Photogrammetry and non-destructive determination of properties of Antarctic meteorites

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.

Tom Harvey in the curation lab imaging one of the meteorite samples. Photo: KJoy

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.

Final model of Antarctic Meteorite OUT 18010, images of which are shown in the above slideshow. Video: Tom Harvey

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!


Three more meteorites classified

Three more meteorites have been classified from the first search season in 2018-2019. If you want to find out more about how we have classified these samples take a look at this blog post.

This time we announce the collection and classification of two samples that are achondrites – meaning they are from parent asteroids that have been differentiated (see here for more details). These two samples are classified as mesosiderites – called OUT 18014 (made up of two separate stones found about 5 m apart from each other) and OUT 18018 (one stone) found ~1.7 km away from the other two samples.

Mesosiderites are an unusual type of meteorite – they are stony-iron, meaning that they are made up of roughly equal parts silicate minerals and iron metal. As of May 2021 there are only 280 meteorites that have been classified as being a mesosiderite, with only 61 of the group having been found in Antarctica. In terms of the Lost Meteorite of Antarctica project goals, finding this sample is very interesting – as it is a stony-iron type of meteorite we would have perhaps expected to find meteorites like this sitting below the ice, rather than on the top ice surface. We are looking into this question as we classify more of the samples we found.

We also report another L-chondrite sample (adding to the list that we previously reported – see below. This one is called OUT 18004 and it looks like this:

These newly classified stones join the first nine chondrite samples we announced early in May 2021: see below for a gallery of the previous meteorites classified from the project.

A list of all the meteorites classified so far can be found here.

UK Antarctic Meteorite Classification – the process

The first of the UK Antarctic meteorites have been classified! Nine rocks retrieved from the Hutchison (HUT) and Outer Recovery (OUT) icefields have been approved and published by the Meteoritical Bulletin. For details of these, please see this table and follow the links.

How were they classified?
The rocks collected in Antarctica were shipped frozen to the UK and kept in temperature monitored freezers. Each rock was thawed in an exisccator chamber to minimise any chemical reaction with residual ice; any ice present should sublimate straight to gas from the solid phase. After thawing, the rocks were all individually weighed and photographed. Measurements were taken of the rock’s magnetic susceptibility and electrical conductivity (see this previous blog post by Tom Harvey, as these properties can give a provisional indication of their possible classification.

Some of the meteorites were chosen to undergo computerised tomography (CT) scanning and photogrammetry. CT-scanning gives a 3D image of the interior of the rock. This provided compositional information and helped us to choose where to attempt to break or cut the rock. Photogrammetry allows a full 3D surface rendering of the meteorite to be made. For more details of this work, please see Tom’s LPSC conference presentation.

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Tom Harvey in the curation lab imaging one of the meteorite samples. Photo: KJoy

Where there were no natural crumbs or fragments of material for further analysis, each rock was split with rock splitters, keeping them inside their sterile bag so that the only contact was with the stainless steel blade. A few meteorites and some of the fragments from the rock splitters were cut with a low speed saw to ensure the material does not get hot enough to cause any alteration.

Fragments were weighed, photographed and then made into epoxy blocks. These have to be polished to a very flat surface, in order to be able to examine the chemical composition using a scanning electron microscope (imaging system) and electron microprobe (used to determine mineral chemistry).

The scanning electron microprobe provides greyscale images (see left below), where the brightness corresponds to the differences in atomic number (number of protons) of the different elements. For example, iron, with an atomic number of 26, will be much brighter compared to sodium, with an atomic number of 11. It can also be used to quantify the difference in this brightness to get an idea of the relative abundance of each element and likely mineral. We use the scanning electron microscope first to get an image and element maps of the whole section (see right image below).

Following this, around 30-40 spots (points) are selected for measurement with the electron microprobe as this allows for more precise and accurate quantification of the mineral composition. In the element map above, the silicate minerals shown in green are usually olivine, and the minerals in light blue are usually pyroxene. The iron content of olivine and pyroxene varies and this is used to distinguish ordinary chondrites between the “H”, “L”, and “LL” classifications (check out where these meteorites sit within the family tree here).  “H” chondrites have the highest iron metal content, but lower iron oxide in pyroxene and olivine. “L” chondrites have lower iron and less metal but higher iron oxide in pyroxene and olivine, whereas “LL” have the lowest total iron but the highest iron oxide in pyroxene and olivine.

Graph showing the iron oxide abundance in pyroxene (Fs) against the iron oxide abundance in olivine (Fa). Yellow crosses show the data collected for five UK Antarctic meteorites, superimposed on a graph of values recommended as distinguishing the H, L and LL ordinary chondrite groups for the Nomenclature committee (Grossman & Rubin, 2006).

Meteorites classified!

We have news…. its a big day for the project as we have our first batch of meteorites that have been formally classified by the Meteoritical Society Nomenclature committee and have now been published in the Meteoritical Bulletin Database!

A list of the newly classified meteorites can be found here https://ukantarcticmeteorites.wordpress.com/meteorite-discoveries/ , and you can click on each meteorite name to learn more and see some pictures of the samples. So far all of those classified from our 1st field season come from parent bodies in the asteroid belt: all are undifferentiated ordinary chondrite types. This means that they come from some from asteroids that represent some of the earliest Solar System building block rocky material that never got big enough to completely melt (hence they are undifferentiated), but they are all are from quite a common type of meteorite group (the ordinary chondrites). So far all the types classified are stony and not metal types – meaning that they are dominantly made up of silicate minerals rather than metal. In case you get lost with all the terms used – an overview of the different types of meteorites can be found at https://ukantarcticmeteorites.wordpress.com/meteorite-classification/

The first classified batch include nine meteorites: two were recovered from from the Hutchison Icefield area (these ones are called HUT), and seven from the Outer Recovery icefield area (these ones are called OUT). The number after the acronym name specifies the particular sample type. If you want to read more about the names of icefields we visited you can read here https://ukantarcticmeteorites.wordpress.com/2021/04/15/new-meteorites-new-names/

This has been the cumulation of a lot of hard work from the field search teams from the 1st 2018-2019 field season (Julie Baum and Katie Joy) and logistics support personnel, the BAS cargo transfer team, the local meteorite lab and classification team – lead by Jane MacArthur with help from Thomas Harvey, Rhian Jones and Katie Joy. A huge thanks to the local analytical lab leads who have kept the instruments we use to image the samples and determine their chemistry (Lewis Hughes and Jon Fellowes), those who support the labs we use to prepare our samples (John Cowpe and Lydia Fawcett) and appreciate key advice from Andrew Smedley, Romain Tartese and Geoff Evatt. Thanks also to all the external help we have had from the Natural History Museum staff in helping these efforts, and to the Meteoritical Society Nomenclature team and Meteorite Bulletin teams for reviewing, approving and sharing the samples’ new names.

You can also read a blog from Jane on the lab curation approach where she will talk about how we go from picking up a sample in the field to working out what type of meteorite it represents.

We also have more samples under review by the nomenclature team so will announce what else we have found in the near future… stay tuned for more meteorites to come…

Photo of the largest sample found in season 1 – now called OUT 18021 (or still affectionately referred to as the melon on account of its large size). the scale cube is 1 cm in size. Image: Lost Meteorites of Antarctica / The University of Manchester

New meteorites need new names…

To be able to give the meteorites we have recovered a formal name we have to go through some procedures…

Dense meteorite stranding zones (areas where lots of meteorites are found) are awarded a name by the Meteoritical Society Nomenclature committee. The meteorites recovered from these areas are then named after these sites – for example the first recognised lunar meteorite Allan Hills (ALHA for short) 81005 is named after the Allan Hills icefield A in Antarctica. Thus, to be given a name we need the place that the meteorites are found to be called something!

Our issue is that the regions we visited in Antarctica had not been formally allocated names by the countries who administrate these areas. So we have gone through two different routes to formally assign names to the field sites we visited so that we can use the names of these geographical features in future research publications and use them to name the meteorites we recovered.

We are happy to announce that our two main field areas have been approved as the Outer Recovery Icefields in Dronning Maud Land by the Norwegian Polar Institute and Hutchison Icefield in Coats Land (British Antarctic Territory ) by the UK Antarctic Place-names Committee. Both of these field sites contain nunataks (mountain tops emerging from the ice), which have also been named after meteorite and meteor scientists (see below for details). The UK site names are included in the UK Antarctic Gazetteer (https://apc.antarctica.ac.uk/) and are available for use on all maps and charts and in all publications. They are also included in the Scientific Committee on Antarctic Research (SCAR) Composite Gazetteer of Antarctica (https://data.aad.gov.au/aadc/gaz/scar/ ).

These names have now also been approved by the Meteoritical Society as dense meteorite collection areas and we will be able to call the meteorites either OUT (for those collected at the Outer Recovery Icefields) and HUT for those collected from the Hutchison Icefield.

Regional context of the fieldsites for the Lost Meteorites of Antarctica project. See below for details of the two areas highlighted with black boaxes. Base map is Landsat Image Mosaic of Antarctica. Image: Katherine Joy.

Outer Recover Icefields Area

Outer Recovery Icefields. named because of its proximity to the Recovery Glacier found adjacent to the northern extent of the area. Link to online Norwegian record.

Halliday Nunatak (81°24’32.97″S, 18° 1’59.88″W): Located in the Outer Recovery Icefields. named after Canadian astronomer Dr Ian Halliday (1928-2018) who was a Canadian astronomer with expertise in meteor (asteroid and comet) delivery rates to the Earth. Link to online Norwegian record

Outer Recovery Icefield area showing locations of the nunatak and four separate blue ice fields. Base map is Landsat Image Mosaic of Antarctica overlain with high resolution Sentinel 2 image. Map scale is 1:250,000 Image: Katherine Joy.

Hutchison Icefield Area

Hutchison Icefield (81°30′ 30″S, 26°10’W): Named after British meteorite scientist Dr Robert Hutchison (1938-2007) who was the Curator of Meteorites at the Natural History Museum, London. He was Head of the Cosmic Mineralogy Research Programme at the NHM, and responsible for the national meteorite collection, one of the most significant meteorite collections in the world. Awarded the Gold Medal of the Royal Astronomical Society in 2002; asteroid 5308 named Hutchison by the International Astronomical Union. Named in association with names of pioneering meteoriticists grouped in this area. Link to online SCAR record.

Turner Nunatak (81°27′ 50.42″S, 26°24’48.88″W): Located in the Hutchison Icefield. Named after Professor Grenville Turner FRS (b. 1936) pioneering lunar and meteorite scientist, Emeritus Professor at the University of Manchester. He established the University of Manchester Isotope Cosmochemistry group and his pioneering work on rare gases in meteorites led him to develop the argon–argon dating technique that demonstrated the great age of meteorites and provided a precise chronology of rocks brought back by the Apollo missions. He was one of the few UK scientists to be a Principal Investigator of the Apollo samples during the time of the US manned Moon missions. Link to online SCAR record.

Pillinger Nunatak (81°34’40″S, 26°24’15″W): Located in the Hutchison Icefield. Named after Professor Colin Pillinger FRS (1943-2014), English planetary scientist who was a founding member of the Planetary and Space Sciences Research Institute at Open University in Milton Keynes, and through his career studied stable isotopes in Apollo Moon samples, martian meteorites and asteroidal meteorites. He was also the Principal Investigator for the British Beagle 2 Mars lander project. Link to online SCAR record.

Map showing Hutchison Icefield area with Turner nunatak to the north and Pillinger nunatak to the south. Karpenko massif is a region of disturbed ice named after a Russian Engineer Aleksei Illaryonovich Karpenko (1940-82). Base map is Sentinel 2 image. Image: Dr Adrian Fox (UK Antarctic Place-names Committee)

With many thanks to Dr Adrian Fox (UK Antarctic Place-names Committee), Dr Oddveig Øien Ørvoll of the Norwegian Polar Institute for all of their help with the naming of these regions and advice from Laura Gerrish at the British Antarctic Survey.

Meanwhile in Manchester…

13 Jan 2020

Readers will have seen and read about a lot of goings on “down south” in Rothera and at the Outer Recovery ice fields, and the results of the team’s searches at the field site near the Shackleton Mountains. This is only part of the story (though a key one)!

Back in Manchester the rest of the Lost Meteorites of Antarctica team have been busy, so we thought it only right that we give a brief overview of the work going on behind the scenes. Recently mentioned, Liam and John provided support to Wouter with the technical glitches and have of course been instrumental throughout the project from its initial design, build, lab testing and field testing.

Patches of blue ice at the base of cliffs in the Theron Mountains. Selecting the right spot is key to finding meteorites. [Credit: Romain Tartese]

In parallel with the detector system build, Andy has been working with lots of data analysis (using satellite datasets and climate model outputs) to figure out whereabouts the team was best searching for meteorites. Antarctica is a big place and meteorites are only found in a few spots. Sometimes people head out there to come back empty handed, so we wanted to do our best for last season to make sure we found a “blue ice area” that harboured meteorites. First of all, a selection of candidate sites were tracked down by Katie (before the current project was funded) and then reduced to a long-list of those accessible on a logistics basis with the help of BAS. Then, using a combination of estimates of snowfall (that tells us something about the rate at which meteorites accumulate in a given area), and the local surface ice flow and wind scouring (that tells about the rate of loss of meteorites), we came up with a prediction of what density of meteorites we expected across these candidate sites. That prediction enabled us to refine and rank our preferred areas for Katie to visit last year. Thankfully she and Julie Baum confirmed our estimates and found some meteorites! Once we had decided on particular areas, Andy was involved in making custom maps for the team’s GPSs from hi-res satellite imagery, more detailed estimates of which individual ice fields to return to (from the data and samples Katie collected last year), and the logistics involved in shipping and planning. At the moment he’s the main contact back in Manchester and has been responsible for posting updates sent through by satellite phone while Geoff, Katie, Wouter and Romain have been at the remote field site.

There’s lots of posts about trying to find meteorites on the blog, but once we find them — what happens to them? That job is being undertaken by Jane and Tom working with members of the isotope group.

Well, we’ve made sure the potential meteorites have all been collected following defined procedures to keep them as free from any contamination as possible, for example, they only come into contact with stainless steel equipment used to get them into polythene bags, and every sample is double-bagged. They are even kept at sub-zero temperatures throughout their journey back to the UK, giving us the best chance of keeping them in pristine condition for future science. Jane, working with Katie, Rhian Jones and with folks at the meteorite group at the NHM, has been working out the necessary steps for the preliminary examination plan for classifying the meteorites, to ensure the samples do not get contaminated, and that every stage of examination is thoroughly documented. In line with this, the first ten samples from last season have now been thawed and she is using “CT-scanning” to look inside the rock and get an initial idea of what it is made of, before deciding how to break or cut the sample. Small pieces will then be mounted on glass slides in order to examine them with microscopes so that they can be formally classified into their different classes.

The “light box” set up used to acquire the images for 3D photogrammetry scans. [Credit: Tom Harvey]

Now the first samples from last year’s reconnaissance trip have been defrosted, Tom has been working to scan the fresh sample exteriors with a technique called photogrammetry. Photogrammetry uses information in pictures of a sample (in this case a meteorite) which show overlapping surface features to position that bit of the sample in 3D space — meaning that we can generate an electronic 3D model of the sample! These models are really useful because they preserve a record of the sample exterior prior to analysis (or, if needs be, cutting), and mean that we can zoom in on parts of the surface that are particularly interesting, which is great for curation and initial characterisation purposes and gives a permanent record of what the meteorite looked like when it was found.

And as this post goes online, it sounds like this year’s samples might just be starting to make the long journey back to the UK… holding the promise of lots more interesting science.