Space Rock in my Wombok?

Back in September 2022, my wife and I sat down for an unusually interesting lunch. That day I had made some coleslaw using the firm-leaved bottom-half of a wombok (大白菜, Wong Ngaa Bak, also known as Chinese cabbage, Napa cabbage, wong bok, siu choy, and 紹菜). I had made the coleslaw in my usual way, finely chopping the cabbage leaves, then adding grated carrot, apple, and mayonnaise. All the veggies and fruit were grown in Australia and purchased at my local Coles supermarket.

During lunch I was chewing a mouthful of the coleslaw and had the discomfort of biting down on something hard (I think I was lucky not to chip my tooth). I removed the small object from my mouth, and after a quick clean, inspected the offending item.

My first impression was that it was shiny, and looked like a small piece of metal or a rock chip.

This is obviously not what you’d expect to find in a meal, so I cleaned it up and had a closer look using my hand lens. I should mention that I am a geologist, so having several hand lenses at home to inspect rocks and minerals is not unusual.

The object was approximately 2mm x 2mm x 1mm, circular in shape like a tiny disc, and a dark grey colour, with reflective metallic luster (shiny) in places. Suspecting it may be a piece of metal, I got my scribe-magnet (another important geological tool).

I was amazed by how magnetic the object was: it actually leapt onto the magnet from several centimeters away.

I have been using a scribe-magnet to identify naturally magnetic minerals (such as magnetite and pyrrhotite), for many years, and this response was highly unusual for a natural mineral. I thought the object must be a shard of iron from some type of farming equipment that had ended up in my lunch.

‘What is this thing?’

With my interest piqued I used my trusty Olympus TG-5 camera (with its excellent microscope-zoom function) to take some close up images. I was blown away to see that the object wasn’t simply a small round chip of metal, but had a rough irregular surface, with concave surfaces on both sides of the disc. The concave sides were also covered in tiny aggregates of blue and pale/white glassy crystals, and there were signs of oxidation or grey-coloured crusting in between the crystals. I thought, as the textures and colours looked much more like a natural object rather than a man-made object or fragment of a farming tool.

Given the unusually magnetic metallic composition, the pitted and concave shape, and the presence of a glassy blue coating, which I thought might be a type of tektite glass, my geologist brain thought that this could possibly be a tiny meteorite, or piece of space junk, which landed in my wombok while it was growing! This may sound far-fetched, but I remembered a favourite factoid from my youth that stated:

'About 100 tonnes of meteors and ‘space dust’ enter the Earth’s atmosphere each day'.

The factoid went on to say that if the dust fell evenly on the Earth, then that volume of material entering the atmosphere would be enough for each cabbage to have one speck of dust land upon it during the growth cycle (NOTE: I have tried to hunt down the book in which I read this, but can’t find the reference, so I may be mis-remembering). NASA also estimates that there could be hundreds of thousands of pieces of space debris in orbit around the Earth ranging in size from the tiny to the huge (e.g. the defunct 8-tonne Envisat satellite (BBC Reference here)

Could my ‘space rock’ be ‘space junk’?

The next step was to try to generate some data to establish whether my object was really a potential wombok-space-rock. Fortunately, being employed through a university, I have access to some fancy analytical equipment. With the assistance of post-graduate researcher Brad Cave, the sample was mounted in a resin puck for further examination (as we do with other samples we are investigating). Brad then polished the uppermost surface to be able to put the sample into the scanning electron microscope (SEM) for a closer look, and to probe the composition using the SEM’s inbuilt non-destructive energy dispersive X-ray spectroscopy (EDS) system. The sample took the polish very well and had a highly metallic reflectance. The sample was then given a very thin carbon coating to aid imaging and analysis in the SEM.

Once loaded, the imaging was easy, although what we saw also surprised me. Samples viewed under the SEM display textures and variations consistent with the composition of the sample, so it is usual to see mineral grains, crystals, and variations in surface texture in detail.

However, away from the pitted edge, the wombok sample had no visible textures at all.

It simply looked like a single piece of polished metal, no crystals, no cracks, no variation of reflectance at all. The edges were uneven, and slightly pitted, but there was no observable variation in the composition of the sample at all; just shiny pale grey metal, with none of the blue, glassy, coating present at the edge.

We then switched over to the EDS system, and analysed a number of ‘spots’ across the sample. The EDS system fires a concentrated beam of electrons at the surface of the sample and measures the response. I was hoping that the analysis would reveal a iron-nickel composition, with between 5-20% nickel, which is typical of iron meteorites.

However, when analysed the sample came back as ~85% iron, ~1.5% rubidium and between 10-15% carbon (from the coating applied to the sample). We analysed a number of spots all over the sample, and every time the results were that the sample was composed of almost pure telluric (native) iron with ~10% carbon (from the coating), a trace of rubidium, but no detectable nickel.

If the composition doesn’t contain nickel, then that should rule out a meteoric origin, but the composition also doesn’t match known telluric (native) iron.

Terrestrial telluric (native) iron is extremely rare, and is associated with mafic basalts. There are only a handful of locations where it can be found including Germany, Kamchatka, and the only significant accumulation Disko Island in Greenland. These are all a very long way from where my wombok was grown. There are two types of telluric iron (Type 1 and Type 2) and both contain under 3% nickel, which distinguishes them from iron meteorites. Type 1 can contain up to 4% carbon, 0.05-4% nickel, making it a brittle form of telluric iron. Type 2 contains around 0.05-4% nickel and less than 0.7% carbon, making it malleable enough for native Inuuit inhabitants to make use of it for tools (which I thought was cool), but this doesn’t really help me to understand the origins of my wombok sample.

The last time I checked, farming tools were also not made of pure iron, and modern steel would have approximately 10-18% chromium and 0.2-2% carbon. Some specialist steels may also contain variable amounts of molybdenum, manganese or vanadium, but our sample does not contain any of those either.

Did I find a space rock in my wombok?

The SEM and EDS data made the story of the origin of this sample, and how it got into my lunch, even less clear. Not being a specialist in extraterrestrial geoscience or iron, I asked a few learned colleagues at the university what they thought, but no-one had any particular ideas, so I put the sample on a shelf to be looked at in the future. I have considered booking a session on the laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) lab to get quantitative data on the composition, but haven’t got around to it, so the mysterious sample has been sitting on a shelf for about 18 months gathering (space) dust.

If you have any theories on what the sample might be, or suggestions for ways to positively identify the sample I’d be pleased if you could add them to the comments!