What is the difference between a simulant and an analogue?
Analogues are representative of a large area of a target body without having well defined parameters. For example, one of the ESA2C analogue materials is basalt that is representative of the basalts present on the Moon and Mars i.e. they are fairly pristine rocks with little alteration from Earth’s water. Many of the field sites are termed ‘analogue sites’ because they can be applicable to many places on Mars or the Moon when conducting field tests (for example, rover field trials). Simulants are representative of a specific planetary body and lithology, with properties that more closely match those of the parent body. A lunar mare regolith simulant, for example, would have a grain size distribution and grain shape similar to lunar regolith returned from the Moon during the Apollo missions. Simulants can be ‘general use’ as previously described, or be more specific and aim to represent a particular sample or localised region on a planetary surface.
How do I know which simulant to choose for my experiment?
It depends on the experiment and what you require from the simulants. Are you using it for mechanical tests, to drive over or test equipment with? Or testing regolith handling and transport mechanisms? If so, physical properties such as density, grain shape and size distribution, cohesion, and internal friction angles are the most relavant properties. For these kinds of tests, the chemistry and mineralogy of the samples are less important.
However, if testing ISRU processes such as oxygen extraction or 3D printing, relevant modal mineralogy (i.e. the proportions of minerals and phases) and mineral chemistry are just as important as grain size distribution and shape. Other physical properties, like internal friction angle or magnetic susceptibility, may not be so important.
Choosing a simulant without some of the relevant properties will likely affect the results of your process/experiment, and the results may not be as representative as you need. Overall, it depends on the experiments that you will be doing. Remember:
What goes into your process ultimately affects what comes out!
What are the most important properties of a simulant?
Again, this largely depends on what you will be using the material for as some properties are likely to be more relevant than others for a specific technique. However, there is a table of simulant properties ranked by importance that can be found in Sibille et al. (2006). This table is one of the outcomes of the 2005 Workshop on Lunar Regolith Simulant Materials and highlights which properties are critical to performing lunar surface activities and, therefore, for development of a lunar regolith simulant.
Although this workshop was specifically focussed on lunar regolith simulants, many of the top-ranked properties are equally important for other simulants and for geologic materials on Earth, so this table is highly useful as a first-order reference for simulant properties to be considered. In order of importance, the top ranked properties are: grain size, grain size distribution, particle density, glass composition, bulk density, modal mineral composition (as a function of grain size), grain shape, bulk chemistry, magnetic properties, mechanical strength, and total modal mineralogical composition (Sibille et al. 2006).
What properties are easily customisable?
Should you require a simulant that has specific properties, some requests will be far easier than others depending on the properties that require customisation. For example, grain size distribution is an easily customisable property as crushing and sieving of feedstock material are the main processes required to alter it. Modal mineralogy is also fairly straightforward provided suitable feedstock material is available.
However, grain shape is not such an easy property to customise, especially when the grains form from complex processes such as micrometeorite impacts. Some of the most difficult properties to address are trace element geochemistry, implanted gas content, and compositions of volatiles that are bound within the mineral structures. These properties are tightly linked to the formation history of the samples and cannot easily (if at all) be recreated or altered in a laboratory environment.
Scale is also an important issue when customising simulants. Many tonnes of material can be mined and given the appropriate grain size distribution, but some of the more challenging properties to replicate may only be possible for grams to milligrams of material and may be very costly (in terms of both money and time) to produce. Large scale simulants (available in tonnes or more) tend to be for ‘general use’ as the material only replicates a small number of the most easily customisable properties.
What are the difficulties with making simulants?
Scale, as previously mentioned, is highly challenging. What may be a useful technique to produce a few grams of material may be very costly and even impossible to produce at the kilogram or tonne range.
Also, no simulant is truly representative of any extraterrestrial geological unit. Each simulant is designed for a specific purpose and focusses on certain physical and/or chemical properties to fulfill its purpose.
Following on from the previous point, some of the properties of lunar and martian surface materials are not known. Some tests, particularly for mechanical properties, require kilograms of material so many of the lunar samples have not been tested for these properties, or only small masses have been tested so the results may not be representative of bulk regolith. Therefore, estimating the properties of materials and then attempting to replicate those properties can lead to large uncertainties. For Mars samples, few mechanical properties are known which results in a high level of difficulty and some ‘best guesstimates’ for simulant creation.
Finally, curating and ensuring standard properties throughout a simulant is a major challenge, particularly for large-mass simulants. If the feedstock material is mined (which is often the case for large-mass simulants), chemical and mineralogical properties may vary with distance or depth through the mine so subsequent batches of the same simulant may have different physical and chemical properties. From a curation standpoint, knowing what processes and tests a simulant has been subject to is difficult to know, particularly with older simulants that have been passed between laboratories and not sufficiently documented. That is why one of the aims of SACF is to provide materials available for use that have been effectively curated, so anyone borrowing the sample knows exactly where it originated and how it has been processed and stored since its creation.
Can I borrow the ESA2C materials?
For most analogues and simulants in the collection, yes. Please fill out a loan form on the ‘ESA2C Loans’ page, and be sure to read the terms and conditions of the loan. As they are loans, the material needs to be returned to the ESA2C collection once you have finished using them.
For small destructive tests, we usually provide material for those too. If possible, we would ask for a product to be returned to us if a simulant has been used for manufacturing purposes. For example, with a recent construction test the borrower very kindly sent 3 of the blocks that were made from the simulant materials for us to curate. These are highly useful to the SACF not only for public outreach but for future tests (if required) and future reference to how our materials have been used.
For some large-mass destructive tests, it may not be possible to provide the material. Our curation protocol states that we cannot provide more than 10% (by mass) of a single sample for destructive tests. However, if you require greater masses than we can provide, we would be happy to put you in contact with our feedstock suppliers so you can obtain the larger masses directly from them. As many of our analogues come from quarries, the feedstock material is readily available in large quantities.
Some of our samples are ‘voucher specimens’ which means we have a small quantity for in-house testing, or for curation for future generations (such as for samples used for component or instrument tests). Other reasons for not lending include:
– That particular sample may already be on loan, or is being used in-house.
– We have health and safety concerns regarding the sample and how it will be used/stored/handled at its destination.
– We have concerns regarding sample curation at it’s destination, such that the samples may be altered or lost during their loan period.
– The reason for requesting the samples is not justifiable based on the science/technology application stated in the submitted loan form.
We do try to accept as many loans as possible to make the best use of the ESA2C collection. If you have any questions about the samples or the loan process, or if you are unsure if you should apply for any of the materials in the collection, please email curation-esa2c@esa.int and we will endeavour to answer any questions that you may have.
