not going to be going north-south,” Panning explained.
You can use that polarization to work out the direction which a wave is coming from. “So if we know how far away the quake is from the timing of the P and S, and we know which direction it came in from the polarization of the waves, that gives you a location,” Panning said.
Peering into the interior
Once you can locate the source of a quake, you can use this information to learn about the planet’s interior. We know that the interior structure of Mars is made up of layers, consisting of a molten core, a mantle, and a crust. But before InSight, we didn’t have a good understanding of how thick each layer was.
As abstract as it may sound, understanding the deep interior of the planet is vital for understanding all sorts of issues from the planet’s history to its state today. “How the planet has cooled and what has happened to it, could it have had a magnetic field in the past, could it have one today – those kinds of questions are critically dependent on the deep interior,” Johnson said.
So InSight took data from marsquakes to measure the depth of the layers. As each layer has different material properties, each one interacts with seismic waves in different waves. And this is what allows researchers to work out the thickness and properties of each layer.
To study the crust, you use a technique called receiver functions. When a P wave hits a boundary, like the edge of the crust, some of it will convert to an S wave. Then you can see this converted S wave energy arriving a little later than the P wave, and that can tell you how thick the crust is.
To study the core, which is molten, you look for the energy that bounces off the boundary between the core and the mantle. A big quake can cause an S wave which hits this boundary, reflects off, and bounces back to the receiver. You can look for incoming waves that have the right polarization to be identifiable as this particular type of wave — called an ScS wave — and this lets you work out the radius of the core.
To study the mantle, researchers wanted to know how quickly waves pass through this layer which lets them know about the mantle’s temperature. For this, you look for waves that have bounced off the planet’s surface, called PP waves. You can see these reflections arrive at your receiver later than the original P waves, which tells you how fast the waves are traveling.
Looking to the future
This is how InSight was able to gather the most accurate information yet on the Martian interior, finding different sub-layers within the crust and pinpointing the size of the core. This is a big step in understanding the planet and was achieved within just a few years of InSight operations. This is the legacy that InSight will leave for Mars science.
“Each mission results in a big leapfrog forward in our understanding — in this case, it’s a big leap in our understanding of the interior of Mars and the surface environment of the lander,” Johnson said.
The greater understanding of the interior of Mars that InSight has provided will support future missions, from the planned Mars Sample Return mission to long-term plans for astronauts to eventually visit the red planet in person. And results from the lander are still being used to make discoveries, such as a recent finding that Mars is spinning faster each year.
So even though the InSight mission has come to a close, Johnson and her team are optimistic about what the mission has achieved and what the future holds for Mars science.
“Any mission is this incredible journey,” Johnson said. “It’s always a sad moment when something comes to an end. But you also spend a lot of time thinking about how the mission has enabled the next step of investigations.”
