Leonice, la flora y el trabajo remunerado

At the microscopic level soils can be defined by the properties of the individual constituent particles. Naturally, the mineralogy and moisture content play a determinant role in soil mechanics.

When looking at Martian terrain, significant levels of regolith water content of around 4% have been estimated based on MARS Express Image Spectrometer measurements, postu- lated to be mostly due to frost-related processes [156]. Although the existence of a thin

3.5. Planetary Soil Simulants Characterisation

liquid water layer between the frost and the regolith can not discarded, it is reasonable to assume that the type of soil relevant to this research can be considered dry. Similarly, widespread terrains with clay minerals have been discovered but are limited to the oldest regions [157], with a large predominance of an-hydrated volcanic terrain. According to these two observations this study focuses on dry hard-granular frictional soils.

Regarding the physical properties of soil particles, the main factors to take into account are: their shape, their size and their compaction state. More angular grains favour particle inter-locking, thus increasing the internal friction of the soil. Weathering processes tend to smooth down sharp corners, decreasing internal friction but enabling more compact packing configurations. On the other hand, larger particle sizes lead in general to lower levels of porosity. Even for uniformly graded soils there is a noticeable variability in the range of particle sizes. Well-graded soils, with wider particle size ranges, facilitate higher densities.

As a result of the particle properties mentioned above, a given soil can adopt a range of compaction states leading to changing values of bulk density (ρ_B), defined as the ratio of total mass (mT) over total volume (VT) of a sample. In the case of dry soils the total mass

is equal to the mass of the solids (m_S) divided by the sum of solids volume (V_S) and voids volume (V_V), resulting in an expression that depends solely on the soil solid particle density (ρ_S) and the void ratio (e_V), as shown in Eq. (3.1).

ρB = mT V_T = mS V_S + V_V = ρS 1 + e_V (3.1)

A value of relative density (ρ_R) can be obtained comparing the sample bulk density with the maximum and minimum bulk densities achievable for that given soil, as in Eq. (3.2). Vari- ations in this relative density value will directly affect the macroscopic physical properties of the sample, as will be discussed in Sub-sections 3.5.2 and 3.5.3.

ρR =

ρ_B − ρ_{M IN} ρM AX− ρM IN

3.5. Planetary Soil Simulants Characterisation

Table 3.2: Soil simulant microscopic and bulk density properties

Property SSC-1 SSC-2 SSC-3 ES-3

Loose Dense Loose Dense Loose Dense Loose Dense

ρ_B [g/cm3] 1.62 1.71 2.23 2.38 1.46 1.64 1.47 1.69

eV 0.48 0.4 0.41 0.32 0.78 0.58 0.79 0.56

Main Component Quartz Garnet Quartz Quartz

ρS [g/cm

3_{] 2.4 4.1 2.6 2.63}

Particle Shape Rounded Angular Sub-Angular Sub-Rounded

d50 [µm] 264.7 53 247 456.14

d₆₀ [µm] 298.7 56.9 258.4 491.2

d10 [µm] 118.7 34 166.6 391.2

U 2.51 1.67 1.55 1.58

The range of soils used in this research have been selected both to test over a variety of different mineralogical compositions and particle sizes and shapes and to be as representative as possible of planetary soils. This selection process took into account limitations both in the sourcing costs and the availability of planetary soil physical properties data. Three of the soil types were readily available from previous in-house planetary soil simulant sourcing studies namely ES-3 [158], SSC-1 and SSC-2 [114]. A fourth soil type, denominated SSC-3, is a naturally occurring sand gathered from a beach near West Wittering, in the South of England.

The microscopic properties of these four types of soil are varied and complement each other well, as summarised in Table 3.2. Regarding the particle shape, ES-3 shows sub-rounded particles, SSC-1 has rounder grains, SSC-3 granules are sub-angular and SSC-2 presents the highest level of angularity. Microscopic images of each soil showing these differences in particle shape can be seen in Fig. 3.10. In terms of particle size, data from dry sieving tests using mesh sizes between 30 µm and 1 mm for all four types of soil are compiled in Fig. 3.11, which plots the cumulative percentage of mass retained versus the mesh size of each sieving step.

3.5. Planetary Soil Simulants Characterisation

(a) (b)

(c) (d)

Figure 3.10: Microscopic images of (a) ES-3, (b) SSC-1, (c) SSC-2 and (d) SSC-3

3.5. Planetary Soil Simulants Characterisation

Most of the soil types used can be classified as sands according to the British Soil Classi- fication System [159] since the size of their particles are contained in the 60 µm - 2 mm range. The only exception is SSC-2, which contains a considerable amount of coarse silt. When comparing the median particle sizes (d50) ES-3 falls in the coarse sand range with

values of ∼ 500 µm while SSC-1 and SSC-3 are in the fine sand region with values of ∼ 250 µm. When looking at the Uniformity Coefficient (U ), defined in Eq. (3.3), similar values of ∼ 1.6 are obtained for all soil types, indicating high particle size uniformity, except for SSC-1 with a value of 2.5, implying a wider range of particle sizes.

U = d60

d10

(3.3)

The differences in the characteristics presented above lead to different levels of compaction depending on the way particles are deposited, i.e. the sample preparation method. Follow- ing the guidelines for consistent soil preparation given in [160], two different preparation methods are used to make the compaction level as low and as high as possible. For loose preparations, the soil is poured at a constant rate from a height of at least 50 cm, so that all particles may reach terminal velocity. For dense preparations the same pouring technique is used but simultaneously vibrating the sample to promote compaction. The bulk density can be calculated as the final volumetric mass density of the sample, weighing its mass (excluding the container) and dividing it by the known volume capacity of the container.

As seen in Eq. (3.1) the bulk density depends on the solid particle density, which may vary significantly depending on the mineralogy of the particles. The solid particle density can be found by measuring the mass of a saturated sample (mSS) in a known volume (VSS) and

completely drying the sample in an oven. The mass difference between the saturated sample and the dry sample corresponds to the evaporated water (mW) which, knowing the value of

water density (ρ_W), can be used to obtain the solid volume of the sample and calculate the solid density as in Eq. (3.4).

ρS = m_S VS = mS VSS− mWρW = mS VSS− (mSS − mS)ρW (3.4)

3.5. Planetary Soil Simulants Characterisation

The calculated solid densities, show how the quartz-based soils take similar values in the 2.4 − 2.7 g/cm3 region. However, the garnet-based SSC-2 soil has a much higher particle solid density which reflects in significantly higher bulk densities, exceeding 2 g/cm3 while the other soil types are constrained to the 1.4 − 1.8 g/cm3 range.

In this case it also translates in generally lower void ratios of 0.32 − 0.41 than the quartz- based soils. Among the latter, SSC-1 shows the lowest void ratios and the least variability between the loosely and densely compacted samples, most likely due as a combination of the lower particle size uniformity and the rounder particle shape.

Overall, the selected soils provide interesting permutations of the different characteristics, covering a wide range of particle sizes and shapes and levels of compaction and density. These variations in the properties will have an effect on the macroscopic physical properties of the different soils that are presented below, enabling a rich and meaningful experimen- tation phase.