Every year, more than 5,000 tons of material with cosmic origins lands on Earth’s surface, with as much as 15,000 tons of this “space dust” making its way into the atmosphere but vaporizing during reentry.
The resulting rain of micrometeorites that reaches our planet consists mostly of tiny objects anywhere from 30 to 200 micrometers in size, based on past studies. But what are the origins of these large volumes of material that accumulate over time as they shower the Earth throughout the year?
According to new research, evidence for one potential source of this cosmic debris has now been found: it could be coming from unidentified near-Earth objects.
Space Dust with Unknown Origins
The problem with tracing the origins of space dust is that most of it that makes its way into Earth’s atmosphere doesn’t survive the intense conditions of atmospheric entry, and those tiny bits that do—known as cosmic spherules—are reshaped as they melt from the friction they encounter while falling.
Although their original mineral structure is compromised as well as their shape, there are a few isotopes—namely oxygen—that these spherules maintain that scientists can use to help trace their origins. Based on their chemical “fingerprints,” cosmic spherules are placed into different groups by planetary scientists, with many being able to be traced back to known varieties of meteorites and other sources.
Among these groups, around ten percent fall into a category possessing unusually small quantities of the isotope oxygen-16. This category, known as Group 4, doesn’t appear to match any known meteorite group—the implication, therefore, is that some of this cosmic debris may originate from an unidentified variety of near-Earth objects.
New Clues to the Origins of Uncorrelated Space Dust
Now, according to new research by Matthias Van Ginneken and colleagues, there are at least some ways that the origins of these spherules can be determined, one of which involves their orbital parameters.
In one case, the team writes in a recent study published in Science Advances, “the mineralogical and textural properties of a subset of cosmic spherules do provide information on the orbital parameters of their precursors, including eccentricity and encounter velocity.”
This group of materials, which is dubbed “CumPo” based on its material consistency of cumulate olivine porphyritic, is “characterized by clustered olivine phenocrysts that increase in size from one side of the spherule to the other,” the team writes.
In short, this unique class of spherules points to equally unique orbital characteristics that may be associated with what the research team surmises could be atypical types of near-Earth objects, or NEOs, that could be near enough to our planet to provide a likely source.
A particular clue offered by these objects is that some of them may have undergone unusually high entry speeds, as evidenced by their surface texture. One factor that could potentially account for this is that they may have entered Earth’s atmosphere at angles consistent with objects possessing high orbital eccentricity.
More Clues from Antarctica
As with past studies, researchers were able to retrieve several samples of these CumPo cosmic spherules from Antarctica, which provides a pristine environment that helps to facilitate their discovery and collection. However, another source location had been far less remote: urban rooftops, where a large amount of cosmic dust lands every year.
With the aid of electron microscopy and microprobe chemistry studies, Ginneken and colleagues examined the mineralogy, chemistry, and textures of these spherules, as well as the oxygen isotopes they contained.
What they found indicated an entirely new subset of spherules, which possess significant amounts of cumulate olivine rich in sulfur, garnering the name “SCumPo.” This unique new class of samples has links to the “Group 4” class and their minimal inclusion of oxygen-16. Other unusual similarities they possess are an overall absence of magnetite and a low presence of nickel in their olivine crystals, paired with an abundance of iron-nickel sulfur and a significant amount of sulfur present within glasses produced from vitrification.
Intriguingly, some spherules appeared to show a combination of signatures that were both poor in oxygen-16 in some areas, while rich in the isotope in others. This suggests that these cosmic materials had likely been mixed from two separate original sources, since such conditions could not be otherwise explained through natural processes.
“We interpret this as strong evidence that the SCumPo precursors were composite materials containing at least two different components,” the team writes in their study. Then, by running numerical simulations of crystal settling to help determine the likely entry speeds of these materials, and thereby their eccentricities while in orbit, the team determined that the settling of olivine is most consistent with entry speeds of as much as 17 kilometers per second, and likely no more than 14 kilometers per second—speeds that would be consistent with NEOs, as opposed to asteroids originating from the main-belt.
Evidence for an Unknown Class of Space Objects
Based on their findings, the team believes that unsampled meteorites from the CM, CO, and CY classifications of carbonaceous chondrites—a group of primitive, carbon-rich meteorites containing some of the oldest materials in our Solar System—are the likely sources for these cosmic materials.
“Given that we have linked their composition to the CM-CO-CY clan of carbonaceous chondrites, it is plausible that their parent body was a primitive carbonaceous asteroid that migrated onto an Earth-crossing orbit—attaining comet-like orbital parameters,” the team writes.
“For instance, one might consider the disrupted fragments of a thermally altered but water-bearing asteroid (like the CY group) that evolved into near-Earth space,” they add.
Although still only inferred from their studies, the team suggests the existence of a hypothetical new variety of NEOs based on the samples they studied, which are unlike any variety of meteorites currently known on Earth.
“These findings point to a previously unsampled, primitive, sulfide-rich CY-like near-Earth asteroid, which represents a ‘missing’ meteorite parent body that contributes distinctive 16O-poor cosmic dust to Earth,” the team writes, noting that any future confirmation of the existence of this unknown class of meteorites would represent “a pivotal discovery.”
The team’s new paper, “16O poor cosmic spherules from near-Earth CY chondrite asteroids,” appeared in Science Advances.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.