Noachian and more recent phyllosilicates in impact craters on Mars

Abstract

Hundreds of impact craters on Mars contain diverse phyllosilicates, interpreted as excavation products of preexisting subsurface deposits following impact and crater formation. This has been used to argue that the conditions conducive to phyllosilicate synthesis, which require the presence of abundant and long-lasting liquid water, were only met early in the history of the planet, during the Noachian period (> 3.6 Gy ago), and that aqueous environments were widespread then. Here we test this hypothesis by examining the excavation process of hydrated minerals by impact events on Mars and analyzing the stability of phyllosilicates against the impact-induced thermal shock. To do so, we first compare the infrared spectra of thermally altered phyllosilicates with those of hydrated minerals known to occur in craters on Mars and then analyze the postshock temperatures reached during impact crater excavation. Our results show that phyllosilicates can resist the postshock temperatures almost everywhere in the crater, except under particular conditions in a central area in and near the point of impact. We conclude that most phyllosilicates detected inside impact craters on Mars are consistent with excavated preexisting sediments, supporting the hypothesis of a primeval and long-lasting global aqueous environment. When our analyses are applied to specific impact craters on Mars, we are able to identify both pre- and postimpact phyllosilicates, therefore extending the time of local phyllosilicate synthesis to post-Noachian times.

Fig. 1.

Thermal stability of phyllosilicates. Laboratory reflectance spectral measurements illustrating the thermal stability of (A) nontronite, (B) montmorillonite, (C) chlorite, (D) kaolinite, (E) serpentine, and (F) prehnite.

Fig. 2.

Physiographic and geochemical setting of Toro crater. (A) MOLA colorized shaded relief map of Mars centered on Toro (red arrow). (B) THEMIS grayscale mosaic of Toro. The hourglass shape represents the location of the CRISM observation shown in C. (C) CRISM observation FRT0000B1B5 in false colors: red, smectites; green, prehnite; blue, chlorites. Yellow and magenta are mixed hydrated phases. (D Top) CRISM I/F corrected for the geometry of the observation and the atmosphere. The thick dashed line represents an example of the function used to remove the continuum. The results of the continuum removal are shown in the Middle. (Bottom) Some continuum removed laboratory reflectance spectra. The green spectrum is compared to RELAB spectrum C1ZE03 of prehnite. The blue spectra are compared to a 50% mixture of clinochlore (dark blue) and chamosite (light blue) (9). The red spectrum is compared to CRIMS Spectral Library spectrum 397F174 of smectite.

Fig. 3.

Central temperatures in the transient Toro crater. (A) A 3.1-km asteroid with a density of 3,000 kg/m³ impacting vertically at 8 km/s. (B) A 2.5-km asteroid at 12 km/s. Higher sizes and/or velocities will yield higher temperatures (see Table 1). Asteroid diameters and impact velocities were chosen to keep the crater diameter constant.

Fig. 4.

Distribution of prehnite inside Toro crater. (A) Portions of HiRISE images PSP_005842_1970, PSP_009270_1970, and ESP_011538_1970 covering Toro’s central uplift complex and surrounding crater floor. CRISM observation FRT0000B1B5 is highlighted in green to indicate the presence of prehnite. The peaks described in B and C are labeled. (B) Digital elevation model of the southern peak. Prehnite seems to be shed from the top of the peak and scattered to the northwest into the central pit. (C) Digital elevation model of the northern peak, showing intact light- and dark-toned layers and fractured and brecciated bedrock. Models derived using SOCET set v5.4.1 (BAE systems) on the PSP_005842_1970 and ESP_011538_1970 stereo pair (NASA/JPL/ASU) at a scale of 2 m per post and a vertical exaggeration of 5×.