Magnetic field at Martian surface ten times stronger than expected
Fluctuations in field provide clues about upper atmosphere
New data gleaned from the magnetic sensor aboard NASA’s InSight spacecraft is offering an unprecedented close-up of magnetic fields on Mars.
In a study published today in Nature Geoscience, scientists reveal that the magnetic field at the InSight landing site is ten times stronger than anticipated, and fluctuates over time-scales of seconds to days.
“One of the big unknowns from previous satellite missions was what the magnetization looked like over small areas,” said lead author Catherine Johnson, a professor at the University of British Columbia and senior scientist at the Planetary Science Institute. “By placing the first magnetic sensor at the surface, we have gained valuable new clues about the interior structure and upper atmosphere of Mars that will help us understand how it – and other planets like it – formed.”
Zooming in on magnetic fields
Before the InSight mission, the best estimates of Martian magnetic fields came from satellites orbiting high above the planet, and were averaged over large distances of more than 150 kilometres.
“The ground-level data give us a much more sensitive picture of magnetization over smaller areas, and where it’s coming from,” said Johnson. “In addition to showing that the magnetic field at the landing site was ten times stronger than the satellites anticipated, the data implied it was coming from nearby sources.”
Scientists have known that Mars had an ancient global magnetic field billions of years ago that magnetized rocks on the planet, before mysteriously switching off. Because most rocks at the surface are too young to have been magnetized by this ancient field, the team thinks it must be coming from deeper underground.
“We think it’s coming from much older rocks that are buried anywhere from a couple hundred feet to ten kilometres below ground,” said Johnson. “We wouldn’t have been able to deduce this without the magnetic data and the geology and seismic information InSight has provided.”
The team hopes that by combining these InSight results with satellite magnetic data and future studies of Martian rocks, they can identify exactly which rocks carry the magnetization and how old they are.
Day-night fluctuations and things that pulse in the dark
The magnetic sensor has also provided new clues about phenomena that occur high in the upper atmosphere and the space environment around Mars.
Just like Earth, Mars is exposed to solar wind, which is a stream of charged particles from the Sun that carries an interplanetary magnetic field (IMF) with it, and can cause disturbances like solar storms. But because Mars lacks a global magnetic field, it is less protected from solar weather.
“Because all of our previous observations of Mars have been from the top of its atmosphere or even higher altitudes, we didn’t know whether disturbances in solar wind would propagate to the surface,” said Johnson. “That’s an important thing to understand for future astronaut missions to Mars.”
The sensor captured fluctuations in the magnetic field between day and night and short, mysterious pulsations around midnight, confirming that events in and above the upper atmosphere can be detected at the surface.
The team believe that the day-night fluctuations arise from a combination of how the solar wind and IMF drape around the planet, and solar radiation charging the upper atmosphere and producing electrical currents, which in turn generate magnetic fields.
“What we’re getting is an indirect picture of the atmospheric properties of Mars – how charged it becomes and what currents are in the upper atmosphere,” said co-author Anna Mittelholz, a postdoctoral fellow at the University of British Columbia.
And the mysterious pulsations that mostly appear at midnight and last only a few minutes?
“We think these pulses are also related to the solar wind interaction with Mars, but we don’t yet know exactly what causes them,” said Johnson. “Whenever you get to make measurements for the first time, you find surprises and this is one of our ‘magnetic’ surprises.”
In the future, the InSight team wants to observe the surface magnetic field at the same time as the MAVEN orbiter passes over InSight, allowing them to compare data.
“The main function of the magnetic sensor was to weed out magnetic “noise,” both from the environment and the lander itself, for our seismic experiments, so this is all bonus information that directly supports the overarching goals of the mission,” said InSight principal investigator Bruce Banerdt of NASA’s Jet Propulsion Laboratory in Pasadena, California. “The time-varying fields, for example, will be very useful for future studies of the deep conductivity structure of Mars, which is related to its internal temperature.”
Seismic activity on Mars resembles that found in the Swabian Jura
The SEIS experiment on board NASA's InSight geophysical station recorded 174 seismic events up to the end of September 2019.
Weak earthquakes – magnitude less than three to four.
Accompanying measurements provide information about the local weather conditions.
In the coming weeks, the Mars 'Mole' is to be assisted more effectively by pressure from above applied with the robotic arm.
Focus: Space, exploration, planetary geophysics
Mars is a seismically active planet – quakes occur several times a day. Although they are not particularly strong, they are easily measurable during the quiet evening hours. This is one of many results of the evaluation of measurement data from the NASA InSight lander, which has been operating as a geophysical observatory on the surface of Mars since 2019. A series of six papers have now been published in the scientific journals Nature Geoscience and Nature Communications. Eight scientists from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have made contributions to these. The papers describe the weather and atmospheric dynamics at the landing site, its geological environment, the structure of the Martian crust and the nature and properties of the planetary surface.
The Seismic Experiment for Interior Structure (SEIS) instrument – a seismometer developed by an international consortium under the leadership of the French space agency CNES – recorded a total of 174 seismic events between February and September 2019. Twenty of these marsquakes had a magnitude of between three and four. Quakes of this intensity correspond to weak seismic activity of the kind that occurs repeatedly on Earth in the middle of continental plates, for example in Germany on the southern edge of the Swabian Jura hills. Although only one measurement station is available, models of wave propagation in the Martian soil have been used to determine the probable source of two of these quakes. It is located in the Cerberus Fossae region, a young volcanic area approximately 1700 kilometres east of the landing site.
"Due to the higher gravity, SEIS could only be tested to a limited extent on Earth. We are all very excited about how sensitive it actually is," says Martin Knapmeyer, a geophysicist at the DLR Institute of Planetary Research, who is involved in SEIS data evaluation. "The seismic activity observed on Mars so far is significantly stronger than that found on the Moon – which is what we expected. How much stronger it actually is and whether there are more powerful marsquakes than those of magnitude four will become clear as the mission continues." However, even now, important and fundamentally new conclusions can be drawn about the planet's internal structure: "Similar to the Moon, the crust seems to be heavily disrupted down to a depth of several kilometres. Nevertheless, the seismic signals are more similar to Earth than to the Moon, but we do not yet understand why. For example, much of the time we cannot identify the cause of the marsquakes. Here, we are breaking new scientific ground." The mission will continue at least until the end of 2020 and will continuously provide further data. "We have not detected any meteorite impacts yet. However, it was clear early on that only expect a very small number of impacts would be expected during the mission."
InSight takes the 'pulse' of the Red Planet
This is the first time that an experiment to record marsquakes has provided such data on a larger scale and over a longer period of time. After the Moon, Mars is only the second celestial body other than Earth on which natural quakes have been recorded. It is true that instruments for performing seismic measurements were also installed on the first landers to visit Mars – the legendary Viking 1 and 2 missions – which arrived there in July 1976. However, these instruments were located on the lander platforms and only provided 'noisy' results, which were not particularly meaningful due to the presence of interfering signals, particularly those caused by the wind.
Following its launch on 5 May 2018, InSight landed on 26 November of the same year in Elysium Planum, four-and-a-half degrees north of the equator and 2613 metres below the reference level on Mars. The InSight team named the landing site ‘Homestead hollow’. More precisely, the landing site is located in an old, shallow crater that is approximately 25 metres across. The crater is heavily eroded and filled with sand and dust. The more distant surroundings of InSight are not very interesting geologically, but that was exactly one of the most important criteria for the selection of the landing site. It needed to be flat and level – and have as few rocks and stones as possible. The entire region consists of lava flows that solidified two-and-a-half billion years ago and were subsequently broken down by meteorite impacts and weathering into what is referred to as 'regolith'. It is thought that there are no large boulders down to a depth of at least three metres.
Magnetic field surprise
InSight, a NASA Discovery-class mission, is the first purely geophysical observatory on another celestial body in the Solar System. Its primary objective is to study the composition and structure of Mars, its thermal evolution and current internal state, and current seismic activity. Forces and energies inside a planetary body 'control', to some extent, geological processes – the results of which are visible on the surface – such as volcanism and tectonic fractures in the rigid crust, over billions of years.
With SEIS and the DLR Heat Flow and Physical Properties Package (HP3) geothermal sensor system, together with a collection of supporting instruments (the Auxiliary Payload Sensor Suite (APSS) – consisting of a barometer, an anemometer, a magnetometer, and two cameras), the HP3 radiometer and the Rotation and Interior Structure Experiment (RISE), InSight takes the 'pulse' of the Red Planet, measuring irregularities in its daily rotation and recording atmospheric parameters and weather at the landing site. One surprising result has been the local detection of a magnetic field that is 10 times stronger than predicted using the results of observations from Mars' orbit. This magnetic field is generated by magnetised minerals in the rock. The magnetisation ultimately came from a planet-wide magnetic field from Mars' early history.
The 'moving' day of a seismometer on Mars
Before the turn of the year 2018/2019, the SEIS experiment was set down on the surface of Mars and, protected from wind and weather by its characteristic dome (resembling a 'Cheese Bell') and perfectly horizontally aligned by a levelling system developed at the Max Planck Institute for Solar System Research in Göttingen, started routine measurement operations in February 2019. The experiment is so sensitive that almost any small change at the landing site is recorded as a signal: Movements of the robot arm, gusts of wind, thermal 'stress' in the lander caused by temperature differences, or of course the vibrations of the hammering Mars Mole right beside it. For this reason, the daily weather patterns, in particular the activity of the wind and the extreme fluctuations in temperature in the day and night rhythm, as well as the vibrations caused by the hammering mechanism of the DLR experiment HP3 were analysed.
"We are dealing with much greater temperature differences at the landing site than those that occur on Earth," explains Nils Müller from the DLR Institute of Planetary Research, who has analysed thermal radiation from the surface using the HP3 radiometer experiment. "At midday, the Sun heats the fine sand on the surface to above zero degrees Celsius on most days, while the thin atmosphere remains 10 to 20 degrees Celsius colder. At night, however, temperature drops to minus 90 degrees Celsius or even lower."
During the day, the increase in temperature always results in a very characteristic weather pattern, with winds first freshening and then easing in the afternoon. The scientists have even identified traces of small tornadoes or 'dust devils', frequent phenomena in the Martian weather pattern, on the ground after their course was recorded from orbit by NASA's Mars Atmosphere and Volatile Evolution (MAVEN) orbiter. These dust devils can even raise the Martian soil a little, which is registered by the seismometer. This allows conclusions to be drawn about the properties of the upper layer of the surface material. At night, the weather calms down noticeably, so the best time window for recording distant marsquakes is in the first half of the night, because almost no atmosphere-induced noise interferes with the experiment.
HP3 delivers results and the Mars mole gets help from above
Measurements and observations performed by DLR's HP3 experiment have also been incorporated into the scientific inventory, including the radiometer data and the soil properties derived from the course of the experiment to date, with the hammering of the Mars Mole serving, among other things, as a seismic source for analysing the upper layer of the soil. However, it has not yet been possible to use the self-hammering thermal probe to penetrate deeper than 38 centimetres into the Martian soil there, with its unusual properties, even for Mars. In autumn 2019, the experiment seemed to be well on its way – the 'Mole' was given lateral support by the scoop on the robotic arm, which provided the friction necessary for driving into the subsurface. "After the Mole was almost completely in the Martian soil, it backed out again a small distance. Subsequently, with repeated lateral pressure from the robotic arm, it has moved a little deeper into the ground again with a recent slight backward movement," explains the Principal Investigator of the HP3 experiment – Tilman Spohn from the DLR Institute of Planetary Research. "In the coming weeks we want to help more effectively by applying pressure from above with the scoop on the robotic arm." For months, DLR researchers and numerous technicians and engineers at the Jet Propulsion Laboratory (JPL) have been working meticulously with the Mole on Mars and with simulations, models and tests on Earth to find a solution. In his P.I. blog, Tilman Spohn explains the current situation and the possibilities for moving deeper into the soil with the Mars Mole.
Banerdt, Smrekar et al. (2020) Initial results from the InSight mission on Mars, Nature Geoscience, in press, DOI : 10.1038/s41561-020-0544-y
Lognonné et al. (2020) Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data, Nature Geoscience, in press, DOI : 10.1038/s41561-020-0536-y
Giardini et al. (2020) The seismicity of Mars, Nature Geoscience, in press, DOI : 10.1038/s41561-020-0539-8
Banfield, Spiga et al. (2020) The atmosphere of Mars as observed by InSight, Nature Geoscience, in press, DOI : 10.1038/s41561-020-0534-0
Johnson et al. (2020) Crustal and time-varying magnetic fields at the InSight landing site on Mars, Nature Geoscience, in press, DOI : 10.1038/s41561-020-0537-x
Golombek et al. (2020) Geology of the InSight Landing Site on Mars, Nature Communications, in press, DOI : 10.1038/s41467-020-14679-1
InSight mole making slow progress into Martian surface
WASHINGTON — An instrument on NASA’s InSight Mars lander that has struggled for more than a year to make its way into the Martian surface is now making steady, but slow progress with the help of the lander’s robotic arm.
The Heat Flow and Physical Properties Package instrument on the InSight lander was to deploy a probe, or “mole,” into the surface of the planet, using a hammering mechanism to burrow as deep as five meters below the surface to measure the heat flow from the planet’s interior. The probe, though, got stuck shortly after it started burrowing in February 2019, getting no deeper than about 30 centimeters.
That approach is working so far. “The mole is going down by its hammering mechanism, but it is aided by the push of the scoop that balances the force of the recoil,” said Tilman Spohn, principal investigator for the instrument at the German space agency DLR, during a May 4 webinar about results from the mission that was part of the European Geosciences Union General Assembly, a conference that moved online because of the coronavirus pandemic.
However, the progress is slow because of the need to reposition the arm as the mole gets deeper. “That is a very tedious operation,” he said. “We can only go like 1.5 centimeters at a time before we have to readjust.”
Another issue is the angle at which the mole is penetrating into the surface. The mole was originally designed to go down vertically, but is now at an angle of nearly 30 degrees from the vertical. “It’s not something we like to see,” he said. If the mole is able to get completely below the surface, he expects that it will “rectify itself to some extent.”
The problems have given scientists some insight into the properties of the surface at InSight’s landing site. There is a “duricrust” about 20 centimeters thick, which he described as sand that has been cemented into place by salt. That duricrust didn’t provide enough friction to keep the mole from recoiling as it tried to hammer into the surface initially.
Another issue, he said, is that there is now a region of compacted sand created by the mole as it hammered in place without moving deeper. That will make it more difficult for the mole to penetrate into the surface, even with the assistance of the robotic arm.
While Spohn didn’t state how long the current effort to get the mole into the Martian surface would last, other project officials have suggested it may take a couple months. The latest effort had just started when Bruce Banerdt, principal investigator for the overall mission, gave a briefing at a meeting of NASA’s Mars Exploration Program Analysis Group April 17, noting that the lander’s other instruments, including its seismometer, were working well.
“We anticipate that we’ll have the mole down flush with the ground within another month or two months,” he said. By then, the arm will no longer be able to help push the mole further into the ground. “At that point, it’s either going to be able to go on its own or not.”
Mini-'Marsquakes' measured by InSight lander show effects of sun and wind
Kyushu University research based on NASA InSight Martian lander seismometer data reveals different sources of different types and frequencies of Martian "microtremors", showing the potential to use such data to image the Martian subsurface
Fukuoka, Japan - Compared with our own planet Earth, Mars might seem like a "dead" planet, but even there, the wind blows and the ground moves. On Earth, we study the ambient seismic noise rippling mainly due to ocean activity to peek underground at the structure of the Earth's interior. Can we do the same on Mars without ocean?
According to a new study by researchers at Kyushu University's International Institute for Carbon-Neutral Energy Research, we're closer than ever to achieving this goal.
The study, published in Geophysical Research Letters, is based on data collected by NASA's InSight ("Interior Exploration using Seismic Investigations, Geodesy and Heat Transport") Martian lander, which landed on Mars on November 26, 2018. The InSight lander placed a seismometer on the surface of Mars and its readings are transmitted back to Earth. Continuous seismic records collected between February and June 2019 revealed the existence of several hundred "marsquakes." Most were much weaker than the quakes typically felt on Earth, although some reached a magnitude of almost 4.
The data from these "microtremors" were analyzed to determine their directions of propagation and directional intensity. Study co-author Tatsunori Ikeda explains, "Our polarization analysis revealed that seismic waves of different frequencies and types showed different patterns of variation over the course of the Martian day. The temporal variations in low-frequency P-waves were related to distant changes in wind and solar irradiation, and the low-frequency Rayleigh waves were related to the wind direction in the region near the lander. Higher-frequency ambient noises were dominated by vibration of the lander itself. Thus, microtremors of different types and frequencies likely have different sources, and some are probably influenced by geological structures."
These important differences between the dominant sources of different types of Martian microtremors may help in efforts to identify geological structures in Mars's interior, as we inferred the lithological boundary beneath the seismometer from high frequency ambient noise.
A single seismometer is not yet enough to reconstruct images of the planet's interior, however. On Earth, data from networks of multiple seismometers must be used together for that purpose. But this analysis of the InSight lander's seismic data is an important step toward achieving that goal on Mars. According to senior author Takeshi Tsuji, "These results demonstrate the feasibility of ambient noise methods on Mars. Future seismic network projects will enable us to model and monitor the planet's interior geological structure, and may even contribute to resource exploration on Mars, such as for buried ice."
NASA's InSight Flexes Its Arm While Its 'Mole' Hits Pause
Now that the lander's robotic arm has helped the mole get underground, it will resume science activities that have been on hold.
NASA's InSight lander has been using its robotic arm to help the heat probe known as the "mole" burrow into Mars. The mission is providing the first look at the Red Planet's deep interior to reveal details about the formation of Mars and, ultimately, all rocky planets, including Earth.
Akin to a 16-inch-long (40-centimeter-long) pile driver, the self-hammering mole has experienced difficulty getting into the Martian soil since February 2019. It's mostly buried now, thanks to recent efforts to push down on the mole with the scoop on the end of the robotic arm. But whether it will be able to dig deep enough - at least 10 feet (3 meters) - to get an accurate temperature reading of the planet remains to be seen. Images taken by InSight during a Saturday, June 20, hammering session show bits of soil jostling within the scoop - possible evidence that the mole had begun bouncing in place, knocking the bottom of the scoop.
While the campaign to save the mole continues, the arm will be used to help carry out other science and engineering work. Here's what you can expect in the months ahead from the mission, which is led by NASA's Jet Propulsion Laboratory in Southern California.
What's next for the mole?
The mole is part of an instrument called the Heat Flow and Physical Properties Package, or HP3, that the German Aerospace Center (DLR) provided NASA. While the scoop on the end of InSight's arm has blocked the mole from backing out of its pit again, it also blocks the arm's camera from seeing the mole and the pit that has formed around it. Over the next few weeks, the team will move the arm out of the way to better assess how the soil and mole are interacting.
The mole needs friction from soil in order to burrow. Ironically, loose soil provides that friction as it collapses around the mole. But the soil beneath InSight has proven to be cement-like duricrust, with dirt granules that stick together. As a result, recoil from the mole's self-hammering action causes it to bounce in place. So the team's next moves may be to provide that friction by scraping or chopping nearby soil to move it into the pit it's in.
More thoughts about the mole's recent progress can be found on a blog written by HP3's principal investigator, Tilman Spohn of DLR.
What's next for InSight's arm?
InSight landed on Mars on Nov. 26, 2018. Its robotic arm subsequently set HP3, a seismometer and the seismometer's Wind and Thermal Shield on to the planet's surface. While the arm has been key to helping the mole, scientists and engineers are eager to use the arm's camera to pan over InSight's solar panels, something they haven't done since July 17, 2019.
It's the dusty season on Mars, and the panels are likely coated with a fine layer of reddish-brown particles. Estimating how much dust is on the solar panels will let engineers better understand InSight's daily power supply.
Scientists also want to resume using the arm to spot meteors streaking across the night sky, as they did earlier in the mission. Doing so could help them predict how often meteors strike this part of the planet. They could also cross-check to see whether data from InSight's seismometer reveals a meteor impact on Mars shortly afterward.
What's next for the seismometer?
InSight's seismometer, called the Seismic Experiment for Interior Structure (SEIS), detected its first marsquake nearly three months after starting its measurements in January 2019. By the fall of 2019, it was detecting a potential quake or two per day. While SEIS has detected more than 480 seismic signals overall, the rate has dropped to less than one per week.
This rate change is tied to seasonal variations of atmospheric turbulence, which creates noise that covers up the tiny quake signals. Despite the protective Wind and Thermal Shield, SEIS is sensitive enough that shaking from the wind hitting the shield can make quakes harder to isolate.
More About InSight
JPL manages InSight for NASA's Science Mission Directorate. InSight is part of NASA's Discovery Program, managed by the agency's Marshall Space Flight Center in Huntsville, Alabama. Lockheed Martin Space in Denver built the InSight spacecraft, including its cruise stage and lander, and supports spacecraft operations for the mission.
A number of European partners, including France's Centre National d'Études Spatiales (CNES) and the German Aerospace Center (DLR), are supporting the InSight mission. CNES provided the Seismic Experiment for Interior Structure (SEIS) instrument to NASA, with the principal investigator at IPGP (Institut de Physique du Globe de Paris). Significant contributions for SEIS came from IPGP; the Max Planck Institute for Solar System Research (MPS) in Germany; the Swiss Federal Institute of Technology (ETH Zurich) in Switzerland; Imperial College London and Oxford University in the United Kingdom; and JPL. DLR provided the Heat Flow and Physical Properties Package (HP3) instrument, with significant contributions from the Space Research Center (CBK) of the Polish Academy of Sciences and Astronika in Poland. Spain's Centro de Astrobiología (CAB) supplied the temperature and wind sensors.
NASA’s Insight Mars lander rescue operation makes progress saving ‘stuck’ probe
The scientists running NASA’s Insight Mars lander found themselves attempting a rescue operation for the mission when one of its instruments became stuck in the red planet’s regolith early last year. After a few assistance maneuvers using a robotic shovel-clad arm in the months since, it appears the team has finally reached a point where there’s light at the end of the dusty tunnel.
“Another short test has my self-hammering mole making gradual progress. Pressing down on the soil above has helped it dig a little further. We’ll do another of these moves soon,” the lander’s official Twitter page announced recently.
So what happened?
“Our leading theory was that the Mole did not move into the subsurface because the regolith did not provide enough friction to balance the recoil force of the Mole,” explained Tilman Spohn on DLR Blog, a site run by one of NASA’s European Insight partners, the German Aerospace Center.
The stuck instrument, named HP3 (Heat Flow and Physical Properties Package), is one of the three main experiment tools on board the lander. It functions as a 40 cm long self-hammering probe that digs straight down into the surface to measure heat flow from the Martian interior and its thermal state. When operating as designed, the thermal conductivity of the soil is measured every 50 cm up to a depth of 5 meters. NASA’s team refers to the device as a ‘Mole’ as a nod to its underground burrowing, and the hashtag ‘#savethemole’ has become its social media rally cry for supporters of the rescue mission.
Ironically, lack of friction to support planned digging is very similar to a key plot point in the movie The Core. The ship used by the main characters to dig to the center of the Earth was engineered to work in a high pressure environment, but in the film the core wasn’t very solid at all. As is the case with Insight, plenty of scientific thinking was employed to overcome the challenge, albeit a bit less dramatically.
A stuck heat probe isn’t Insight’s only issue of late, either. Weather sensors on board the lander stopped sending data back to Earth a couple of weeks ago, and NASA’s team suspects an electronics issue is to blame. The instruments are currently in safe mode and await a reset while scientists assess what data is available to troubleshoot the issue in the meantime.
You can watch an in-depth video about the HP3 instrument below:
Surprise on Mars
NASA’s InSight mission provides data from the surface of Mars. Its seismometer, equipped with electronics built at ETH Zurich, not only records marsquakes, but unexpectedly reacts to solar eclipses as well. When the Martian moon, Phobos moves directly in front of the sun, the instrument tips slightly to one side. This miniscule effect could aid researchers in determining the planet’s interior.
An observer standing on Mars would see the planet’s moon Phobos cross the sky from west to east every five hours. Its orbit passes between the sun and any given point on Mars about once each Earth year. Each time it does so, it causes from one to seven solar eclipses within the space of three days. One place where this happens is the site of NASA’s InSight lander, stationed in the Elysium Planitia region since November 2018. In other words, the phenomenon occurs much more frequently than on Earth, when our moon crosses in front of the sun. “However, the eclipses on Mars are shorter – they last just 30 seconds and are never total eclipses,” explains Simon Stähler, a seismologist at ETH Zurich’s Institute of Geophysics. Photos taken by NASA’s two Mars rovers, Opportunity and Curiosity, also show a sharp-edged lump against the backdrop of the sun.
Photographs are not the only way to observe these transits. “When Earth experiences a solar eclipse, instruments can detect a decline in temperature and rapid gusts of wind, as the atmosphere cools in one particular place and air rushes away from that spot,” Stähler explains. An analysis of the data from InSight should indicate whether similar effects are also detectable on Mars.
Waiting for 24 April 2020
In April 2019, the first series of solar eclipses were visible from InSight’s landing site, but only some of the data it recorded was saved. Initial indications from that data prompted Stähler and an international research team to prepare excitedly for the next series of eclipses, due on 24 April 2020. They published the findings from their observations in August in the journal Geophysical Research Letters.
As expected, InSight’s solar cells registered the transits. “When Phobos is in front of the sun, less sunlight reaches the solar cells, and these in turn produce less electricity,” Stähler explains. “The decline in light exposure caused by Phobos’s shadow can be measured.” Indeed, the amount of sunlight dipped during an eclipse by 30 percent. However, InSight’s weather instruments indicated no atmospheric changes, and the winds did not change as expected. Other instruments; however, delivered a surprise: both the seismometer and the magnetometer registered an effect.
Unusual signal from the seismometer
The signal from the magnetometer is most likely due to the decline in the solar cells’ electricity, as Anna Mittelholz, a recent addition to ETH Zurich’s Mars team, was able to show. “But we didn’t expect this seismometer reading; it’s an unusual signal,” Stähler says. Normally, the instrument – equipped with electronics built at ETH – would indicate quakes on the planet. So far the Marsquake Service, led by John Clinton and Domenico Giardini at ETH, has recorded about 40 conventional quakes, the strongest of which registered a magnitude of 3.8, as well as several hundred regional, shallow quakes.
What was surprising during the solar eclipse was that the seismometer tilted slightly in a particular direction. “This tilt is incredibly small,” Stähler notes. “Imagine a 5-franc coin; now, push two silver atoms under one edge. That’s the incline we’re talking about: 10-8.” As slight as this effect was, it was still unmistakable. “The most obvious explanation would be Phobos’s gravity, similar to how Earth’s moon causes the tides,” Stähler says, “but we quickly ruled this out.” If that were the explanation, then the seismometer signal would be present for a longer period of time and every five hours when Phobos makes its pass, not only during eclipses. Researchers determined the most likely cause of the tilt: “During an eclipse, the ground cools. It deforms unevenly, which tilts the instrument,” says Martin van Driel from the Seismology and Wave Physics research group.
As it happens, an infrared sensor did indeed measure a cooling of the ground on Mars of two degrees. Calculations revealed that in the 30 seconds of the eclipse, the “cold front” could penetrate the ground only to a depth of micro- or millimetres, but the effect was enough to tug at the seismometer.
Experiments in an old silver mine
An observation back on Earth supports Stähler’s theory. At the Black Forest Observatory, located in an abandoned silver mine in Germany, Rudolf Widmer-Schnidrig discovered a similar phenomenon: during a seismometer test, someone neglected to turn out the light. The heat given off by a 60-watt bulb was apparently enough to warm the topmost layer of granite deep below ground, so that it expanded slightly and caused the seismometer to tilt slightly to one side.
Scientists should be able to use the tiny tilt signal from Mars to map Phobos’ orbit with more precision than was previously possible. InSight’s position is the most accurately measured location on Mars; if the scientists know exactly when a transit by Phobos here begins and ends, they can calculate its orbit precisely. This is important for future space missions. For example, Japan’s space agency JAXA plans to send a probe to the moons of Mars in 2024 and bring samples from Phobos back to Earth. “To do that, they need to know exactly where they’re flying to,” says Stähler.
What precise orbit data reveals
Precise data on Phobos’s orbit could also shed more light on the inner workings of Mars. While our moon continues to gain angular momentum and is steadily moving away from Earth, Phobos is slowing down and gradually falling back to Mars. In 30 to 50 million years, it will crash onto the planet’s surface. “We can use this slight slowdown to estimate how elastic and thus how hot the Martian interior is; cold material is always more elastic than hot,” explains Amir Khan, also at ETH Zurich’s Institute of Geophysics. Ultimately, the researchers want to know if Mars was formed of the same material as Earth, or if different components could explain why Earth has plate tectonics, a dense atmosphere and conditions that support life – characteristics that Mars is lacking.