The surface magnetic field environment from InSight

Anna Mittelholz,Catherine L. Johnson,Matthew O. Fillingim, Steve Joy,Benoit Langlais, Shea N. Thorne,Mark Wieczorek,Sue Smrekar,W. Bruce Banerdt

crossref（2022）

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摘要

InSight landed on Mars in November 20181 and carries the InSight FluxGate Magnetometer, IFG2 which has provided the first surface magnetic field measurements on Mars1,3. Previous magnetic field measurements taken at orbital altitudes have provided global coverage, with limited spatial resolution. Laboratory analyses of meteorites provide information on magnetic properties of martian rocks, but without detailed local context for their provenances4. Advances from the IFG are thus unique and complementary science, specifically characterizing the crustal ambient static and external time-varying fields at a single location on Mars. External fields provide information on the planet’s interaction with the interplanetary magnetic field and the ionosphere. Crustal magnetic fields carry information about the ancient dynamo and crustal conditions at the time at which magnetization was acquired, and on how the crust has been modified by subsequent exogenic and endogenic processes4. IFG data for sols 14-736 were collected almost continuously, with some data gaps from electronics anomalies. After sol 736, the magnetometer was operational for shorter periods due to power constraints (Fig. 1) 5. A range of studies have been enabled by IFG data, supported by results from other instruments such as the seismometer, and we summarize those.Strong crustal fields provide evidence for an ancient dynamo. The IFG measured a surface magnetic field strength of ~2000 nT, ~10x stronger than predicted from satellite data3,6 and consistent with an ancient Earth-like dynamo3. The strong surface field indicates that magnetization at wavelengths shorter than those resolvable from current satellite data (~150 km) contribute substantially to the overall magnetic field. Characterization of the crust through seismic measurements7 and geologic inferences3 of subsurface layering allow assessment of magnetization of the crust (Fig. 2). Depending on the depth at which the magnetization is carried, specifically whether it is in the seismically-determined deep layer of Noachian origin or also in the shallow Hesperian-aged crust, the minimum magnetization required to explain the surface field is ~2 A/m or ~0.4 A/m. Seismic characterization of crustal structure8 indicates a deep subsurface layer (> 20 km, Fig. 2) of no porosity, while the upper crust (<20 km, Fig. 2) is less porous8. Magnetization of these layers require an early active dynamo (>~4 Ga).  Fractured, less porous material could have provided pathways for hydrothermal circulation and chemical remanent magnetization4,8,10. Magnetization of the most surficial layer of Hesperian age would be consistent with a long-lasting (up to ~3.7 Ga) dynamo9.  IFG data also reveal time-varying fields at the planetary surface that include contributions with different periods and origins. External fields have been observed and characterized from orbit11–13. However, the degree to which external fields penetrate to and interact with the surface could not be studied prior to the InSight landing. Static and long-duration observations from a surface magnetometer are advantageous because, unlike satellite measurements, temporal variability in the field is not mixed with spatial variability. Here, we summarize different external magnetic field phenomena, transient and periodic that have been observed in IFG data (Fig. 3). Periodic variations include short period waves (100s-1000s3,14), diurnal variations15, the ~26 sol Carrington period associated with solar rotation16,  and seasonal15,17 fluctuations. Transient events are observed in response to space weather18 and dust movement19,20.The inclusion of the magnetometer on InSight has provided unique and substantial scientific contributions to the overall mission results, as well as a starting point for future planetary surface magnetic field investigations. To overcome limitations of current data sets, we look forward to Mars sample return, as well as possible near-surface investigations. Including magnetometers on future missions at a variety of surface locations for long duration observations will be of great value in understanding a range of external field phenomena, including the influence of crustal magnetic fields on ionospheric currents and the effects of space weather during different phases of a solar cycle. We further advocate for regional investigations for example via a helicopter20 that can provide local magnetic field measurements at a spatial scale commensurate with detailed geological knowledge, to further constrain evolution of Mars’ ancient dynamo and explore the magnetic properties of the crust. <img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.efa5905fb48266986682561/sdaolpUECMynit/2202CSPE&app=m&a=0&c=aadd4ac595c9efb226f8701a8b59b97d&ct=x&pn=gnp.elif&d=1" alt="" width="928" height="716">Figure 1: a) Martian years 1 (blue) and 2 (red) of the magnetic field amplitude, B, versus solar longitude (ls). All data up to sol 1106 of InSight operations are included (PDS release 13). The blue vertical dashed line marks the beginning of the mission. (b) Corresponding sol numbers. <img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.52cfec5fb48268096682561/sdaolpUECMynit/2202CSPE&app=m&a=0&c=2685cbe79396fe714178815a7051c88e&ct=x&pn=gnp.elif&d=1" alt="" width="853" height="557">Figure 2: The minimum magnetization required by B=2013 nT (within its 99% confidence intervals)21 for the crust below InSight8. Burial depth describes the depth extent of the unmagnetized layers above the top of the magnetized layer. A burial depth of 200 m (blue), corresponds to burial beneath the young (H: Hesperian, HNt: Hesperian-Noachian transition) near-surface lava flow3 and magnetizations are at least ~0.4 A/m if the entire underlying crust is magnetized. A burial depth of 10 km (blue) requires magnetizations >1 A/m, hosted by Noachian units. The velocity profiles show the seismically-determined interface depths7. <img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.11662e6fb48266296682561/sdaolpUECMynit/2202CSPE&app=m&a=0&c=a901e5d8cb2787758f4b9f1efb44430f&ct=x&pn=gnp.elif&d=1" alt="" width="856" height="601">Figure 3: A composite power spectral density (PSD) plot for the surface magnetic field strength at the InSight landing site. Estimates for longer periods are derived using a Lomb-Scargle algorithm (black), shorter periods (purple) show a Welch spectrum.[1] Banerdt, W. et al. Nat. Geosci. (2020).[2] Banfield, D. et al. SSR (2019). [3] Johnson, C. L. et al.  Nat. Geosci.(2020). [4]Mitteholz, A. & Johnson, C. L. Frontiers (2022). [5] Joy, S. et. al. (2019). [6] Smrekar, S. et al. SSR (2018). [7] Knapmeyer-Endrun, B. et al. Science (2021). [8] Wieczorek, M. et al. JGR (2022). [9] Mittelholz, A. et al. Sci. Adv. (2020). [10] Gyalay, S. et al. GRL (2020). [11] Mittelholz, A. et al. JGR (2017). [12] Ramstad, R. et al. Nat. Astron. (2020).  [13] Brain, D. et al. JGR (2003). [14] Chi, P. et al. LPSC (2019). [15] Mittelholz, A. et al. JGR (2020). [16] Luo, H. et al. JGR (2022). [17] Mittelholz, A. et al. LPSC (2021). [18] Mittelholz, A. et al. GRL (2021). [19] Thorne, S. et al. PSS (2022).  [20] Bapst, J. et al. AAS (2021). [21] Parker, R. JGR (2003).

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