PREVENCIÓN Y CONTROL DE MICOTOXINAS EN ALIMENTOS

The Coriolis Effect instantaneously affects any particles from dust grains to landslides moving with a perpendicular component to the rotation axis of Vesta. However, the LAMO resolution of 20 m/pixel sets a limit on the observable deflection. To estimate the travel distance and travel time of a mass, it is assumed that a deflection is identifiable if it crosses over at least

5.3. Coriolis Effect on Mass Movements five pixels (∼100 m) and that the Rossby number (Equation 3.27) equals one, meaning that the Coriolis and centrifugal forces are equivalent (Section 3.2.4). Hence, a mass motion at a latitude of 45◦S requires a minimum velocity of

v = lf Ro ≈ 5 cm/s and a minimum travel time of ∼30 minutes at that speed

to result in a deflection observable in LAMO resolution. In most cases masses move with larger velocities than the minimum velocity, however the travel times are generally shorter.

In the following paragraphs, the different types of mass wasting, including intra-crater mass-wasting features associated with young craters, flow-like and creep-like patterns of shocked and fractured material, landslides, slumps of compact material and curved radial ridges from the early formation stage of Rheasilvia as described in Otto et al. [2013] (Appendix C) will be analysed and the Rossby number (Equation 3.27) will be derived to examine the influence of the Coriolis force.

Most features associated with intra-crater mass-wasting regions, such as spurs, boulders, dark patches and talus material do not show any evidence of the Coriolis Effect because they do not preserve the trajectory of a mass motion. Instead, spurs, dark patches and talus material are immobile features. Only the rolling trace of a falling boulder could indicate the trajectory of a movement. However, because the observed boulders were carried within landslides, the rolling traces were immediately covered by landslide material.

Intra-crater landslides, however, preserve their trajectories along the trav- elled path. They have been detected in craters of up to 50 km diameter and thus the run-out length has a maximum of ∼25 km, but is shorter for smaller craters or if the landslide stopped before reaching the crater centre. As an example of how the Coriolis force affects intra-crater landslides, we consider the dark landslide in crater Canuleia (Figure 2(b) in Appendix C, latitude ϕ ≈ 34◦_{S, slope α ≈ 24}◦_{, length l ≈ 3 km). The Coriolis frequency (Equation}

3.18) is f = 2Ω sin(ϕ) ≈ 2.7·10−4s−1. Using the modelling approach ofLucas et al. [2014], the wasting velocity can be estimated: v ≈qgVD cos(α), where

gV is Vesta’s gravitational acceleration and D the mean thickness of the landslide. The mean thickness cannot be resolved in the DTM, but the lower limit can be estimated from the size of the largest blocks in the landslide (∼100 m). Thus, applying a thickness of 100 m results in a minimum wasting velocity of ∼4.8 m/s. Consequently, the lower limit for the Rossby number (Equation 3.27) is Ro = v/(lf ) ≈ 5.9, indicating that the Coriolis force had only a minor effect on the mass motion. Furthermore, the effect is reduced for landslides in regions of lower latitude and for those moving in the direction of rotation (for details see Section 3.2).

(a) The flow-like feature within the area of 20◦S to 45◦S and 35◦E to 95◦E.

(b) Close-up image of striations developed on the flow-like feature.

Figure 5.6.: The flow-like feature north of the Matronalia Rupes scarp (a) and a close-up image of its striations (b). The arrows indicate the the flow direction along which striations developed. The dashed white line in (a) shows the extent of the feature and the rectangle illustrates the position of the close-up image (b). The images are in a stereographic projection with the scale bar applicable at the centre of the image. Image (a) is overlaid by a DTM referenced to a 285 km × 229 km biaxial ellipsoid.

5.3. Coriolis Effect on Mass Movements When considering flow-like features, it is unreasonable to determine the Rossby number to find whether they may be affected by the Coriolis force. This is because the flow velocity is unknown as acoustic fluidisation (Sec- tion 3.1.5) may reduce the internal friction and increase the velocity [Collins and Melosh, 2003; Melosh, 1986]. However, analysing the flow-like feature opposite the Matronalia Rupes scarp (Figure 5.6(a)), it is noticeable that the main lobe shows a slight deflection to the east in accordance with the Coriolis Effect. On the other hand, the striations developed on the surface do not show a common deflection but rather follow the topography (Figure 5.6(b)). Hence, it remains disputable whether the Coriolis force is responsible for the deflected lobe.

Creep-like mass movement produces a pattern of small mounds which migrate slowly downhill (Figure 6 in Appendix C). However, the terrain is unlikely to maintain any Coriolis-affected deflection because the creep trajectory is not preserved in the morphology of the mounds. Additionally, the relatively slow creep velocity (in the order of millimetres to centimetres per day) is below the minimum velocity associated with the data resolution and therefore a deflection is not detectable.

Large landslides on Vesta usually spread during their migration (Fig- ure 11(b) in Appendix C). The extent of the spreading overruns the morpho- logic evidences of the Coriolis force. However, following the calculations above for intra-crater landslides, the narrow landslide along the Matronalia Rupes scarp (Figure 2.14(c), latitude ϕ ≈ 55◦S, slope α ≈ 40◦, length l ≈ 25 km) yields a Rossby number of ∼0.3 and has thus probably been influenced by the Coriolis Effect. Indeed, the slide shows a curvature in agreement with the effect. It is, however, important to note that the topography also constrained the motion of the slide. In other regions, the landslides seem to be following the curved pattern of the ridge and groove terrain which is more likely to be related to the substructure rather than the Coriolis Effect on the specific landslides.

Slumping blocks and the related curved concentric ridges are much larger than intra-crater landslides or creep-like features. However, the distance of their motion is only approximately as long as the height of the main drop. For the Matronalia Rupes slumping block (Figure 5.2(a)) at a latitude of ∼55◦_{S this drop is ∼5 km. Because the velocity of the slumping process is} unknown it is not possible to derive a Rossby number. However, the slumping maximum velocity, when considering that the Rossby number needed to observe a Coriolis deflection has to be below 1, is ∼3 m/s. Nevertheless, even if these conditions were met, the block as a whole is embedded in the surface material which restricted the motion in any other than the main mass-wasting

direction.

As described in Section 5.2, the curved structures of the radial ridges appear to be remnants of the early gravitational crater collapse. The strength properties of material during the early formation stage were plastic, with negligible internal friction [Melosh, 1989], which made the mass motion relatively fast. Using a latitude of ∼40◦S, a length of ∼200 km and a velocity of ∼50 m/s, Jutzi et al.[2013] estimated a relatively low Rossby number of ∼0.5 for the curved radial ridges within the Rheasilvia basin, which indicates that the material wasting downhill was influenced by the Coriolis force.

In accordance with the Coriolis Effect, the curvature points against the direction of rotation for motion towards the rotation axis, and in the direction of rotation for motion away from the axis (Figure 5.3). The curvature on the crater wall of Rheasilvia would therefore have been generated by a mass motion towards the rotation axis, which nearly agrees with the centre of Rheasilvia. The patterns on the central peak suggest motion away from the axis. This is consistent with the idea of masses moving downhill into the basin, off both the crater rim and central peak, as the crater collapses.

The Coriolis Effect was capable of generating curved structures across the entire Rheasilvia crater. However, in some regions, for example the central peak and the intersection of craters Rheasilvia and Veneneia, additional mechanisms may have produced curved features.

On Rheasilvia’s central peak, an oblique impact might have been able to create curved scarps and ridges. A method for determining impact directions was devised by Scherler et al. [2006], who investigated the structure of ridges and faults of the central peak of Upheaval Dome (Utah, USA) to infer an impact direction. The Rheasilvia central peak exhibits bent fractures and faults visible as scarps from which material moved downhill (Figure 5.7). The analysis of the central imbrication structure of Rheasilvia after Scherler et al. [2006] is restricted by the small number of features and does not yield a definite impact direction or obliquity. However, the curved main scarps of the central peak, instead of straight radial expanded structures, may indicate a non-vertical stress component, e.g. an oblique impact (Figure 5.3).

At the intersection of Rheasilvia and Veneneia, the crater collapse of a two-layered target might have been able to produce the observed spiral pattern. The development of curved strike-slip faults has been demonstrated in experiments byAllemand and Thomas[1999]. They performed experiments in a two-layered target with a lower ductile layer and an upper brittle layer consisting of silicon and sand, respectively. A circular hole was cut through the layers and the relaxation process with variable layer thickness was observed. The collapse process produced spiral strike-slip faults for low brittle-ductile

5.3. Coriolis Effect on Mass Movements

Figure 5.7.: Scarps and ridges of Rheasilvia’s central peak (from Otto et al.

[2013]). The minor scarps indicate slumping from previously wasted material and ridges represent degraded scarps and material accumulations. The main scarps are slightly curved suggesting a non-vertical stress component of the impact. The centre of Rheasilvia (301◦E and 75◦S [Jaumann et al., 2012]) and the South Pole are labeled CR and SP, respectively. The image is in a stereographic projection with the scale bar applicable at the centre of the image. It is overlaid by a DTM referenced to a 285 km × 229 km biaxial ellipsoid.

thickness ratios. These faults crossed in a characteristic V-shaped pattern, meaning that the faults intersected at angles above 130◦ (Figure 5.8). The spirals developed on the crater wall, but more prominently beyond the crater rim.

The brittle-ductile thickness ratio of the intersection of Rheasilvia and Veneneia has likely been reduced, because the Veneneia impact may have removed parts of the brittle surface prior to the Rheasilvia impact. The

Figure 5.8.: Curved ridges at the intersection of Rheasilvia and Veneneia in the area within 25◦S to 65◦S and 110◦E to 210◦E (fromOtto et al.[2013]). The dashed lines mark the most prominent structures. The V-shaped crossings are indicated with black chevrons. They might be caused by the relaxation of a bi-layered target with ductile and brittle components. The image is in a stereographic projection with the scale bar applicable at the centre of the image.

ratio might have been sufficiently low to allow spirals, represented by curved ridges, to have formed during the relaxation process. Furthermore, the spiral pattern in this region expands beyond the Rheasilvia rim as expected by the

Allemand and Thomas [1999] theory.

The Coriolis Effect is thus the only process that can explain the curved pattern at all locations, however the oblique impact and crater relaxation theories cannot be completely ruled out for the central peak and Rheasilvia- Veneneia intersection, respectively.

6. The Coriolis Model