5:15 PM - 6:45 PM
[MIS16-P04] Spatial distribution and existing period of subsurface ice east of Erebus Montes by investigating rootless cones
Keywords:Mars, Erebus Montes, rootless cone, ice
The meteorite impact that triggered a Marsquake (S1094b) discovered by InSight on December 24, 2021, scattered water ice around the impact site (Posiolova et al., 2022; Dundas et al., 2022). It is notable that this ice exposure is located in the lowest latitude.
By our detailed observation of the area around the impact crater where the surrounding ice was discovered, rootless cone-like cone features have been found (hereafter, EEM cone). Rootless cone is a type of pyroclastic cone which formed by explosive interactions between lava and water (e.g., Thorarinson, 1951). As the lava flows over a water-saturated layer, subsurface water is heated, vaporized, and expanded. This causes an increase in pressure under the lava and induces an explosion. As these explosions occur continuously, the ejecta deposits in the surrounding area, form cone-like features (i.e., rootless cones).
In this study, we investigate geomorphometry and spatial distribution of EEM cones discovered in this study to clarify their origin. If EEM cones are rootless cones, subsurface ice is regarded to have been consumed in their formation. Following this hypothesis, we aimed to constrain the spatial distribution and existence period.
To characterize the spatial distribution of EEM cones, we performed nearest neighbor and Voronoi analyses. The nearest neighbor analysis is a method to investigate the spatial distribution pattern from the distance between an object and the object closest to it in a population using two values, R and c. If R>1 and c>1.96, it indicates that the population is dispersed. When R=1 and -1.96<c<1.96, it indicates that the population is randomly distributed. Furthermore, when R<1 and c<-1.96, the population is dense. Voronoi analysis is a tool that examines the affected region of an object in a population using cells (Voronoi cells). Va is the average area of Voronoi cells in the population under study, Ve is the area of Voronoi cells when all nearest neighbor distances are Ra, and V is Va/Ve.
This study revealed that EEM cones have similar morphometric and spatial distributional characteristics as rootless cones. The edifice diameters of EEM cones ranged from 82.6 to 126.0 m. This is consistent with the edifice diameter of terrestrial rootless cones, 5-450 m (Fagents et al., 2007), and inconsistent with those of scoria cones, tuff cones, and maars. EEM cones are widely distributed, with the closest distance from the S1094 epicenter-crater being 3 km and even more distant than that. The R and c values showed a dense distribution trend with calculation over large areas containing multiple cone clusters. This is consistent with rootless cones and pingos (Bruno et al., 2006), rather than secondary craters or pedestal craters. On the other hand, individual cone clusters showed an increase in R and c and a tendency toward random or repelled distribution patterns. Voronoi analysis also revealed that some cone clusters tended to be equally spaced. This may indicate the contribution of the self-organization process in the spatial distribution of rootless cones proposed by Hamilton et al. (2010). EEM cones showed a tendency for R to decrease rapidly as the area of clusters increased. This trend is more consistent with rootless cones than with pingos. These results suggest that EEM cones are likely to be rootless cones.
The presence of subsurface ice in EEM is definite because it was found in the impact crater that caused S1094b. if EEM cones are rootless cones, subsurface ice was consumed by their formation at EEM cone localities. The estimated surface age in EEM is 250 Ma at the oldest location. This implies that the subsurface ice in EEM has existed since at least 250 Ma.
By our detailed observation of the area around the impact crater where the surrounding ice was discovered, rootless cone-like cone features have been found (hereafter, EEM cone). Rootless cone is a type of pyroclastic cone which formed by explosive interactions between lava and water (e.g., Thorarinson, 1951). As the lava flows over a water-saturated layer, subsurface water is heated, vaporized, and expanded. This causes an increase in pressure under the lava and induces an explosion. As these explosions occur continuously, the ejecta deposits in the surrounding area, form cone-like features (i.e., rootless cones).
In this study, we investigate geomorphometry and spatial distribution of EEM cones discovered in this study to clarify their origin. If EEM cones are rootless cones, subsurface ice is regarded to have been consumed in their formation. Following this hypothesis, we aimed to constrain the spatial distribution and existence period.
To characterize the spatial distribution of EEM cones, we performed nearest neighbor and Voronoi analyses. The nearest neighbor analysis is a method to investigate the spatial distribution pattern from the distance between an object and the object closest to it in a population using two values, R and c. If R>1 and c>1.96, it indicates that the population is dispersed. When R=1 and -1.96<c<1.96, it indicates that the population is randomly distributed. Furthermore, when R<1 and c<-1.96, the population is dense. Voronoi analysis is a tool that examines the affected region of an object in a population using cells (Voronoi cells). Va is the average area of Voronoi cells in the population under study, Ve is the area of Voronoi cells when all nearest neighbor distances are Ra, and V is Va/Ve.
This study revealed that EEM cones have similar morphometric and spatial distributional characteristics as rootless cones. The edifice diameters of EEM cones ranged from 82.6 to 126.0 m. This is consistent with the edifice diameter of terrestrial rootless cones, 5-450 m (Fagents et al., 2007), and inconsistent with those of scoria cones, tuff cones, and maars. EEM cones are widely distributed, with the closest distance from the S1094 epicenter-crater being 3 km and even more distant than that. The R and c values showed a dense distribution trend with calculation over large areas containing multiple cone clusters. This is consistent with rootless cones and pingos (Bruno et al., 2006), rather than secondary craters or pedestal craters. On the other hand, individual cone clusters showed an increase in R and c and a tendency toward random or repelled distribution patterns. Voronoi analysis also revealed that some cone clusters tended to be equally spaced. This may indicate the contribution of the self-organization process in the spatial distribution of rootless cones proposed by Hamilton et al. (2010). EEM cones showed a tendency for R to decrease rapidly as the area of clusters increased. This trend is more consistent with rootless cones than with pingos. These results suggest that EEM cones are likely to be rootless cones.
The presence of subsurface ice in EEM is definite because it was found in the impact crater that caused S1094b. if EEM cones are rootless cones, subsurface ice was consumed by their formation at EEM cone localities. The estimated surface age in EEM is 250 Ma at the oldest location. This implies that the subsurface ice in EEM has existed since at least 250 Ma.