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Lander Offers New InSights on Martian Magnetic Field

Feb. 24, 2020

mars magnetic fields catherine johnson

Contributions to the Magnetic Field Measured by the InSight Fluxgate Magnetometer (IFG). Time varying fields are either of external origin (orange), including the interplanetary magnetic field, ionospheric currents and weather events such as dust devils; they can also be of lander origin (blue), e.g., due to movement of the arm, radio-science (RISE) communications, Solar Array Currents, or Martian temperature variations, measured by the temperature sensors on the lander. The Martian static crustal field (red) results from crustal magnetization, represented schematically here as subsurface dipoles. A steady field is also associated with the lander itself (green). Inset shows the IFG sensor box mounted under the lander deck.

 

Investigations by NASA’s InSight lander have found that the Martian magnetic field is 10 times stronger than expected, while experiencing unexpected pulses, a new paper by Planetary Science Institute Senior Scientist Catherine Johnson says. 

“We know from previous work based on satellite data that about 4 billion years ago Mars had a global magnetic field that is no longer present,” said Johnson, lead author on the paper “Crustal and Time-Varying Magnetic Fields at the InSight Landing Site on Mars” that appears in Nature Geoscience. 

This field magnetized ancient rocks that can still be found on the surface, though most are now between 200 feet (61 meters) to several miles below ground. InSight has found that the magnetic field at Homestead hollow is 10 times stronger than expected based on data from orbiting spacecraft. Those previous measurements were averaged over a couple of hundred kilometers; InSight's ground-level sensor — the first on Mars — provides a view of the local magnetic field. 

“We can’t learn more about the timing of that magnetic field from InSight alone, but we have discovered that those ancient magnetized rocks, that satellite data tell us are mainly seen in the southern hemisphere of Mars, are also present beneath the InSight landing site called Homestead hollow,” Johnson said. 

Because most rocks at the surface at InSight's location are too young to have been magnetized by the planet's former field, "this magnetism must be coming from ancient rocks underground," Johnson said. "We're combining these data with what we know from seismology and geology to understand the magnetized layers below InSight. How strong or deep would they have to be for us to detect this field?" 

The magnetic field also pulses mysteriously, typically around midnight on Mars. It's likely these pulses originate in space above Mars. Before InSight, scientists weren't sure whether weak signals from space could be detected at the Martian surface. In the future, the InSight team wants to observe the surface magnet field at the same time that NASA’s MAVEN orbiter passes over InSight, several hundred kilometers above the surface, allowing the team to compare data. 

If a rocky planet or moon has a global magnetic field like on Earth either today or sometime in the past, it tells us about the planet’s deep interior at that time. In particular that the deep iron metallic region or core has to be hot enough – thousands of degrees – to be at least partly liquid. 

“So if, when, and for how long a planet had a global magnetic field tells us about the history and composition of the planet – how it has cooled over time, and whether the core is pure iron or iron with elements we find in rocks like sulfur, or oxygen, in it,” Johnson said. “Big open questions for Mars are if, and how, the changes in climate over billions of years and the presence or absence of a global magnetic field are linked.” 

There are some similarities between what is seen at Homestead hollow on Mars and on Earth. “If you measure the magnetic field somewhere like Hawaii it will mostly look like the Earth’s global magnetic field now – pointing toward the north magnetic pole,” Johnson said. “But there will also be a contribution due magnetization of the volcanic rocks that make up the islands. The rocks were magnetized when they erupted and cooled a few million years ago. They are essentially a tape recorder of the Earth’s magnetic field history, like the rocks buried beneath InSight.” 

InSight – Interior Exploration using Seismic Investigations, Geodesy and Heat Transport – is a Mars lander designed to give the Red Planet its first thorough checkup since it formed 4.5 billion years ago. It is the first outer space robotic explorer to study in-depth the "inner space" of Mars: its crust, mantle, and core.

 

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SCIENCE CONTACT:

Catherine Johnson

Senior Scientist

cjohnson [at] psi.edu 

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