Tom Harvey writes:
Throughout ejection from their parent body, transportation through space and subsequent arrival on Earth, meteorites undergo extreme pressures and temperatures. Some meteorites, despite having survived entry to Earth, can be extremely fragile. Some of the analytical techniques that provide a wealth of information about meteorite geochemistry and physical properties can be damaging to a meteorite sample, either exposing it to some forms of contamination, or destroying it entirely. Given the opportunity to analyse relatively pristine samples from Antarctica, we opted to develop a procedure for understanding some of the fundamental properties of the returned meteorites that was entirely non-destructive.
Density (the mass per unit volume of a material) is a useful property to understand because it can allow us to make inferences about parent body composition, in addition to being an important metric in understanding thermal evolution models, as well as the survivability of materials in impacts, and during atmospheric entry to Earth. Non-destructive, high-precision measurement of the mass of a meteorite is straightforward to undertake using lab scales (i.e., weighing balances).
However, the determination of sample volume is not so straightforward. Traditional “Archimedean” methods for the determination of volume involve measurement of the displacement of a well-characterised medium (for example water, ceramic beads or inert gases) when the sample is emplaced into (see this previous study Consolmagno et al. 2008). Depending on the medium that the meteorite might come into contact with, it is possible that the sample might be chemically altered or mechanically damaged. Volume determination has also been performed by laser scanning of meteorite samples, to produce a three-dimensional (3-D) computer model of the sample surface topography (for example, see McCausland et al. 2011). A potential barrier to performing this sort of study is the requirement for expensive and highly specialised laser and camera technology.
An alternative to these methods that we have chosen to develop is the production of 3-D computer models of the samples returned by the Lost Meteorites of Antarctica project. This method involves the use of a suite of photographs of an object to computationally generate a three-dimensional colour model of that object. The method is non-contaminating, scalable, and the only necessary hardware was a DSLR camera, some small portable lights, and a computer with the appropriate software installed. Furthermore, the production of an accurate computer model of the samples’ surface topography and colour makes an excellent curatorial record which can be used in lab decision making, and to understand the orientation relationship between sub-splits in the future.
First, the meteorites were placed into a controlled light environment. This meant that we could control the amount of reflection and shadow on the sample surface, which is important for producing a good model. By using the appropriate sterile tools and portable lights, this method did not expose the samples to any alteration or contamination. Photographs were taken using a high-resolution camera at 5º rotational intervals around the sample (and in multiple orientations to capture) to ensure that we had captured all the details of the sample surface.
These photographs were then loaded into Agisoft Metashape, a professional software for photogrammetry applications, which were used to produce the models. This process is broken into several key steps which involve a pixel matching algorithm, and alignment of the various orientations of the sample to generate a shape file for the sample, followed by the production of a mosaic derived from the sample photographs to map over the shape file to bring it to life in colour.
In order to determine the volume of these models, the shape files were imported into a computer-aided design (CAD) software, 3DS Max, and scaled according to known external sample dimensions, measured in the lab with a Vernier calliper. In order to understand how well this method determined the true volume of the sample, we produced models of wooden cuboid blocks of known size and found that the computed volumes of these blocks were within 2% of their true, measured volume.
Using the data from this method, we will be able to compare our computed volumes with measurements derived from other methods such as computed tomography scans of the sample. The photogrammetry derived volume measurements and the sample masses measured in the lab allowed us to compute a density value for each meteorite in the study. Once all of the meteorites are formally classified, we will also be able to compare the densities derived from our photogrammetry-based method, with literature density values for meteorites of the same type as the ones recovered by the Lost Meteorites of Antarctica team to assess the usefulness of the method.
One of the potential drawbacks of our method is that it can be fairly time consuming, taking several days to carefully photograph, and computationally process a meteorite from start to finish. Furthermore, the method may be challenging to undertake on highly reflective metallic samples – the software relies on identifying matching features in different pictures, and so a sample that predominantly reflects light might cause issues.
However, it is possible to make high fidelity models of meteorites of a range of a size, shape and colours. These can be useful curational tools and should also provide a dataset of information about the samples that would otherwise be difficult to attain whilst maintaining the pristine nature of the samples.
This study was the topic of a 2021 LPSC abstract which you can find here. You can find the various models we’ve made dotted about the individual meteorite information pages from the first season, or you can find a selection of them below!