Search

When the Hale impact crater penetrated the martian cryosphere 1Ga, landforms indicating post-impact volatile mobilisation were generated. We have found landforms in the ejecta blanket of Hale Crater similar to 'permafrost molards' found in periglacial environments on Earth, and probably related to the past or present presence of volatiles at/near the surface. Permafrost molards are conical mounds of debris associated with landslide deposits, resulting from the degradation of blocks of ice-rich material mobilised by a landslide in periglacial terrains. Here we analyse the spatial and topographic distribution of conical mounds around the Hale crater at regional and local scales, and compare them to those of molards on the deposits of the Mount Meager debris avalanche in Canada. Hale Crater's conical mounds are located at the distal boundary of the thickest ejecta blanket, which is the closest to the main crater. We observe a similar spatial arrangement of molards along the distal parts of the terminal lobe of the Mount Meager debris avalanche. We then compare the morphology and morphometrics of the conical mounds on Hale Crater to those of terrestrial molards on the Paatuut and Niiortuut rock avalanches in western Greenland. We find that morphology and setting of conical mounds within Hale Crater ejecta are consistent with the formation pathway of molards on Earth. We infer that they originated from blocks of ice-cemented regolith that were produced by the Hale-crater-forming impact, transported by the ejecta flows, and finally degraded to cones of debris (molards) on loss of the interstitial ice. The similarities in distribution between the ejecta flows of Hale and Mount Meager debris avalanche on Earth suggest that the mounds resulted from the rheological separation of the ejecta flows, with a relatively fluid-poor phase that allowed the volatile-rich blocks to survive transport. This supports the prevailing hypothesis that the Hale impact event penetrated the martian cryosphere, providing important constraints on the rheology of martian ejecta deposits. • Conical mounds reveal the history of the cryosphere near the Argyre impact basin. • Conical mounds within the ejecta of Hale Crater are similar to terrestrial molards. • Spatial distribution and morphometry of terrestrial and martian molards are compared. • Conical mounds in Hale reveal volatile-rich ejecta at the time of emplacement. [ABSTRACT FROM AUTHOR]

Small impact events recorded on the surface of Earth are significantly underrepresented based on expected magnitude-frequency relations. We report the discovery of a 36-m-diameter late Holocene impact crater located in a forested area near the town of Whitecourt, Alberta, Canada. Although undetectable using visible imagery, the presence of the crater is revealed using a bare-Earth digital elevation model obtained through airborne light detection and ranging (LiDAR). The target material comprises deglacial Quaternary sediments, with impact ejecta burying a late Holocene soil dated to ca. 1100 14C yr B.P. Most of the 74 iron meteorites (0.1-1196 g) recovered have an angular exterior morphology. These meteorites were buried at depths <25 cm and are interpreted to result from fragmentation of the original projectile mass, either at low altitude or during the impact event. Impact of the main mass formed the simple bowl-shaped impact structure associated with an ejecta blanket and crater fill. The increasing availability of LiDAR data for many terrestrial surfaces will serve as a useful tool in the discovery of additional small impact features. [ABSTRACT FROM AUTHOR]

Archaean ferropicrites have been under-appreciated in the past because they have been frequently misidentified as enriched komatiites or Al-depleted komatiites. To investigate the nature of Archaean ferropicrite magmatism, we sampled ferropicrites from the Steep Rock, Lumby Lake, Grassy Portage Bay, and Dayohessarah Lake greenstone belts in the Western Superior Province, Ontario, Canada. Ferropicrite samples that are thought to approximate liquid compositions have ∼18 wt % Fe2O3* at ∼19 wt % MgO, and frequently contain less than 5 wt % Al2O3. They are enriched in Ti and high field strength elements relative to komatiites, and have fractionated trace element profiles (La/Yb ∼11). These distinctive geochemical characteristics require that ferropicrites and komatiites have different mantle sources, with that of the ferropicrites being Fe- and incompatible element-enriched compared with that of komatiites. A consideration of recent 5 GPa melting experiments on pyrolite and Fe-rich Martian mantle compositions indicates that Archaean ferropicrites could be generated by melting of an olivine-dominated mantle source with a Mg-number of ∼85 at ∼5 GPa. The high densities calculated for the ferropicrite magmas (e.g. 3·33 g/cm3) suggest that more Fe-rich magmas would have difficulty rising to the Earth's surface and would tend to stagnate or sink within the mantle. [ABSTRACT FROM PUBLISHER]

Molards are cones of debris that result from the disaggregation of ice-cemented blocks transported by mass movements (e.g., Cruden, Can. J. Earth Sci 1982; Milana, PPP 2016). Recently, the origin of molards has been directly linked to permafrost degradation (Milana, PPP 2016; Morino et al., EPSL 2018). On Earth, permafrost degradation has accelerated in periglacial environments (e.g., Brown and Romanovsky, PPP 2008), but few landforms exist to track this process over time, and molards are a rare example that can serve this purpose. A process similar to permafrost degradation is thought to occur also on Mars, where water ice is known to exist as ground ice polewards of 30-40° in both hemispheres (Byrne et al., Science 2009; Feldman et al., JGR 2004), and where volatiles (H2O, CO2, etc.) can easily change phase (Fanale and Cannon, JGR 1974). Volatile loss should play an important role in landscape evolution of Mars, so finding distinctive landforms that testify this under-debate process can aid in understanding the processes that shape the surface of the planet. Here, we present a comparative study of molards that we have found in the Mount Meager debris-avalanche deposits on Earth with conical landforms that we have identified in the flows emanating from the ejecta deposits of the one billion year old Hale crater on Mars. The Mount Meager debris avalanche (British Columbia, Canada) is the largest landslide in the Canadian history, and permafrost degradation is thought to have contributed to the release of the failure (Roberti et al., Geosphere 2017) and in the development of the molards. We compare morphometric and planimetric measurements of molards on Earth with those for the molard-like features identified in the ejecta of the Hale Crater on Mars, whose ice-rich nature has already been proposed based on other landforms (Jones et al., Icarus 2011). The visual similarity of the molards of the Mount Meager debris avalanche and of the ejecta flow of Hale crater suggest that they are both formed via the degradation of ice-rich surface material. The finding of candidate molards on Mars adds to the evidence that the impact of Hale Crater was into ice-rich materials, and these landforms have the potential to constrain both the initial ground ice content at the Hale impact site, but also the conditions during ejecta emplacement. Our study shows that permafrost degradation/volatile loss can leave a significant geomorphic fingerprint when combined with rapid mass movements in periglacial terrains on Earth and on Mars. [ABSTRACT FROM AUTHOR]