2I/Borisov: A Remarkably Pristine Interstellar Comet

The beauty of comet 2I/Borisov, the second interstellar object discovered in our Solar System, is that it looks and acts more or less like, well, an interstellar comet, without the puzzling characteristics of its predecessor, the still controversial ‘Oumuamua. 2I/Borisov’s cometary nature is clear in the latest observations from the European Southern Observatory’s Very Large Telescope, data from which also tell us that this is one of the most undisturbed relics of a circumstellar disk ever found. Scientists believe it never passed close to any star before its 2019 passage by the Sun.

We don’t know around which star it formed, but Stefano Bagnulo (Armagh Observatory and Planetarium, Northern Ireland), lead author of one of two new papers on the object, says that 2I/Borisov “could represent the first truly pristine comet ever observed.” Bagnulo’s team used the FORS2 instrument on the VLT (FOcal Reducer and low dispersion Spectrograph), an instrument that can take spectra as well as measuring polarization.

That later capability, called polarimetry, helps astronomers understand the chemistry of comets by studying how sunlight is polarized by a comet’s dust. The technique has been used on small bodies in the Solar System including comets, making for interesting comparisons. For 2I/Borisov’s polarimetric properties differ from Solar System comets with the exception of one, comet Hale-Bopp, which was likewise one of the most pristine comets observed to that time.

Image: Taken with the FORS2 instrument on ESO’s Very Large Telescope in late 2019, when comet 2I/Borisov passed near the Sun. Here the background stars appear as streaks of light as the telescope followed the comet’s trajectory. The colours in this composite image are the result of combining observations in different wavelength bands. Credit: ESO/O. Hainaut.

You may recall Hale-Bopp from the late 1990s, when it was a naked eye object (I remember a total stranger offering up a view in his small telescope on a nearby parking lot one night). A comet like this would be little affected by the solar wind and other radiation, thus having a composition similar to the original cloud of gas and dust that produced it. But Hale-Bopp was thought to have made one pass by the Sun before its recent visitation, while 2I/Borisov shows every sign of being a complete newcomer to the inner regions of a star.

Two interesting points from the paper:

…at the time of our observations, comet 2I/Borisov was polarimetrically homogeneous, showing no sign of active areas contributing to the coma formation. Prior to its recent perihelion passage, comet Hale-Bopp probably was near the Sun at least once, and possibly only once, ~2250?BC; at the time of that first approach, the original material was removed from the surface and active areas were open, hence Hale-Bopp could manifest activity during its recent perihelion passage. Comet 2I/Borisov instead, most likely never passed close to the Sun or any other star, and may represent the first truly pristine comet that has ever been observed.

That’s useful information in the context of what follows, as we learn in this paper that 2I/Borisov’s composition is consistent with objects that emerged close to home:

The similarity between the polarimetric properties of the two comets must depend upon the microscopic structure and composition of the aggregates, and not on their macroscopic characteristics, as the two comets are quite different in size: the analysis of the photometric profile of the inner coma suggests that comet Hale-Bopp belongs to the class of giant comets, with the diameter of the nucleus being estimated between 20 and 35?km, while 2I/Borisov’s nucleus size is ? 0.4?km. The close similarity between the polarimetric behaviour of the comet 2I/Borisov and Hale-Bopp suggests that, whatever astrophysical environment in which comet 2I/Borisov originated in, such environment had properties which led to the formation of a body bearing significant analogies with those accreted in the outer regions of our Solar System, a remarkable result on its own.

Image: Image of comet C/1995 O1 (Hale-Bopp), taken on 1997 April 04, with a 225mm f/2.0 Schmidt Camera (focal length 450mm) on Kodak Panther 400 color slide film with an exposure time of 10 minutes. The field shown is about 6.5°x6.5°. At full resolution, the stars in the image appear slightly elongated, as the camera tracked the comet during the exposure. Credit: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria (http://www.sternwarte.at) – Own work, CC BY-SA 3.0.

In a second paper on the interstellar comet, ESO astronomer Bin Yang used data from the Atacama Large Millimeter/submillimeter Array (ALMA) to study 2I/Borisov’s dust grains. Here the key finding is that the coma of the comet surrounding the nucleus contains dust grains of one millimeter or larger. The relative amounts of carbon dioxide and water also changed as the comet neared perihelion, an indication that mixing of materials occurred where it formed. We can begin to deduce interesting things about the home system of this interstellar comet:

Our ALMA and VLT observations indicate that 2I/Borisov’s home planetary system, much like our own Solar system, had experienced efficient radial mixing from the innermost parts of its protoplanetary disk to beyond the frost line of CO. Among a number of probable mechanisms that have been proposed for the origin of ISOs [interstellar objects], gravitational interactions between planetesimals in the protoplanetary disk and growing giant planets is favored, as it can explain both the ejection of ISOs from their home systems as well as account for the strong radial transport of materials in the disk. While the most common planets in other exoplanetary systems seem to be super-Earths and mini-Neptunes, our study suggests the presence of giant planets in the home system of 2I/Borisov.

The first paper is Bagnulo, et al., “Unusual polarimetric properties for interstellar comet 2I/Borisov,” Nature Communications 12, No. 1797 (30 March 2021). Abstact/Full Text. The second paper is Bin Yang et al., “Compact pebbles and the evolution of volatiles in the interstellar comet 2I/Borisov,” Nature Astronomy 30 March 2021. Abstract.


Shaping Circumstellar Disks

The circumstellar disks that give rise to planets occur in huge variety depending on the nature of star formation around them. Such disks form early as stars emerge, according to some recent work appearing within 10,000 years after the birth of the star. New work out of Leiden University in The Netherlands homes in on the environmental factors shaping the evolution of these disks, giving us a sense of how stellar systems differentiate as their planetary configurations form.

The work, led by Francisca Concha-Ramírez, offers up a model of circumstellar disk formation in young star-forming regions. The formation model is a mathematical treatment that begins with the collapse of a giant molecular cloud and the subsequent formation of stars in a variety of masses, velocities and positions within a cluster. The disk formation model sets stellar evolution into motion at the same time as disk formation to study the interactions between the two as star-forming regions of varying densities emerge.

So a variety of astrophysical processes are at work simultaneously in these simulations, including not just stellar dynamics as the molecular cloud collapses but the effects of nearby young stars through processes like photoevaporation. In the latter, the disk surface evaporates due to radiation from the host star, or just as significantly, from bright stars nearby in the cluster. Areas of a disk can be cleared by host star radiation, while radiation from nearby stars can deplete in particular the outer regions of disks, where material is less bound to the host star.

No wonder we see the variety displayed in the image below. If nearby stars deplete a circumstellar disk too swiftly, planet formation becomes impossible.

Image: Circumstellar discs in the Orion Nebula, observed with the Hubble Space Telescope. The ‘comet tail’-like structures show disks being evaporated by nearby bright stars. Credit: NASA/ESA, L. Ricci (ESO).

As you would imagine, this work gives us a wide variety of potential disk-modifying events. In young star clusters, close encounters from stellar flybys can affect the size and shape as well as surface density of the disks, sometimes forcing the appearance of spiral arms. Disks can exchange mass and tidal streams can emerge, all found in recent disk observations. The outer edge of a disk can become truncated by the passage of a nearby star. In fact, lead author Concha-Ramírez thinks this may have occurred in our own early Solar System:

“A collision may have taken place between our circumstellar disc and another disc. We can see proof of this at the edge of our Solar System, in the region of the planet Neptune. Here there are suddenly much fewer asteroids, which suggests that another disc could have nabbed material. And there is another interesting clue that there might have been a collision between discs: asteroids that, in relation to the Earth, orbit the sun on a different plane. These asteroids probably come from another disc.”

That’s a controversial ‘probably,’ but Concha-Ramírez is pointing to the so-called Sednitos that move in the outer regions of the Solar System between the Kuiper Belt and the Oort Cloud, whose origin is still up for grabs. Other observational evidence of disk-shaping by the environment can be found in the Orion Nebula, where we see ‘proplyds,’ circumstellar disks that are swathed in the radiation of tightly spaced stars. Here we can find cometary tail-like structures, and mass loss as disks closest to massive stars experience photoevaporation.

Image: The Trapezium cluster, in the Orion Nebula. This is similar to the regions Concha-Ramírez simulated. Credit: NASA; K.L. Luhman (Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.); and G. Schneider, E. Young, G. Rieke, A. Cotera, H. Chen, M. Rieke, R. Thompson (Steward Observatory, University of Arizona, Tucson, Ariz.)

In general, disk masses decrease as the density of nearby stars increases, but the authors are quick to add that massive disks can still form in regions like these. About 60 percent of the disks are destroyed by photoevaporation within the first 100,000 years of evolution in high density star regions, but in these simulations, massive radiating stars do not all form at the same time, and tend to emerge later than low mass stars. This would give some massive disks more time to evolve. Gas present in the clusters also has a protective effect and helps to explain the proplyds found in Orion, which have found a path to persist despite intense external radiation.

The catalog of disks emerging out of all this seems to be as broad as the catalog of the planets that emerge within them. Making the call on which disks will survive and in what form is a matter of sorting all of these factors on a case by case basis. From the paper:

…while photoevaporation is an important process for the depletion of disc masses, other factors such as the morphology of the star-forming regions and the length of the star formation process can allow for circumstellar discs to survive for long periods of time, and to remain massive enough to form planets. This could help explain the great variety of disc mass distributions observed in star-forming regions of different ages and configurations, and could also be a factor in solving the ‘proplyd-lifetime problem’, where massive discs are observed in regions of high stellar density and background radiation.

The paper is Concha-Ramirez et al., “Evolution of circumstellar discs in young star-forming regions,” submitted to Monthly Notices of the Royal Astronomical Society (preprint).


Theia: Tracking Remnants of the ‘Big Whack’

The ‘Big Whack,’ as I’ve heard it called, is the impact of a planetary embryo of perhaps Mars-size (or larger) that is thought to have struck the Earth during the latter era of planet formation. Or we can call it the ‘Giant Impact,’ as Arizona State scientists did in a presentation at the virtual Lunar and Planetary Science Conference recently concluded. Whatever the name, the event offers a model for the formation of the Moon, one that explains the latter’s small iron core and the anomalous high degree of angular momentum of the Earth-Moon system.

The impact of the protoplanet called Theia would have been a fearsome thing, blasting pieces of both worlds into space that later coalesced into the Moon. Think When Worlds Collide, the 1933 science fiction novel written by Philip Wylie and Edwin Balmer, whose cover is irresistible and thus must be reproduced here. Better known, of course, is the 1951 film of the same name, produced by George Pal. Neither has anything to do with the Moon but vast objects running into each other offer possibilities Hollywood was bound to seize at some point.

The new work on the Moon’s formation is less photogenic, but it draws on a detectable feature within the mantle of the Earth, so-called Large Low Shear Velocity Provinces (LLSVPs), which have been confirmed through seismic wave detections. The slowing of seismic waves that encounter LLSVPs is an indication that the material they’re made of is denser than the rest of the mantle, and indeed they seem to rest on the rim of the outer core, which is itself telling.

The work is complicated by the lack of agreement on the size of Theia and questions about its composition, as a description in the Lunar and Planetary Science Conference materials (LPI Contrib. No. 2548) makes clear:

…one of the most critical issues related to this scenario is that no evidence has been found for the existence of the hypothesized planetary embryo Theia. This is in part because of its widely debated size ranging from 0.1~0.45 Earth mass (M?) [5] and enstatite to carbonaceous chondrite composition [6]–[9]. Moreover, whereas it is mostly agreed that the core of Theia promptly merged with the proto-Earth core shortly after the impact [3], what fraction of and how the Theia mantle was preserved into the Earth mantle remain elusive. This post-impact process is not only responsible for the initial thermal and compositional structures of the Earth, but also significantly affects Earth’s long-term chemical evolution.

Image: The Giant Impact hypothesis for the origin of the LLSVPs. Credit: Li et al.

Did intact pieces of Theia’s mantle survive inside the Earth? The Arizona State study indicates that some of these materials account for the Large Low Shear Velocity Provinces, sinking to the bottom of Earth’s mantle in the process. The authors use numerical modeling experiments to support the idea, working with mantle materials from both Theia and the Earth as key components and producing models where the Theia materials sink and survive.

The key here is the density of the Theia mantle, allowing the material to survive mixing by convection within Earth’s mantle as long as it is denser than the material around it. The analysis suggests that the Theia materials were rich in iron and several percent denser, allowing them to sink to Earth’s lowest mantle and accumulate into the LLSVPs. The initial thickness of the Theia mantle materials in this model is in a range between 350 kilometers and 500 kilometers, existing as chunks of a protoplanet in the form of LLSVPs beneath Africa and the Pacific Ocean.

For more, see Yuan et al., “Giant Impact Origin for the Large Low Shear Velocity Provinces,” 52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548), available here. The paper is in process at Geophysical Research Letters.


Flow in Enceladus’ Internal Ocean

New thinking about the shape of the ice shell on Enceladus raises the interesting possibility that the ocean believed to lie beneath is churning with currents. That would be a departure from the accepted view that the ocean — thought to be 30 kilometers deep or more, as opposed to Earth’s average of 3.6 kilometers — is relatively homogenous.

Remember, this is a body of water buried beneath perhaps 20 kilometers of ice, locked within a moon that is 500 kilometers in diameter. We know it’s there because a 2014 flyby of the Cassini Saturn orbiter collected data on geyser activity erupting through the striking fissures in the south polar ice. That immediately put Enceladus high on the list of astrobiologically interesting targets along with, of course, Jupiter’s moon Europa.

Image: A masterpiece of deep time and wrenching gravity, the tortured surface of Saturn’s moon Enceladus and its ongoing geologic activity tell the story of the ancient and present struggles of one tiny world. A new theory examines circulation within the ocean beneath the surface, with implications for future searches for life there. Credit: NASA/JPL-Caltech.

The differences from Earth’s oceans are obvious enough, including the fact that the Enceladus ocean is being heated from below at its interface with the core, while being cooled at the top, which argues for a great deal of vertical movement affecting its properties. The paper on this work points out that variations in the thickness of the ice shell would involve separate regions of freezing and melting at the upper ocean/ice interface.

To examine the resulting circulation, Caltech graduate student Ana Lobo, working with colleague Andrew Thompson and JPL’s Steven Vance and Saikiran Tharimena, has applied Thompson’s work on water/ice interactions in Antarctica to the Enceladus environment. The researchers used a computer model to demonstrate the likelihood of ocean currents beyond the expected vertical upwellings. This would be a process that transports heat and introduces a zone of freshwater in the moon’s polar regions, and is partially driven by salinity, a factor that Thompson has found to be affecting ocean circulation near Antarctica.

That’s something to consider as we ponder whether sampling directly from the Enceladus plumes actually reflects the broader conditions found within the global ocean. Cassini has already shown us that the Enceladus ice shell is thinner at the poles than at the equator, a likely indication of areas of melting and freezing that directly affect ocean currents. The reason: Freezing salty water releases salts and causes surrounding water to sink, with the opposite effect happening in regions where the ice is melting. “Knowing the distribution of ice allows us to place constraints on circulation patterns,” according to Lobo.

The researchers’ model implies that ocean circulation on Enceladus connects these areas of freezing and melting in a flow that goes from pole to equator. We will need to take such currents into account as we look into the potential for Enceladus to support life. From the paper:

The circulation will also impact how nutrients from the ocean-mantle boundary are distributed. If nutrients are transported upward primarily by ocean plumes, we would expect detrainment to release these nutrients in the ocean interior. In Earth’s ocean, it has been shown that adiabatic mixing, or stirring along isopycnals [lines connecting points of a specific density], can be an important pathway for nutrient delivery to the surface and modulate productivity there [26].

Thus, the distribution of freshwater fluxes at the ocean-ice interface, by setting global patterns of upwelling, could influence nutrient fluxes and provide insight into which regions of an icy world have resources for life to potentially flourish.

The study of oceans beneath the ice on outer system moons is in its infancy. The authors of the study, which appears in Nature Geoscience, close their paper by noting other ocean worlds where variability in the ice thickness may be large enough to alter the density structure of the ocean. Titan, with an ice shell anywhere from 50 to 100 kilometers thick, is a case in point, and the Dragonfly mission will carry a geophysical package that should yield further clues to the ocean beneath. At Jupiter, we have both Europa Clipper and JUICE to look forward to, helping to constrain variations in the thickness of the shell while probing the properties of the subsurface ocean.

The paper is Lobo et al., “A pole-to-equator ocean overturning circulation on Enceladus,” Nature Geoscience (25 March, 2021). Abstract / Preprint.


Pluto: Musings on Scale

The news that Ingenuity, the helicopter aboard NASA’s Perseverance rover, contains a swathe of fabric from the 1903 Wright Flyer underscores the conflation in time I’ve been feeling all week, ever since looking at the images below. The first, released by the New Horizons mission team, shows a speck to the right of Pluto. This is 486958 Arrokoth, the Kuiper Belt Object visited by New Horizons at the beginning of 2019 and superimposed in the Pluto image just for scale.

Image: The main exploration targets of New Horizons: Pluto (2015) and the much smaller Arrokoth (2019), to scale. Other exploration targets included Pluto’s five moons. Credit: NASA/Johns Hopkins APL/Southwest Research Institute.

The point is strikingly made: Pluto itself, which had been no more than a speck in our field of view in 1930, when what was deemed the ninth planet was discovered, has now become a world, and even the tiny Arrokoth is a well imaged object, the farthest ever visited by a spacecraft. Such adjustments to scale are a marker for our success at pushing instruments into the outer Solar System, and they all but force philosophical musings. What would the bicycle makers from Ohio have made of something they touched that windy day on a North Carolina beach winding up on the fourth planet from the Sun? What would Clyde Tombaugh have thought about Sputnik Planitia and the abundant terrain we can now see on Pluto?

Image: A plates from the original discovery of Pluto in 1930. Credit: Lowell Observatory Archives.

New Horizons is very much alive, as principal investigator Alan Stern’s reports continue to remind us, the latest of which can be found here. We might see the imagery above as partially in celebration of an upcoming event, the crossing of the 50 AU distance from the Sun, which will occur on April 17 (or 18th, as time zones are accounted for). That’s another reminder that we’re in a region where the Sun, as Stern notes, is smaller in the sky than Jupiter from Earth. Only four other operating spacecraft ever made it out here, 7.5 billion kilometers from home.

Up next for New Horizons is the proposal of a new mission and science plan to NASA, one that would fund the craft for another three years. The proposal is due early next year, and will advocate the continued exploration of the outer Kuiper Belt, a place devoid of other spacecraft. Stern’s report is a bit philosophical itself, and why not. He notes that by the late 2030s, New Horizons will be too low on power (its plutonium power supply produces 3.3 watts less every year) to run the primary spacecraft systems, at which point it will be nearing 100 AU.

But even once the spacecraft is derelict—either because it runs out of power or fuel, or for any other reason—it will continue to coast outward, into the galaxy at a speed of nearly 3 AU (about 300 million miles) per year. In fact, even when the day comes in a few billion years that the Sun goes red giant and engulfs Earth, New Horizons, like the Pioneers and Voyagers, will still be out there, outliving even its home planet!

With those lines, Stern reaches into Olaf Stapledon territory. But there is potentially a lot to do before we lose this craft. The New Horizons team is working on a flight plan and command load to study three Kuiper Belt Objects in May, a reminder that to this point the spacecraft has studied almost 30 KBOs, close enough to some of them to work at resolutions beyond what the Hubble Space Telescope can provide. Continuing observations are planned for September and December.

Meanwhile, a summer search for a new KBO flyby target will take place. The hunt for Arrokoth took fully four years, but Stern says that this one may be a good deal quicker:

…we’re applying a new tool—artificial intelligence. Using machine-learning software, mission co-investigator JJ Kavelaars and collaborating scientist Wes Patrick have sped up and made those searches far more productive. In fact, when they reran the 2020 search data through their new software tools, it not only worked 100 times faster, but it turned up dozens of new KBOs that human searchers had not found in the search images! We’ll be taking advantage of this important new tool again later this year, and next year and after that as well.

With most of the new flight software upgrades scheduled for July uplink (the one for the SWAP solar wind instrument is already aboard), New Horizons will continue to produce new science. No one should ever take a mission extension for granted, especially in economic times like these, but it would be a spectacular failure of imagination should we turn away from a functioning spacecraft in a part of the Solar System where we have no other assets, and my instincts tell me we’ll be talking about another New Horizons KBO encounter down the road.

Here is a final image for today, one that never fails to stun me. We’re looking at Pluto’s surface just after the spacecraft’s closest approach. The scale jumps again from the Pluto image at the top of the post, in this case turning a world into a landscape. Note the terrain, the atmospheric haze, the detail that makes a remote object a setting for observation and awe.

Image: Surface of Pluto just after New Horizons’ closest approach. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.