‘The line is getting fuzzier’: asteroids and comets may be more similar than we think

As anyone who has ever tried to clean a home knows, ridding yourself of dust is a Sisyphean effort. No surface stays free of it for long. It turns out that space is somewhat similar. Space is filled with interplanetary dust, which the Earth constantly collects as it plods around the sun – in orbit, in the atmosphere, and if it’s large enough, on the ground as micrometeorites.

While specimens may not be large, it turns out such dust particles are reforming scientists’ conception of asteroids and comets and are enough to reconstruct entire scenes in the history of the solar system.

Asteroids and comets are primitive bodies left over from early in solar system formation, so the more we can know about their composition, the more we know about where they formed. Those asteroids that formed in the same neighbourhood as comets tend to be closer in composition to them.

Trying to break down the asteroid-comet continuum and categorise how similar asteroids could be to comets is what Dr Pierre Beck is doing in the SOLARYS project at France’s University of Grenoble Alpes.

There are about a million asteroids registered officially and there should be many more, he explains.

‘Traditionally, these objects have been thought of as the most primitive in the solar system. You can look at the ingredients and see what was there, how they were accreted and how they were formed a long time ago.’

Similar primordial material that formed Earth or Mars has experienced geological activity and been fundamentally changed by conditions like heat, pressure and erosion.

‘The most primitive objects therefore don’t come to Earth in the form of rocks, but in the form of dust,’ he said. ‘While the expected (amount) of meteorites that come to Earth in a year may be 5-6 tonnes – for dust it is 40,000 tonnes.’

Using samples of interplanetary dust collected from high in our stratosphere and micrometeorites from pristine locations like Antarctica, Dr Beck is using a new method of infrared spectroscopy combined with atomic force microscopes to examine their spectra and properties on the micrometre-scale.

Like an archaeologist placing artefacts from a dig site, he can then compare those results to existing data from asteroids in space. ‘When you’re a geologist and you find a rock, you have an outcropping and try see the rock in its context,’ Dr Beck said.

‘In the past we thought asteroids are rocks, comets are icy. But now we see that there are comets that are almost inactive…and there are asteroids that are active.’

Dr Jessica Agarwal, Technical University of Braunschweig, Germany

Compounds

Using changes in infrared laser light on samples that are just 10-20 micrometres, his team can for the first time pick out silicate minerals and organic compounds without using harsh chemicals that would disturb the material. They also construct bigger models of the samples in the lab to refine what to look for to identify and categorise asteroids and comets with ground-based telescopes.

What they have found in the dust are complex organic polymers, rich in hydrocarbons and elements like nitrogen and oxygen or sometimes deuterium (heavy water).

‘There is a big debate on how these extra-terrestrial organics were formed. One hypothesis is that ice mixtures were irradiated, but in that case different types of ice mixtures should yield different types of organics,’ said Dr Beck.

Studying the chemical composition of these samples should help him to learn more about asteroids’ origins as well as the difference between D-type asteroids, dark and difficult to detect bodies, some with icy interiors, which originate around Jupiter and beyond, and icy comets.

‘If we understand that, it will tell us what the outer solar system is made of and more about the initial stuff that came into the solar system.’

Knowing where certain organic dust types can be found could even help future space probes.

‘You could view some of these asteroids as a fuel source,’ he said. If there are reduced organic compounds, he says, they could be used as a source of energy.

Comets

The presence of such compounds in interplanetary dust is just one thing making scientists wonder if asteroids and comets aren’t necessarily so different after all. Dr Jessica Agarwal at the CASTRA project thinks there may be overlap for other reasons, too.

Using data from the European Space Agency’s Rosetta probe that studied Comet 67P/Churyumov–Gerasimenko and from astronomical telescopes, Dr Agarwal and her team at the Technical University of Braunschweig in Germany looked at how comets and asteroids actively emit material into space.

‘We aim to better understand the processes that lead to changes in the surfaces and interiors of comets and asteroids,’ she said. ‘We also hope to better understand their primitive nature, or how they were 4.5 billion years ago.’

Using data from several instruments onboard Rosetta, Dr Agarwal’s team has been able to model the properties of cometary dust in the environment of Comet 67P. They found that the dust particles could be loose aggregates of micron-sized silicate and sub-micron-sized carbonaceous components.

‘We are also observing huge boulder-size materials coming out from Comet 67P, coming from certain specific places on the surface…a fountain of boulders,’ Dr Agarwal explained.

Active asteroid

Comets are not the only bodies to emit material. Take the case of asteroid 288P. A so-called active asteroid that emits dust, from a distance it looks like a comet with a dusty tail.

‘The weird thing about 288P was that its nucleus looked double…and in the end, I thought, well maybe it’s a binary?’ Dr Agarwal said. ‘We had to wait a couple of years to reobserve it from close up, and then in 2016 we got more Hubble time and really saw that it was two components.’

Their measurements determined that this first-of-its-kind asteroid to be observed is comprised of two similarly-sized pieces, orbiting each other 100 kilometres apart.

‘We found it by chance. We don’t know if there are more systems like it that we don’t see,’ Dr Agarwal said.

They theorise that the asteroids were irradiated by the sun and begin to rotate, splitting in two when they spun too fast to hold together. The distance between the pair may be due to a jet of gas vaporising from the surface that propelled one rock away like a rocket. They are still trying to figure out what causes the tail.

Scientists have long thought that asteroids mainly evolved through collisions, but it’s possible that for smaller asteroids, fast rotation plays just as much of a role.

Their research has revealed a range of active asteroids, from those which have a one-off burst of activity (as if from an impact), to those that emit bursts of dust repeatedly.

‘There is some process happening more or less randomly that triggers the eruption of dust clouds,’ Dr Agarwal said, referring to asteroids which emit the repeated dust bursts. ‘We think maybe it is fast rotation that triggers landslides or something like that.’

The upshot of all of this is that the distinction between comets and asteroids may be more of a spectrum than a hard divide.

‘The line is getting fuzzier. In the past we thought asteroids are rocks, comets are icy. But now we see that there are comets that are almost inactive…and there are asteroids that are active. There is more of a transition between those two populations than we thought in the past,’ Dr Agarwal said.

The research in this article was funded by the EU’s European Research Council.

Originally published on Horizon Magazine

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Q&A: Nanobots could explore human cells – but their size is an engineering challenge

Scientists are developing virus-sized robots that could defuse blood clots, explore human cells or even scrub water of impurities.

But as these inventions get smaller, the laws of motion that govern these machines are not very intuitive, so researchers are drawing inspiration from nature, says Brad Nelson, professor of robotics at ETH Zürich, Switzerland, who focuses on these tiny intelligent machines down to nanometres in size.

’We look at what kinds of microorganisms there are out there…to get some insight into the way nature works and figure out whether we have some technology or engineering possibilities that we could use to solve these problems,’ he said.

What are these tiny robots like?

There are two kinds. Microrobots are usually smaller than a millimetre but as large as a micron — that’s between a 1,000th of a metre to a millionth of a metre in size. Nanorobots are even more minute — smaller than a millionth of a metre, or about a 100^th the width of a hair. In other words, microrobots are roughly bacteria sized, while nanorobots are about the size of a virus.

They’re made of different chemicals, including carbon and metallic elements — and given these robots are in many cases being developed for use in the human body, they’re typically composed of materials that are compatible with our innards. For example, we like to use iron molecules because iron tends to be biocompatible, and we can also use magnetic fields to make them move.

The ones we’re working on look like little metal cages made out of iron that are flanked by polymer branches that hang out. The devices look almost organic, like some kind of weird little microorganism — and with our magnetic fields they are equipped to ‘walk.’

Other robots look like tiny blobs and some of them are even uninteresting looking, like a little speck, except there’s a lot of chemistry going on inside.

What is the potential of nanorobots?

Nanorobots are really more research topics right now, there’s not probably going to be any near-term medical applications of them due to the engineering and medical challenges associated with developing the technology at such a tiny scale.

Some groups are working on emulating size and swimming capacity of the flagellated tails of certain bacteria like E. coli — the tails have a little rotary motor about 40-50 nanometres in diameter that spins at several 100 revolutions per minute. That’s a sort of a nanorobotic mechanism that nature has evolved.

Even the Covid vaccines that use microRNA — the instructions that get taken into your cells to provoke the production of antibodies to fight infection — you could think of it as a type of nanorobotic mechanism. What our field is trying to do is make more complicated devices that can mimic some of these molecular mechanisms.

But I think the biggest promise of nanorobots is in the understanding of how biological systems work. It’s more of a tool for exploring cells — understanding the mechanics of what’s going on inside of the cell. I wouldn’t say they’re going to be therapies in the near term — they’re going to be more like tools for doing research.

Are there any other uses?

When we were tried to create a nano-machine to help us deliver drugs to treat, for example, a tumour — what we kind of accidentally discovered was that we were actually destroying the drug. This was exactly what we didn’t want to do, but it gave the team an idea that the approach could be used instead to destroy micropollutants in water, so we now are working on that.

We’re also working on building micro-catheters that can deliver microrobots near the location of disease — and then the microrobots can then deliver nanorobotic devices that will cause disruption that could, for instance, help break down plaque or defuse a blood clot.

‘I think the biggest promise of nanorobots is in the understanding of how biological systems work.’

Professor Brad Nelson, ETH Zürich, Switzerland

What are the biggest challenges in the field?

Since these devices are incredibly small, the physics that govern their motion is very non-intuitive — so we try to find microorganisms that are navigating these limitations, for instance, by changing their shape during their lifecycle. Then it’s about understanding the physics of that and figuring out whether I have the technology that would allow me to mimic that or be ‘bio-inspired.’

Another big issue is what materials to use to make the devices. Some materials such as cobalt and some rare earth metals have properties that are desirable, but they are toxic to the human body.

On the robotic side, it’s about coordinating magnetic fields to get the device to go just where I want it, and to navigate through complicated blood vessels. A lot of what we do is surprisingly similar to the people working on self-driving cars. If I’m in my office in Zurich and I want to get across the river over to the other side — I’ve got to plan a path and have a roadmap of how to get there. We do the same thing when we’re inside an organ, we create roadmaps of all the blood vessels, and then try to navigate through the pathways.

And there’s the need for funding. It takes a lot of investment to get these things right — government money only takes you so far, but you really need the kinds of investment that large companies can do to take you across the regulatory finish line.

We also need to work closely with the medical doctors, because they’re the ones that will be using this technology to treat patients. Often, they’ll have unrealistic expectations of what maybe we can deliver — and we can also have unrealistic expectations of what they’re really doing; so, bringing together the medicine, the science and technology is a challenge.

Overall, the longer I work in the field, the more excited I get, and the more I’m convinced that we’ll see these devices being used to improve human health within my lifetime, and certainly within my professional lifetime.

Prof. Nelson has received funding from the EU’s European Research Council for his SOMBOT project.

Originally published on Horizon Magazine

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Baby Mantis Shrimp Don’t Pull Their Punches

Mantis shrimp don’t need baby food. They start their life as ferocious predators who know how to throw a lethal punch.

A new study appearing April 29 in the Journal of Experimental Biology shows that larvae of the Philippine mantis shrimp (Gonodactylaceus falcatus) already display the ultra-fast movements for which these animals are known, even when they are smaller than a short grain of rice.

Their ultra-fast punching appendages measure less than 1 mm, and develop right when the larva exhausts its yolk reserves, moves away from its nest and out into the big wide sea. It immediately begins preying on organisms smaller than a grain of sand.

Although they accelerate their arms almost 100 times faster than a Formula One car, Philippine mantis shrimp larvae are slower than larger adults, which goes against the theoretical expectation that smaller is always faster.

“They’re producing amazing speeds and impressive accelerations relative to their body size, but they’re not as fast as adults,” said Jacob Harrison, a PhD candidate in biology at Duke University and lead author of the study.

Mantis shrimps achieve their ultra-fast movements through a tiny spring-actuated mechanism hidden in their punching appendage. A muscle contracts, deforming a tiny segment of their exoskeleton –the rigid cuticle that covers their body. This contraction allows elastic energy to be stored in the locked joint. Once the latch releases, the exoskeleton springs back into its natural position, violently propelling the appendage forward with ultra-fast speeds.

Engineering and physics models predict that smaller organisms, who have a smaller mass to displace, will be faster than larger, heavier, organisms. Mantis shrimp larvae show that biology doesn’t always follow the theory.

“Theoretically, they should be producing the highest acceleration,” said Harrison, “but we don’t find that.”

Harrison explains that this discrepancy may be due to multiple factors. The larvae muscles may be too small to effectively load a very stiff spring, or the water resistance at their small size may be too high for their punches to reach the speed that larger individuals reach, among other possibilities.

“There are limitations to these spring and latch structures that we don’t fully understand,” said Harrison. “But whenever biology moves away from theoretical models it highlights some pretty interesting areas for us to learn.”

Mantis shrimp larvae are an interesting system not only due to their small size, but also due to their color, or lack thereof.

Adult mantis shrimps have opaque exoskeletons, rendering the inner working of their spring-latch mechanisms impossible to observe in action. The exoskeleton of larvae, however, is much thinner and fully transparent, allowing researchers to see precisely how these animals manage to store so much elastic energy in their tiny appendages simply by watching them through a microscope.

“One of the trickiest parts of researching spring-actuated mechanisms is that a lot of those elements are working inside the animal. We can look outside of the animal and see the behavior, measure the kinematics, dissect the animal, and say the mechanism looks like it works like this, but there are always levels of assumption,” said Harrison.

“(Transparency) sets up larval mantis shrimps as systems where we can look at how each of these elements work in concert together,” said Harrison. “It removes assumptions and allows us to understand it on a finer scale.”

Larval mantis shrimps are therefore doubly interesting. They highlight discrepancies between physics and biology, and also offer a true window into a better understanding of the mechanism behind ultra-fast movements.

“When something doesn’t match your predictions, the first gut reaction is always to be incredibly frustrated, but this is actually what highlights new areas of research,” said Harrison.

This work was supported by the Company of Biologists Traveling Fellowship (JEBTF181185), by the National Science Foundation (NSF IOS 1439850 and NSF EPSCoR RII 455 1738567), by the Office of Naval Research (N00014-19-1-2035 and N00014-454 17-1-2062), and by the University of Hawai’i at Mānoa. This material is based upon work supported by, or in part by, the U.S. Army Research Laboratory and the U.S. Army Research Office under contract/grant number W911NF-15-1-058.

CITATION: “Scaling and development of elastic mechanisms: the tiny strikes of larval mantis shrimp,” J. S. Harrison, M. L. Porter, M. J. McHenry, H. E. Robinson, S. N. Patek. Journal of Experimental Biology, April 29, 2021. DOI: 10.1242/jeb.235465

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Spongy Trails

Sluggishly you sweep
across the sea,
ceaselessly stimulating
your supposed sessility
as you secrete spiky streams
through dense, porous skeletons.

Interwoven spicules that
suggest clandestine movements,
swaying in the currents
deep
deep
beneath the surface.

A hidden sanctuary of shifting
shapes that stray from sight;
strategies of survival
below submerged peaks
and shattered sentiments.

Trails left by sponges as they crawl across the seafloor (Image Credit: AWI OFOBS team, PS101).

This poem is inspired by research, which has found that sponges leave trails on the sea floor in the Arctic deep sea.

A sponge is a simple animal with many cells, but no mouth, muscles, heart, or brain. The basic body plan of a sponge is a jelly-like layer sandwiched between two thin layers of cells, and there are over 10,000 species of sponge, most of which live in the ocean and feed on bacteria and other microorganisms, although a few of them also eat tiny crustaceans. Adult marine sponges are usually thought to be stationary, picking a spot on the seafloor while still in their larval stage and sticking to it. Lacking muscles to move around, they are generally referred to as ‘sessile’ (i.e. fixed in one place), as opposed to ‘motile’ (i.e. capable of motion) marine creatures. However, new research has found distinct trails across the Arctic seafloor, made of brown spicules (the spikes that belong to the sponges and which provide them with structural support), indicating that some adult sponges are actually capable of movement.

The researchers in this study examined videos taken by the icebreaker Polarstern on a 2016 mission around the Langseth Ridge in the Arctic Ocean. These videos revealed a densely populated community of sponges, 10° further North than any previously reported observations. Over 70% of the images containing sponges also featured trails, observed as densely interwoven spicules connected directly to the underside of individual sponges, thereby suggesting that they were indeed traces of motility. Three-dimensional modelling of the trails also revealed that the sponges changed direction as they moved, and that they moved uphill, implying that the sponges were moving themselves, rather than being shifted by gravity or strong ocean currents. This is the first time that abundant sponge trails have been both observed in a sponge’s natural habitat and directly attributed to sponge mobility. Further research is now needed to better understand exactly why these sponges would move, although it is believed that they may need to do so in order to better access food in the sparse Arctic environment.

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How to get salt out of water: Make it self-eject

About a quarter of a percent of the entire gross domestic product of industrialized countries is estimated to be lost through a single technical issue: the fouling of heat exchanger surfaces by salts and other dissolved minerals. This fouling lowers the efficiency of multiple industrial processes and often requires expensive countermeasures such as water pretreatment. Now, findings from MIT could lead to a new way of reducing such fouling, and potentially even enable turning that deleterious process into a productive one that can yield saleable products.

The findings are the result of years of work by recent MIT graduates Samantha McBride PhD ’20 and Henri-Louis Girard PhD ’20 with professor of mechanical engineering Kripa Varanasi. The work, reported today in the journal Science Advances, shows that due to a combination of hydrophobic (water repelling) surfaces and heat, dissolved salts can crystallize in a way that makes it easy to remove them from the surface, in some cases by gravity alone.

Play video

When the researchers began studying the way salts crystallize on such surfaces, they found that the precipitating salt would initially form a partial spherical shell around a droplet. Unexpectedly, this shell would then suddenly rise on a set of spindly leg-like extensions grown during evaporation. The process repeatedly produced  multilegged shapes, resembling elephants and other animals, and even sci-fi droids. The researchers dubbed these formations “crystal critters” in the title of their paper.

After many experiments and detailed analysis, the team determined the mechanism that was producing these leg-like protrusions. They also showed how the protrusions varied depending on temperature and the nature of the hydrophobic surface, which was produced by creating a nanoscale pattern of low ridges. They found that the narrow legs holding up these critter-like forms continue to grow upward from the bottom, as the salty water flows downward through the straw-like legs and precipitates out at the bottom, somewhat like a growing icicle, only balanced on its tip. Eventually the legs become so long they are unable to support the critter’s weight, and the blob of salt crystal breaks off and falls or is swept away.

The work was motivated by the desire to limit or prevent the formation of scaling on surfaces, including inside pipes where such scaling can lead to blockages, Varanasi says. “Samantha’s experiment showed this interesting effect where the scale pretty much just pops off by itself,” he says.

“These legs are hollow tubes, and the liquid is funneled down through these tubes. Once it hits the bottom and evaporates, it forms new crystals that continuously increase the length of the tube,” McBride says. “In the end, you have very, very limited contact between the substrate and the crystal, to the point where these are going to just roll away on their own.”

McBride recalls that in doing the initial experiments as part of her doctoral thesis work, “we definitely suspected that this particular surface would work well for eliminating sodium chloride adhesion, but we didn’t know that a consequence of preventing that adhesion would be the ejection of the entire thing” from the surface.

One key, she found, was the exact scale of the patterns on the surface. While many different length scales of patterning can yield hydrophobic surfaces, only patterns at the nanometer scale achieve this self-ejecting effect. “When you evaporate a drop of salt water on a superhydrophobic surface, usually what happens is those crystals start getting inside of the texture and just form a globe, and they don’t end up lifting off,” McBride says. “So it’s something very specific about the texture and the length scale that we’re looking at here that allows this effect to occur.”

This self-ejecting process, based simply on evaporation from a surface whose texture can be easily produced by etching, abrasion, or coating, could be a boon for a wide variety of processes. All kinds of metal structures in a marine environment or exposed to seawater suffer from scaling and corrosion. The findings may also enable new methods for investigating the mechanisms of scaling and corrosion, the researchers say.

By varying the amount of heat along the surface, it’s even possible to get the crystal formations to roll along in a specific direction, the researchers found. The higher the temperature, the faster the growth and liftoff of these forms takes place, minimizing the amount of time the crystals block the surface.

Heat exchangers are used in a wide variety of different processes, and their efficiency is strongly affected by any surface fouling. Those losses alone, Varanasi says, equal a quarter of a percent of the GDP of the U.S. and other industrialized nations. But fouling is also a major factor in many other areas. It affects pipes in water distribution systems, geothermal wells, agricultural settings, desalination plants, and a variety of renewable energy systems and carbon dioxide conversion methods.

This method, Varanasi says, might even enable the use of untreated salty water in some processes where that would not be practical otherwise, such as in some industrial cooling systems. Further, in some situations the recovered salts and other minerals could be salable products.

While the initial experiments were done with ordinary sodium chloride, other kinds of salts or minerals are expected to produce similar effects, and the researchers are continuing to explore the extension of this process to other kinds of solutions.

Because the methods for making the textures to produce a hydrophobic surface are already well-developed, Varanasi says, implementing this process at large industrial scale should be relatively rapid, and could enable the use of salty or brackish water for cooling systems that would otherwise require the use of valuable and often limited fresh water. For example, in the U.S. alone, a trillion gallons of fresh water are used per year for cooling. A typical 600-megawatt power plant consumes about a billion gallons of water per year, which could be enough to serve 100,000 people. That means that using sea water for cooling where possible could help to alleviate a fresh-water scarcity problem.

“This work shows a remarkable and interesting phenomenon,” says Neelesh Patankar, a professor of mechanical engineering at Northwestern University, who was not associated with this research. The findings, he says, “may lead to an entirely new approach to mitigate mineral fouling in industrial processes. Not only is this work interesting from a fundamental science perspective, in my opinion it is also of practical importance.”

The work was supported by Equinor through MIT Energy Initiative, the MIT Martin Fellowship Program, and the National Science Foundation.

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Nearly $500 million a year in Medicare costs goes to 7 services with no net health benefits

A UCLA-led study shows that physicians frequently order preventive medical services for adult Medicare beneficiaries that are considered unnecessary and of “low value” by the U.S. Preventive Services Task Force — at a cost of $478 million per year.

The researchers analyzed national survey data over a 10-year period, looking specifically at seven preventive services given a “D” rating by the task force, and discovered that these services were ordered more than 31 million times annually.

BACKGROUND

The U.S. Preventive Services Task Force, an independent panel appointed by the Department of Health and Human Services, makes recommendations on the value of clinical preventive services. Services rated D are considered to have no likely health benefit to specific patients and may even be harmful to them. Overall, the utilization of a variety of services considered unnecessary by the task force drives up health care spending by billions of dollars each year.

METHOD

The researchers examined data covering the years 2007 to 2016 from the annual National Ambulatory Medical Care Survey to determine how often, and at what cost, seven specific grade D services were utilized:

• Asymptomatic bacteriuria screening in non-pregnant women
• Cardiovascular disease screening in low-risk adults (rest or stress ECG)
• Cervical cancer screening in women over age 65 (Papanicolaou or HPV test)
• Colorectal cancer screening in those over age 85 (colonoscopy or sigmoidoscopy)
•  Chronic obstructive pulmonary disease screening in asymptomatic patients (peak flow or spirometry)
• Prostate cancer screening in men 75 and older (prostate-specific antigen test)
• Vitamin D supplementation for fracture preventing in postmenopausal women

The researchers note some limitations to the study. For instance, their method of estimating Medicare spending on these services may lack clinical details and therefore might misclassify some instances of appropriate care as low value.

IMPACT

Medicare could save nearly $500 million per year and protect patients against potential harm by no longer providing reimbursements for these services. Under the Affordable Care Act, the secretary of health and human services is authorized to prohibit payment for services rated D by the Preventive Services Task Force. In February 2021, the U.S. Office of Personnel Management’s Federal Employees Health Benefits Program stopped covering (PDF) grade D services.

AUTHORS

Dr. Carlos Irwin Oronce, Dr. Joseph Ladapo, Dr. Catherine Sarkisian and Dr. John Mafi, all of UCLA, and Dr. A. Mark Fendrick of the University of Michigan. Dr. Mafi is also a researcher with Rand Corp.

JOURNAL

The study is published in the peer-reviewed Journal of General Internal Medicine.

FUNDING

The study was funded by the Veterans Affairs Office of Academic Affiliations (through the VA/National Clinician Scholars Program); the National Institute on Drug Abuse; the National Heart, Lung, and Blood Institute; the National Institute on Minority Health and Health Disparities; the National Institute on Aging, the UCLA Resource Center for Minority Aging Research/Center for Health Improvement of Minority Elders; the National Center for Advancing Translational Science; the UCLA Clinical and Translational Science Institute; the UCLA Vatche and Tamar Manoukian Division of Digestive Diseases; and the National Institute on Aging’s  Beeson Emerging Leaders career development award funded this study.

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‘The line is getting fuzzier’: asteroids and comets may be more similar than we think

As anyone who has ever tried to clean a home knows, ridding yourself of dust is a Sisyphean effort. No surface stays free of it for long. It turns out that space is somewhat similar. Space is filled with interplanetary dust, which the Earth constantly collects as it plods around the sun – in orbit, in the atmosphere, and if it’s large enough, on the ground as micrometeorites.

While specimens may not be large, it turns out such dust particles are reforming scientists’ conception of asteroids and comets and are enough to reconstruct entire scenes in the history of the solar system.

Asteroids and comets are primitive bodies left over from early in solar system formation, so the more we can know about their composition, the more we know about where they formed. Those asteroids that formed in the same neighbourhood as comets tend to be closer in composition to them.

Trying to break down the asteroid-comet continuum and categorise how similar asteroids could be to comets is what Dr Pierre Beck is doing in the SOLARYS project at France’s University of Grenoble Alpes.

There are about a million asteroids registered officially and there should be many more, he explains.

‘Traditionally, these objects have been thought of as the most primitive in the solar system. You can look at the ingredients and see what was there, how they were accreted and how they were formed a long time ago.’

Similar primordial material that formed Earth or Mars has experienced geological activity and been fundamentally changed by conditions like heat, pressure and erosion.

‘The most primitive objects therefore don’t come to Earth in the form of rocks, but in the form of dust,’ he said. ‘While the expected (amount) of meteorites that come to Earth in a year may be 5-6 tonnes – for dust it is 40,000 tonnes.’

Using samples of interplanetary dust collected from high in our stratosphere and micrometeorites from pristine locations like Antarctica, Dr Beck is using a new method of infrared spectroscopy combined with atomic force microscopes to examine their spectra and properties on the micrometre-scale.

Like an archaeologist placing artefacts from a dig site, he can then compare those results to existing data from asteroids in space. ‘When you’re a geologist and you find a rock, you have an outcropping and try see the rock in its context,’ Dr Beck said.

‘In the past we thought asteroids are rocks, comets are icy. But now we see that there are comets that are almost inactive…and there are asteroids that are active.’

Dr Jessica Agarwal, Technical University of Braunschweig, Germany

Compounds

Using changes in infrared laser light on samples that are just 10-20 micrometres, his team can for the first time pick out silicate minerals and organic compounds without using harsh chemicals that would disturb the material. They also construct bigger models of the samples in the lab to refine what to look for to identify and categorise asteroids and comets with ground-based telescopes.

What they have found in the dust are complex organic polymers, rich in hydrocarbons and elements like nitrogen and oxygen or sometimes deuterium (heavy water).

‘There is a big debate on how these extra-terrestrial organics were formed. One hypothesis is that ice mixtures were irradiated, but in that case different types of ice mixtures should yield different types of organics,’ said Dr Beck.

Studying the chemical composition of these samples should help him to learn more about asteroids’ origins as well as the difference between D-type asteroids, dark and difficult to detect bodies, some with icy interiors, which originate around Jupiter and beyond, and icy comets.

‘If we understand that, it will tell us what the outer solar system is made of and more about the initial stuff that came into the solar system.’

Knowing where certain organic dust types can be found could even help future space probes.

‘You could view some of these asteroids as a fuel source,’ he said. If there are reduced organic compounds, he says, they could be used as a source of energy.

Comets

The presence of such compounds in interplanetary dust is just one thing making scientists wonder if asteroids and comets aren’t necessarily so different after all. Dr Jessica Agarwal at the CASTRA project thinks there may be overlap for other reasons, too.

Using data from the European Space Agency’s Rosetta probe that studied Comet 67P/Churyumov–Gerasimenko and from astronomical telescopes, Dr Agarwal and her team at the Technical University of Braunschweig in Germany looked at how comets and asteroids actively emit material into space.

‘We aim to better understand the processes that lead to changes in the surfaces and interiors of comets and asteroids,’ she said. ‘We also hope to better understand their primitive nature, or how they were 4.5 billion years ago.’

Using data from several instruments onboard Rosetta, Dr Agarwal’s team has been able to model the properties of cometary dust in the environment of Comet 67P. They found that the dust particles could be loose aggregates of micron-sized silicate and sub-micron-sized carbonaceous components.

‘We are also observing huge boulder-size materials coming out from Comet 67P, coming from certain specific places on the surface…a fountain of boulders,’ Dr Agarwal explained.

Active asteroid

Comets are not the only bodies to emit material. Take the case of asteroid 288P. A so-called active asteroid that emits dust, from a distance it looks like a comet with a dusty tail.

‘The weird thing about 288P was that its nucleus looked double…and in the end, I thought, well maybe it’s a binary?’ Dr Agarwal said. ‘We had to wait a couple of years to reobserve it from close up, and then in 2016 we got more Hubble time and really saw that it was two components.’

Their measurements determined that this first-of-its-kind asteroid to be observed is comprised of two similarly-sized pieces, orbiting each other 100 kilometres apart.

‘We found it by chance. We don’t know if there are more systems like it that we don’t see,’ Dr Agarwal said.

They theorise that the asteroids were irradiated by the sun and begin to rotate, splitting in two when they spun too fast to hold together. The distance between the pair may be due to a jet of gas vaporising from the surface that propelled one rock away like a rocket. They are still trying to figure out what causes the tail.

Scientists have long thought that asteroids mainly evolved through collisions, but it’s possible that for smaller asteroids, fast rotation plays just as much of a role.

Their research has revealed a range of active asteroids, from those which have a one-off burst of activity (as if from an impact), to those that emit bursts of dust repeatedly.

‘There is some process happening more or less randomly that triggers the eruption of dust clouds,’ Dr Agarwal said, referring to asteroids which emit the repeated dust bursts. ‘We think maybe it is fast rotation that triggers landslides or something like that.’

The upshot of all of this is that the distinction between comets and asteroids may be more of a spectrum than a hard divide.

‘The line is getting fuzzier. In the past we thought asteroids are rocks, comets are icy. But now we see that there are comets that are almost inactive…and there are asteroids that are active. There is more of a transition between those two populations than we thought in the past,’ Dr Agarwal said.

The research in this article was funded by the EU’s European Research Council. 

Originally published on Horizon Magazine

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