Johnson, and 23 co-authors, reported in 2020 that magnetic fields provide a window into a planet’s interior structure and evolution, including its atmospheric and space environments. Satellites at Mars have measured crustal magnetic fields indicating an ancient dynamo. These crustal fields interact with the solar wind to generate transient fields and electric currents in Mars’s upper atmosphere. Surface magnetic field data play a key role in understanding these effects and the dynamo. Here the authors report measurements of magnetic field strength and direction at the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) landing site on Mars.
The authors found that the field is ten times stronger than predicted by satellite-based models. They infered that magnetized rocks beneath the surface, within ~150 km of the landing site, consistent with a past dynamo with Earth-like strength. Geological mapping and InSight seismic data suggest that much or all of the magnetization sources are carried in basement rocks, which are at least 3.9 billion years old and are overlain by between 200 m and ~10 km of lava flows and modified ancient terrain. Daily variations in the magnetic field indicate contributions from ionospheric currents at 120 km to 180 km altitude. Higher-frequency variations are also observed; their origin is unknown, but they probably propagate from even higher altitudes to the surface. They propose that the time-varying fields can be used to investigate the electrical conductivity structure of the martian interior.
The authors concluded that magnetic fields caused by temperature changes, solar array currents and lander activity need to also be estimated and removed because they exhibit diurnal and seasonal variations distinct from time-varying magnetic fields of martian origin. These corrections are more challenging because of the lower or intermittent sampling rates of auxiliary data (cf. IFG vector data), and they are described in detail in ref. 40. However, empirical corrections for both temperature and diurnal variations in solar array currents suggest that these diurnal variations are known to within a few nT to ~15nT (S. Joy, personal communication).
Signals due to the solar array currents are currently characterized empirically and likely contribute the largest uncertainty, in particular when these currents are changing rapidly in the early morning hours. The short-period continuous pulsations are easily seen in the data, and these observations, combined with the clear lander-induced signals at very short periods (Extended Data Fig. 5), indicate that signals with timescales of seconds to a few minutes of 1nT can be robustly identified. The authors noted that a timing offset of ~73min was discovered in the temperature calibration applied to the data in the June Planetary Data System data release. This has been corrected in the data shown here and in subsequent Planetary Data System releases. Aperiodic signals are challenging to investigate because of spacecraft activities. Many transient signals of different types are associated with lander operations and are often up to 10nT in amplitude (Extended Data Fig. 4). At frequencies above 0.2Hz, IFG data show considerable noise that appears to be correlated with increased high-frequency variability in the solar array currents between 10:00 and 16:00 Mars local solar time. Thus, detecting small-amplitude (a few nT) transient.signals during the day is challenging.
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