Reinterpretation of weak basin magnetizations on Mars
Our results demonstrate that the martian dynamo was active 4.5 and 3.7 Ga ago. The existence of a dynamo field before and after the large basins Hellas, Utopia, Isidis, and Argyre requires an explanation for the general absence of magnetic fields over those basins. The impact demagnetization hypothesis is based on the argument that magnetization is absent within, but present around, the basin. Although this is true, unexplained observations worth noting are as follows: (i) Large tracts of Noachian crust surrounding the basins Hellas and Argyre are also unmagnetized or very weakly magnetized (fig. S7). Shock demagnetization can affect the basin exterior (
27) but fails to explain the heterogeneity of magnetization around the basin or the extensive Noachian aged areas in the southern hemisphere with similarly weak or no magnetization. (ii) Short-wavelength signatures may be present in the interior of the basins (fig. S7) (
16,
17), although lower-altitude tracks or surface measurements are necessary to confirm this.
Can the absence of magnetic field signatures over the basins be explained if a dynamo was operating during basin formation? At least two possibilities exist: (i) The giant impacts excavated large fractions of the crust, possibly removing material capable of carrying strong magnetizations. For crater diameters,
D, up to ~500 km, the excavation depth,
d, is ~0.1
D, i.e., up to 50 km (
37). Transient crater diameter estimates for Argyre, Isidis, and Hellas range from 750 to 1400 km (
38). Although the
d/
D ratio for such large basins is uncertain, the depths would exceed 50 km, effectively penetrating and removing magnetized crust. The observations of very weak fields over the BB, cf. the surrounding southern highlands, suggest that this is plausible. Weak, small-scale signals may exist within the Argyre, Isidis, Hellas, and Utopia basins but require more lower-altitude observations for definitive identification. Material excavation, with only weak or small-scale subsequent magnetization, would produce a magnetic field signature at MGS and MAVEN altitudes barely distinguishable from basin-localized demagnetization. (ii) We also cannot exclude a fortuitous scenario in which a dynamo field at the time of basin formation was substantially weakened or intermittent, as a result of a reversing dynamo field (
39). (iii) Alternatively, the dynamo was inactive during the time of basin formation, for example, because of inherently changing dynamo processes (i.e., from a thermally to a compositionally driven dynamo).
Implications of a dynamo 4.5 and 3.7 Ga ago
Evidence for a dynamo both ~4.5 and ~3.7 Ga ago has major implications for Mars’ evolution. Assuming a thermo-chemically driven magnetic dynamo, Mars must have sustained sufficiently vigorous core convection at its very earliest times and at the time of LP flow emplacement. Furthermore, the observations at LP suggest that a substantial fraction of the magnetization is carried in a thin, shallow magnetized unit. The resulting magnetizations are consistent with magnetization of pyroclastic flows in a 3.7-Ga old surface field with a strength similar to that of Earth’s present field. Excavation during large impacts may have played a key role in establishing a heterogeneous distribution of magnetic carriers in the martian crust, particularly removing magnetic minerals from the interior of major basins. This scenario allows a dynamo to plausibly persist from 4.5 to 3.7 Ga ago, thereby opening the possibility for a range of new magnetization processes to affect the martian surface, including depositional and crystallization remanence. For example, morphological evidence for water in the form of valley networks at the surface of Mars is dated between the Noachian and the Early Hesperian (
3), before and overlapping with the timing of formation of LP and hence the dynamo. Water circulating in the martian crust in the presence of a field could have resulted in hydrothermal alteration facilitating magnetization or remagnetization of magnetic minerals (
40).
Furthermore, the results link to current and planned missions’ e.g., the interior structure is a primary goal of the InSight mission currently operating on the martian surface (
41). The dynamo timing results presented here provide a major step forward in understanding Mars’ thermal evolution, especially when combined with existing constraints on heat flow, mantle temperature, interior composition, and physical models of structure of the martian core. Also, if a global magnetic field protects the atmosphere from solar wind energetic particles, a prolonged dynamo would delay the effects of some of the atmospheric removal processes and hence have implications for martian atmospheric loss rates (
42). This is important for addressing one of the main MAVEN goals of atmospheric escape rates through time (
42). The collection of martian samples and their return to the Earth will finally be underway with sample collection by the Mars 2020 rover to be launched next year. An extended dynamo, consistent with the new results here, is of key importance for the Jezero landing site selected for Mars 2020, because units that could be sampled might have formed at a time of an active dynamo field (
43). Future laboratory investigation of return samples will be the next major step in Mars exploration and, if magnetized, for planetary paleomagnetism.
