Using drones and machine learning to search for an observed meteorite in Australia's Western Nullarbor.

Meteorites can tell us much about the history of the Solar System, having formed in the protoplanetary nebula from which that system formed. They provide useful insights into the physical and chemical compositions of asteroids, and even larger Solar System bodies, but can be hard to find, and begin to be affected by processes on Earth from the moment they arrive. In recent years, fireball observatory networks have been set up in a number of areas, each of which uses a series of cameras to track meteor falls, enabling scientists to track their trajectories and therefore an approximate idea of where any meteorites might have fallen, as well as to calculate their original orbital paths, providing valuable insights into the connection between the composition of meteorites and their origin within the Solar System. However, locating meteorites is still a labourious process, usually involving a team of trained searchers walking in a line 5-10 m apart, sweeping the area of the fall until the meteorite is found, or, more often, the search is abandoned; only about 20% of such searches are successful.

In a paper published on the arXiv database at Cornell University on 3 March 2022, Seamus Anderson, Martin Towner, John Fairweather, Philip Bland, Hadrien Devillepoix, Eleanor Sansom, Martin Cupak, Patrick Shober, and Gretchen Benedix of the Space Science and Technology Centre at Curtin University present the results of a study which used drones and machine learning to search for an observed meteorite in Australia's Western Nullarbor Desert.

The observed meteor fell over the Lintos Paddock area of Kybo Station in Western Australia on the evening of 1 April 2021. This was observed by two cameras operated by Curtain University's Desert Fireball Network, located at Mundrabilla Station and O'Malley Siding, 149 km and 471 km to the east of the end point of the meteor's trajectory. The object was traced from an initial altitude of 87 km until it was only 25 km above the ground, during which time it slowed from 25.4 km per second to 8.4 km per second over a period of 3.1 seconds, while travelling on a slope of 64°. Because of the distance from the observation sites, and the lack of triangulation due to both sites being in the same direction, Anderson et al. created a series of models to predict the final end point of the object.

 

The DFN 09 meteorite fall at Kybo Station, Western Australia. (Clockwise from top) Fireball observations from DFN camera stations at Mundrabilla Station and O’Malley Siding, and their location within Western Australia; The 90% certainty searching area (transparent white), the best fit fall line (red markers), and the location of the recovered meteorite (yellow star); Pre-impact orbit for the DFN 09 meteoroid. Anderson et al. (2022).

Anderson et al. calculated that the object would have a mass of between 150 g and 700g, and identified an area 5.1 km², within which there was a 90% certainty of the meteorite having fallen. This was a high enough certainty to warrant a visit to the area, resulting in a three-day field trip, during which the area was surveyed with a drone and the data from the survey processed by machine learning, resulting in the eventual recovery of a 70 g meteorite.

Favourable and unfavourable prediction distributions from two images. Given a 70% confidence threshold, Image/Distribution (A) will return 3 meteorite candidates, while Distribution (B) will return over 100 candidates. (B)-like images are later used for retraining. For clarity, the number of detections displayed in image (B) is capped at 50. Anderson et al. (2022). 

A DJI M300 drone with a Zenmuse P1 camera was used to study the target area, producing 57 255 images with a 20% overlap. Of these, Anderson et al. were able to process 5096 on site, producing a total of 46 501 000 tiles for their machine learning algorithm to analyse, by comparing patterns to a database of known Nullarbor objects. From these tiles, the algorithm identified 56 384 first stage candidates, which were then studied with a 3x3 grid graphical user interface to eliminate obvious false positives. This reduced the number of candidate tiles to 259 second stage candidates, which were inspected with a second user interface, this time allowing toggling and zooming, to identify more likely objects. This reduced the number of candidates to 38, which were then revisited by the drone for closer inspection, finally reducing the number of candidate objects to four, which were then visited directly by Human researchers.

 

The four stage process for eliminating false positives and verifying meteorite candidates. (From Left to Right) (1) Grid GUI. (2) Zoom-pan GUI. (3) Drone visit. (4) In-person visit.

The meteorite was found less than 50 m from the ideal line predicted by Anderson et al. in the 88th image taken on by the third flight of the drone on the first day of the study. While it has not formally been studied or classified yet, it is a 70 g object measuring approximately 5 x 4 x 3 cm, with a preferentially smoothed side, and a fusion crust typical of chondritic meteorites that have passed through the atmosphere.

 

The recovered meteorite as seen in person (top two), and from the survey drone (bottom one). For scale, a 15 cm long felt pen is placed next to the meteorite (top right). The yellow box in the bottom image is 22 cm on one side. Anderson et al. (2022).

Although their software enabled them to find and recover the meteorite rapidly, Anderson et al. are at pains to emphasise that they have not developed a meteorite-detection program, but rather a program for detecting anomalous objects in images of an area, in this case the Western Nullarbor. As well as the meteorite, the program detected a range of other anomalies, including tin cans, bottles, Snakes, Kangaroos, and piles of bones from multiple Animals. They also note that the program detected equipment used by the surveyors and left around their camp, and which also had not been included in the training data given to the program. They hope that in future the software can be used not just to identify meteorites, but also in fields such as wildlife monitoring, or search and rescue.

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Preserved magnetic field in a mineral grain from the Bishunpur Meteorite.

Magnetic fields are believed to have played an important role in the accretion of material within the protoplanetary disk from which our Solar System formed, but finding evidence of such ancient fields has proved difficult. In theory, traces of ancient magnetic fields could be preserved in certain minerals within unequilibrated chondrites (asteroids that formed within the protoplanetary disk, and which have not undergone any significant subsequent alteration. The best candidates for this are low-nickel (i.e. high iron) kamacite grains with uniform magnetic fields, preserved within dusty olivine crystals from unequilibrated chondrites, though finding such mineral grains requires some dedication, so that ancient uniform magnetic fields have been found to date in mineral grains from only a single source, the Semarkona Meteorite, a chondritic meteorite that fell to Earth in Madhya Pradesh, India, in October 1940. All other such grains examined to date have possessed non-uniform magnetic fields, the long-term stability of which is uncertain.

In a paper published in the journal Nature Communications on 21 March 2018, a team of scientists led by Jay Shah of the Department of Earth Science and Engineering at Imperial College London, the Department of Earth Sciences at the Natural History Museum, and the Department of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology, describe a magnetized kamacite grain from the Bishunpur Meteorite, an unequilibrated ordinary chondrite that is thought to have formed early in the history of the Solar System (about 4.6 billion years ago) and not undergone significant subsequent alteration before landing in Uttar Pradesh, India, in April 1895.

Like all other previously examined low-nickel kamacite grains, except those from the Semarkona Meteorite, the Bashipur Meteorite grains appear to have a non-uniform magnetic field. Shah et al took nineteen such grains, 150–600 nm in size, and examined them using a scanning tunnelling electron microscope to perform off-axis electron holography scans, first at room temperature and then at 100°C intervals until 800°C was reached, with the samples being held at each temperature step for ten minutes before being scanned.

Using this method, Shah et al. were able to establish that the non-uniform magnetic fields were in fact vortexes, and that they remained stable when heated to 600°C, twice as much heating than they are likely to have encountered due within an asteroid over the course of the history of the Solar System (after this the grain began to react chemically with the surrounding olivine).

Magnetic induction map of a Bishunpur kamacite grain. Magnetic induction map of a kamacite grain in dusty olivine reconstructed from electron holograms acquired at room temperature. The contour spacing is π radians. The direction of the projected in-plane magnetic induction is indicated by the arrows and the colour wheel. Scale bar is 200 nm. Shah et al. (2018). 

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Österplana 065: A unique meteorite from the Middle Ordovician of Sweden.

Ordinary chondrites, or stony meteorites, are the most abundant group of meteorites falling on Earth today, forming about 85% of all such objects. They are divided into three subgroups, based upon their mineralogy and chemical composition, H (42% of the total), L (47% of the total) and LL (11% of the total), which are thought to be derived from three different parent bodies, asteroids broken up by some ancient collision or other event. The advent of isotopic dating methods has enabled scientists to calculate the ages of these meteorites, with type H and LL chondrites all being older than 3.5 billion years (i.e. older than these methods can effectively date, and probably dating from close to the origin of the Solar System, but type L chondrites have proved to be much younger, dating from about 470 million years ago, the Middle Ordovician on Earth. Interestingly this coincides with a great increase in the number of meteorites and micrometeorites falling to Earth, as recorded in marine sediments from the time, an increase which has been linked to the Great Ordovician Biodiversification Event, a sudden increase in the turnover and evolution of many marine invertebrate groups, which led to the replacement of many dominant invertebrate groups from the Cambrian with others more typical of the later Palaeozoic. 

In a paper published in the journal Nature Communications on 14 June 2016, Birger Schmitz of the Astrogeobiology Laboratory at Lund University and the Hawai’i Institute of Geophysics and Planetology at University of Hawai’i at Manoa, Qing-zhu Yin and Matthew Sanborn of the Department of Earth and Planetary Sciences at the University of California at Davis, Mario Tassinari, also of the Astrogeobiology Laboratory at Lund University and Caroline Caplan and Gary Huss, also of the Hawai’i Institute of Geophysics and Planetology at University of Hawai’i at Manoa, describe a unique meteorite from the Middle Ordovician limestone deposits of Thorsberg Quarry in southern Sweden.

Micrometeorites, which have very low masses and are very numerous, are fairly common in the rock record, but larger meteorites are very rare, and when found are often badly deformed by their impact and later geological movements, and in addition often have most or all of their minerals replaced, (as in biological fossils). The limestone deposits at Thorsberg Quarry are remarkable in that they preserve a large number of meteors that fell into a shallow sea with a seafloor covered by soft lime clays, which are preserved without any deformation. As with other ancient meteorites, most of the original minerals have been replaced, however some spinels and other minerals have been preserved, allowing some analysis of the meteors. To date over a hundred meteors from Thorsberg Quarry have been studied, all of which have been L type ordinary chondrites.
 

The new meteorite described by Schmitz et al., Österplana 065, does not conform mineralogicaly or chemically with an L type chondrite, or any other previously described meteor type. It appears to lack any of the type of chromite mineral grains which are common in ordinary chondrites, nor does it have any aluminium spinels, the most common type in L chondrites, having a mixture of chromium spinels and chromium-titanium spinels. Furthermore no silicate minerals could be found within the new meteorite, but large rutile grains could; this combination of chromium spinels plus rutile is not known from any of the approximatwly 52 600 meteorites examined and classified by scientists to date.

The Österplana 065 fossil meteorite from the Glaskarten 3 bed. The meteorite is 8 × 6.5 × 2 cm large. It is surrounded by a grey reduction halo, in the otherwise red limestone. Oxygen was consumed when the meteorite weathered on the sea floor. The coin in the image has a diameter of 2.5 cm. Schmitz et al. (2016).

While Österplana 065 is mineralogically different to the type L chondrites it was found alongside, it does share one other feature with them. Cosmic-ray exposure age dating is a method used to date meteorites, by studying nuclides (atoms of a specific isotope of an element) produced by cosmic rays striking minerals on the surface of the rock. Since a rock burried within the centre of an asteroid will not be exposed to cosmic rays, the amount of such nuclides can be used to estimate the time that such a rock has been exposed to space, presumably by the break-up of a larger body. All of the Thorsberg Quarry meteorites, including Österplana 065, yielded cosmic-ray exposure ages of less than a million years, suggesting that they had been released from inside a larger body by a collision less than a million years before falling to Earth and being buried again. Schmitz et al. suggest that this could imply Österplana 065 is a relict of the body which collided the parent body of the L type chondrites, in the collision that resulted in the formation of those bodies, a body of which we have to date discovered no other traces.

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The nature of the Košice Meteorites.

On 28 February 2010 a meteor shower fell over Slovakia, accompanied by a bright fireball and a series of sonic booms. Subsequently a number of meteorites were recovered from the area to the northwest of the city of Košice, in the east of the country, most within four weeks of the observed shower (meteors are ‘shooting stars’ observed in the sky, a meteorite is an actual piece of rock of presumed extra-terrestrial origin).

In a paper published in the April 2014 edition of the journal Planetary and Space Science and on the arXiv database at Cornell University Library on 4 April 2014 a team of scientists led by Tomáš Kohout of the Department of Physics at the University of Helsinki and the Institute of Geology of the Academy of Sciences of the Czech Republic, describe the results of an examination of 67 of these meteorites for bulk and grain density, porosity and magnetic susceptibility (a proxy for iron content). These are non-destructive methods, unlike conventional mineralogical and chemical methods, which enabled the examination of a greater number of samples.

A fusion encrusted meteorite from the Košice Meteorite Shower. Jon Taylor/Wikimedia Commons.

Kohout et al. found that the Košice Meteorites had an mean bulk density of 3.43 g/cm3, and a mean grain density of 3.79 g/cm3. This is typical for an H chondrite (High iron ordinary chondrite) type meteorite . This composition appeared to be true throughout all of the meteorites, with no grains of any material that did not conform, indicating that the meteorites were of a homogenous nature, and therefore presumably originated from a homogenous body.

The samples had a mean porosity of 9.88%, though this was more variable, ranging from 4.2% to 16.1%. This is again within the range of typical values for H chondrites, though the mean value is a little on the high side. There was a relationship between fragment sizes and porosity, with saller samples tending to have higher porosities. This is in line with prediction for such bodies, as an object breaking up in the atmosphere would tend to fracture more freely in areas with higher porosity, with the result that such areas will produce more numerous, smaller fragments.

The meteorites had a mean magnetic susceptibility of 5.35, again within the typical range of H chondrites. 

From this data Kohout et al. conclude that the parent body for the Košice Meteorites was a H chondrite with a homogenous composition (unhomogenous bodies are thought to be fragments of larger bodies, that had enough gravity for the separation of minerals to have occurred during their formation).

It had been suggested  that the Košice Meteorite shower was the product of two bodies breaking up in the atmosphere rather than one; Kohout et al.’s findings suggest that if this were to have been the case then the two bodies must have had a recent common origin; i.e. have been fragments of a parent body that broke up recently enough that these fragments had not had time to separate significantly.

See also….

 Ancient meteorite found in Minnesota field.

 Second meteorite from New Haven County, Connecticut.






 Connecticut house struck by meteorite.

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The origin of the Bosumtwi Impactor.

Lake Bosumtwi is an 8 km diameter roughly circular lake about 30 km to the southeast of Kumasi in the Ashanti Province of Ghana. It is thought to have been created by a large meteorite impacting the Earth roughly 1.07 million years ago during the Late Pleistocene; the impact crater being roughly 10.5 km in diameter, with the lake sitting in a central depression. Tektites (glassy particles thought to be formed as ejecta from impact craters) associated with this site are found over a large area in neighboring  Côte d'Ivoire.

In a paper published on the online arXiv database at Cornell University Library on 15 May 2013, a team of scientists led by Mattia Galiazzo of the Institute for Astrophysics of the University of Vienna examine the origin of the Bosumtwi Impact Crater, and attempt to determine the origin of the object that caused it.

The Lake Bosumtwi impact crater, Ghana. University of Texas at Austin.

It has perviously been calculated that the Bosumtwi Impactor was probably an ordinary chondrite, 750 m - 1 km in diameter that impacted at a velocity in excess of 15 km per second, and probably over 20 km per second. It has furthermore been suggested that in order to create the field of tektites in Côte d'Ivoire, it must have impacted from the northeast at an angle of between 30° and 45°.

The location of Lake Bosumtwi. Google Maps.

Working from this data Galiazzo et al. attempted to build a model of the position of the Earth and direction of impact at the time of the impact event, in order to determine the origin of the impactor. Based upon this model they conclude that the impactor probably originated within the Middle Main Asteroid Belt, and furthermore that it probably had an orbit highly inclined to the plain of the Solar System, possibly as much as 75°. This places it within a group of asteroids including 2002 MO3, 2009 XF8, 2002 SU and 2010 RR30, which have not been studied as a cluster, but which may well share a common origin.

The orbit of 2002 MO3. JPL Small Body Database Browser.

See also The origin of the Chelyabinsk Meteor, Asteroids in retrograde orbits, The nature and history of 'Quasi-Hilda Object' 2000 YN30, The dust tail of 3200 Phaethon and The ejecta of Main-Belt Comet P/2012 T1 (PANSTARRS).

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Connecticut house struck by meteorite.

Larry Beck of Wolcott, Connecticut, was disturbed by a loud crashing noise at 10.30 pm local time on Friday 19 April 2013 (2.30 am on Saturday 20 April GMT), which was accompanied by the appearance of a crack in the ceiling of his kitchen. When he investigated the next morning he found a hole in his roof and a chunk of rock on the floor of his attic. He was persuaded by a friend to take this to the Peabody Museum in New Haven, where Mineralogy Collections Manager Stefan Nicolescu confirmed the rock to be a meteorite.

The Wolcott Meteorite. 7 News.

Nicolescu described the meteorite as an ordinary chondrite, with a thin black fusion crust from its passage through the atmosphere. Ordinary Chondrites (or Stoney Chondrites) are the most abundant form of meteorite, forming roughly 87% of all known specimens. They contain a mixture of iron, iron oxides and silicates, this one appeared to be have a fairly high iron content, and was able to attract a magnet (again, this is quite common). Nicolescu also suggests that due to the time when this meteorite fell that there is a good chance that it is part of the Lyrid Meteor Shower, which would also imply that it is originally a chunk of Comet C/1861 G1 Thatcher (named after the astronomer A. E. Thatcher, not the politician).

Stefan Nicolescu demonstrating the magnetic properties of the Wolcott Meteorite. Melanie Brigockas/Yale.

See also The Lyrid Meteors, Fireball over Wyoming, Fireball over the northeastern United States, Comet C/2011 L4 (PANSTARRS) to reach its closest point to Earth this week and 99942 Apophis to fly by the Earth.

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