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‘Oumuamua: A Shard of Nitrogen Ice?

I’m only just getting to Steven Desch and Alan Jackson’s two papers on ‘Oumuamua, though in a just world (where I could clone myself and work on multiple stories simultaneously) I would have written them up sooner. Following Avi Loeb’s book on ‘Oumuamua, the interstellar object has been in the news more than ever, and the challenge it throws out by its odd behavior has these two astrophysicists, both at Arizona State, homing in on a possible solution.

No extraterrestrial technologies in this view, but rather an unusual object made of nitrogen ice, common in the outer Solar System and likely to be similarly distributed in other systems. Think of it as a shard of a planet like Pluto, where nitrogen ice is ubiquitous. Desch and Jackson calculated the object’s albedo, or reflectivity, with the idea in mind, realizing that the ice would be more reflective than astronomers had assumed ‘Oumuamua was, and thus it could be smaller. As the authors note: “Its brightness would be consistent with an albedo of 0.64, which is exactly consistent with the albedo of the surface of Pluto, which is > 98% N2 ice.”

That’s a useful finding because a nitrogen ice object would behave like ‘Oumuamua was observed to do. Recall the salient problem this interloper presented as it left the system. It moved away from the Sun at a slightly larger velocity than an average comet should have. Desch and Jackson discovered that if it were made of nitrogen ice, and thus smaller (and more reflective than thought), the so-called ‘rocket effect’ could be accounted for. A tiny object is affected by a small amount of escaping gas to a greater extent than a larger, more massive one.

The effect can be calculated by examining how different kinds of ices sublimate, moving from a solid to a gas with no intervening liquid state. As to how an object constituted of nitrogen ice might have gone interstellar, the astronomers worked out the rate that breakaway nitrogen ice pieces would have been produced through collisions in the outer system of an exoplanet. Says Jackson:

“It was likely knocked off the surface by an impact about half a billion years ago and thrown out of its parent system. Being made of frozen nitrogen also explains the unusual shape of ‘Oumuamua. As the outer layers of nitrogen ice evaporated, the shape of the body would have become progressively more flattened, just like a bar of soap does as the outer layers get rubbed off through use.”

That question of ‘Oumuamua’s shape continues to intrigue me, though, as I ponder my bar of Irish Spring. We’ve never observed anything of this shape in the Solar System. And exactly what would have happened at perihelion? I turned to the first of the two papers for more:

Our modelling shows that, perhaps surprisingly, an N2 ice fragment can survive passing the Sun at a perihelion distance of 0.255 au, in part because evaporative cooling maintains surface temperatures less than 50 K. Despite being closer to the Sun than Mercury, ‘Oumuamua’s surface temperatures remained closer to those of Pluto.

Even so, surely nitrogen ice would have been a huge factor in its behavior:

The volatility of N2 did, however, lead to significant mass loss – we calculate that by the time ‘Oumuamua was observed, a month after perihelion, it retained only around 8% of the mass it had on entering the solar system. This loss of mass is key to explaining the extreme shape of ‘Oumuamua: isotropic irradiation and removal of ice by sublimation increases the axis ratios, a process also identified by Seligman & Laughlin (2020). Between entering the Solar system and the light curve observations the loss of mass from ‘Oumuamua increased its axis ratios from an unremarkable 2:1 to the extreme observed value of around 6:1.

In other words, we are dealing with a ‘flattening’ that occurred in our own system and not at place of origin. I suspect this flattening process is going to receive a thorough vetting in the community, key as it is to explaining a salient oddity about ‘Oumuamua.

And so we wind up with a theory that presents ‘Oumuamua as shown below, an unusual aspect ratio to be sure (and yes, reminiscent of what could be a lightsail), but Desch and Jackson think their theory of nitrogen ice matches every aspect of ‘Oumuamua’s behavior without the need for invoking alien technology. Desch comments:

“…it’s important in science not to jump to conclusions. It took two or three years to figure out a natural explanation — a chunk of nitrogen ice — that matches everything we know about ‘Oumuamua. That’s not that long in science, and far too soon to say we had exhausted all natural explanations.”

Image: This painting by William K. Hartmann, who is a senior scientist emeritus at the Planetary Science Institute in Tucson, Arizona, is based on a commission from Michael Belton and shows a concept of the ‘Oumuamua object as a pancake-shaped disk. Credit: William Hartmann.

Thus ‘Oumuamua, in the eyes of Desch and Jackson, might be considered a chunk of an exo-Pluto, which in itself opens the topic of studying interstellar objects for information about their parent systems. We’ve never observed an exo-Pluto before, so ‘Oumuamua may probe the surface composition of worlds like this.

Moreover, if our first identified interstellar interloper is made of nitrogen ice, then the existence of exo-Plutos must be common, although we’ll have to decide exactly how we want to define the term (and I suppose we can now invoke the ruckus about Pluto’s planetary status by asking whether exo-Plutos are actually to be referred to as ‘exo-dwarf planets’).

As the Vera Rubin Observatory/Large Synoptic Survey Telescope in Chile comes online, regular surveys of the southern sky will doubtless up the number of interstellar objects we can identify, helping us home in further on their composition and likely origin.

Image: Illustration of a plausible history for ‘Oumuamua: Origin in its parent system around 0.4 billion years ago; erosion by cosmic rays during its journey to the solar system; and passage through the solar system, including its closest approach to the Sun on Sept. 9, 2017, and its discovery on October 2017. At each point along its history, this illustration shows the predicted size of ‘Oumuamua, and the ratio between its longest and shortest dimensions. Credit: S. Selkirk/ASU.

The first paper is Jackson et al., “1I/’Oumuamua as an N2 ice fragment of an exo‐Pluto surface: I. Size and Compositional Constraints,” Journal of Geophysical Research: Planets (16 March 2021). Abstract / Preprint. The second is Desch et al. “1I/’Oumuamua as an N2 ice fragment of an exo‐pluto surface II: Generation of N2 ice fragments and the origin of ‘Oumuamua,” Journal of Geophysical Research: Planets (16 March 2021). Abstract / Preprint.


Comments on this entry are closed.

  • Michael March 19, 2021, 10:43

    First hydrogen now nitrogen and what next Ice 9 from outer space ?

    • Patient Observer March 19, 2021, 13:17

      Cool! Seems plausible once you get past about the part about a collision flinging out a chuck of N2 ice out of another planetary system. But what about spectroscopic evidence?

      The observable interstellar object population is expected to be dominated by comet-like bodies in agreement with our spectra, yet the reported inactivity of ‘Oumuamua implies a lack of surface ice. Here, we report spectroscopic characterization of ‘Oumuamua, finding it to be variable with time but similar to organically rich surfaces found in the outer Solar System. We show that this is consistent with predictions of an insulating mantle produced by long-term cosmic ray exposure4.


      The above does not seem to describe a chunk of nitrogen.

      • Robin Datta March 20, 2021, 0:28

        “Cool! Seems plausible”…
        Solid N₂? Cold! ~Shiver!~
        An intelligent origin is yet to be unequivocally demonstrated. But juicy speculation is so much tastier.

        • James Jason Wentworth March 20, 2021, 13:14

          I have corresponded with Dr. Avi Loeb (and have read his book), and he doesn’t say that ‘Oumuamua is artificial; he says that this hypothesis is the one that the data collected on ‘Oumuamua best fit. Spectrograms were taken of ‘Oumuamua, and the “hydrogen iceberg” hypothesis–and the “dust bunny” hypothesis–of what it is both involve things that we have never seen in nature (they explain a mystery with either of two enigmas). Gaseous nitrogen, like hydrogen, is very “shy” about revealing itself to spectrographs viewing it from very far away, but pure nitrogen–like pure hydrogen–natural objects haven’t been observed; there is always some frozen methane, cyanogen, ammonia, water, etc. mixed in with the nitrogen, and these compounds, in gaseous form, would have been seen by spectroscopes.

          • Alex Tolley March 20, 2021, 17:32

            Is the frozen N2 on Pluto’s Sputnik Planitia confirmed to have such mixtures of volatiles detected by New Horizon’s instruments? IIRC, one of the papers argues that the other volatiles would be at very low concentrations and the emitted amounts too low to be detectable.

            • Patient Observer March 21, 2021, 10:20

              If the nitrogen atmosphere of Pluto freezes out during the portion of its orbit furthest from the sun, then a thin layer of pure nitrogen snow could be expected. Not much potential for large shards of pure frozen nitrogen flung into interstellar space resulting from a collision, it would seem. I’m leaning more toward the frozen sherbet hypothesis.

              • Alex Tolley March 21, 2021, 12:04

                My understanding is that the frozen N2 on Pluto is primarily from cryovolcanism. One can argue over whether an impact that creates a shard directly or with subsequent collisions, is a likely event, but the proposed mechanism seems justifiable to explain the object. ‘Oumuamua is not from Pluto, but from an exo-Pluto-like body where conditions may be more conducive to such an event.

                • Patient Observer March 22, 2021, 23:17

                  Yes, forgot about cryo-volcanism. Solid nitrogen mechanical properties include very low compressive and tensile stress ratings. It would seem likely a shard could no form from an impact. Rather the nitrogen solid would pulverize. Perhaps a mass of solid nitrogen could gravitate to form a powdery snowball but that is not what passed through our solar system.


      • EricSECT March 20, 2021, 4:16

        A shard from a collision seems extremely improbable, given the vastness of interstellar space.

        Sublimating N2 does explain the slight acceleration….. Sorry if I missed it…. but if it was sublimating N2 as it neared Sol, where was the comet like tail and halo?

        And I agree with Patient O, per this Nature article, the object did not show the spectrum of N2 …..but it does look as if the object was rotating (maybe in 2 axis), Bert Malloy.

        • Alex Tolley March 20, 2021, 13:03

          where was the comet like tail and halo?

          Isn’t the halo and tail composed of reflective particles? Pure N2 sublimation as a gas would be invisible.

          What I would like to know is what other comet experts make of these 2 papers.

          N2 has a poorly detectable spectrum, and teh small size of the modeled object indicates that the gas would be below the detection size.

          Neither of the papers addressed the apparent reddish color of ‘Oumuamua, which might still need to be explained by the frozen N2 hypothesis.

          The 2 papers make a plausible hypothesis, certainly more plausible (to me) than the fragment reaggregation of frozen H2 into a cigar shaped object with a low probability trajectory.

    • Andrei March 22, 2021, 17:58

      While I did like the hydrogen ice hypothesis, I did so for the simple that someone had put some thought of the nature of Oumuamua.
      Just imagine, one entirely natural object from another system fly by and cause a huge fraction of a planet population to yell ‘aliens’ and start a space force and a media nonsense.
      Nitrogen is a better suggestion, based on one of the few facts we know about Oumuamua and that detail is important for the study presented here – the red colour which is various organics compounds.
      (Perhaps I should not have said that about organics so the followers of the Loeb cult don’t state we got spectroscopic evidence of the aliens themselves.)

  • DCM March 19, 2021, 12:42

    So maybe it’s not an alien spacecraft….?

  • Hal Schirmer March 19, 2021, 13:04

    How about a slab of tholins knocked off a Pluto analog? Or something that’s basically a “slab of stellar asphalt” composed of nitrogen-ice-aggregate held together by tholin-tar?

  • Bruce D Mayfield March 19, 2021, 13:43

    This theory is the most plausible one yet, at least in my insignificant opinion. It explains much using only common material (N2 ice) and event (impact). No need to invoke exotic beings at all, (except to garner attention and sell books, etc.)

  • Alex Tolley March 19, 2021, 13:51

    While I will admit to some strong bias in preferring natural explanations, this paper really gives a lie to Loeb’s Sherlockian claim that once you have eliminated the possible…. Clearly, Loeb had not looked at all the possible natural explanations. IDK whether this one is correct or not, but it is a useful hypothesis to consider before jumping to alien spacecraft explanations. As for the flattened shapes, we saw that the KBO object Arrokoth was 2 flattened objects, snd so I expect this disc shape maybe be common.

    A frozen N2 shard is an interesting novelty, and it would be good if we could find others, even a shard from Pluto itself out in the Kuiper belt.

  • Bert Molloy March 19, 2021, 15:50

    It seems very surprising to me that this object does not appear to be showing any (reported ?) signs of rotating.

    If it originated as a more-or-less planar fragment ejected from an extrasolar planet then travelled for ~0.5 GYr through space before entering our Solar System, how likely is it for there to be no rotation ?

    Is there any plausible mechanism whereby any initial rotation would be reduced ?

    Bert Molloy
    Richmond, B.C., Canada

    • Whomever1 March 19, 2021, 23:15

      The original description implied it was rotating, since it was varying in brightness.

  • Whomever1 March 19, 2021, 17:21

    ’Oumuamua was reported to be reddish in color, as is frozen nitrogen (according to a quick search). But the descriptions of Pluto said the reddish color was due to tholins. I’m just confused if there is any red nitrogen on Pluto, or tholins on ’Oumuamua.

    • Andrei March 22, 2021, 17:51

      Smaller objects in the more distant parts of the solar system have also been noted for being red. So yes it’s the same process that cause the colour.

    • Curious March 23, 2021, 22:02

      fwiw optical surfaces like Mylar, such as used on the Ikaros solar sailer, also have a reddish orange color https://en.wikipedia.org/wiki/IKAROS

  • Geoffrey Hillend March 19, 2021, 17:34

    I’ve read William K. Hartmann’s book Moons and Planets for my college planetary science textbook. It covers some of the geology of the planets, asteroids and their spectra. Informative book. I liked it.

  • Hamilton1 March 19, 2021, 19:16

    The idea that the first observed interstellar object lost 92% of its mass during its flyby doesn’t seem to conform with Occam’s razor.

    • Michael Gregory Million March 20, 2021, 9:04

      Could we have Hyperion type body, with around a density of 500 kg per cubic meter.

  • Michael Fidler March 20, 2021, 9:42

    Oh, it was also encrusted with magic mushrooms.

  • Douglas Loss March 20, 2021, 10:06

    It would be interesting to see some responses to the Desch and Jackson papers, particularly from Avi Loeb as he’s the scientist consistently heaped scorn upon for daring to think a bit outside the box. From what I’ve seen, he’s been generally misquoted and his work has been good quality. I’ll admit, I think he probably enjoys stirring the hornets nest occasionally. But that’s no reason for automatically discounting what he says.

  • Patient Observer March 21, 2021, 20:01

    The video linked provides entertaining support for Oumuamua being an alien artifact. The most interesting point for me was the calculation that the object could only be millimeters thick to display the rate acceleration from solar radiation (assuming no propulsive outgassing).

    But why the tumbling if it were an alien probe? Or, was it an alien probe that was derelict?

    I want to believe…


    • Gary Wilson March 22, 2021, 17:44

      Just watched the video. It certainly is thought provoking and raises a number of questions about the validity of the more mundane explanations. I don’t really believe it looks like a rusty Millenium Falcon as one commenter said either. The directed acceleration which allowed an additional 100K km to accumulate during its travel outward past the orbit of Jupiter is puzzling as outgassing of an irregular shaped object should produce irregular acceleration. It’s extreme brightness and lack of detection of a heat signature or gas and particulate matter behind it are also puzzling. The observation that it was travelling at the local zero point velocity relative to the galaxy also seems non random. Taken altogether it makes me question the seemingly desperate attempts to turn it into a red nitrogen ice pancake or a hydrogen ice cigar.

    • Robin Datta March 22, 2021, 21:49

      If a lightsail, did it still have a payload attached? If such can be the case, could there an interstellar consilience yielding semiotics that would offer a way to signal a possible alien craft at the next opportunity?

      • ljk March 25, 2021, 10:07

        I have read elsewhere, including in this blog, that the solar sail might have been detached after delivering its payload here, left to drift free in the same way the final booster stages that lobbed our first five probes beyond the Sol system were left to drift off into the galaxy.

        So where is the payload…

  • downerczx March 26, 2021, 0:32

    The thing the Avi Loeb mentions in his book is that if it were outgassing, it would lose mass, which means the rotational period would have changed from 8 hours, which it did not after approaching the sun. Wouldn’t outgassing from a nitrogen iceberg cause the rotational period to change?

  • Edwin March 29, 2021, 19:56

    Dr Stephen Curran: Avi Loeb, aliens and other ‘utter crackpottery’

  • ljk March 31, 2021, 12:59

    Why Oumuamua, the interstellar visitor, looks eerily familiar

    An artist’s conception of a reddish, cigar-shaped rock gained widespread circulation

    International New York Times, MAR 30 2021, 03:18

    ISTUPDATED: MAR 30 2021, 03:18 IST


  • wdk April 5, 2021, 21:47

    Getting caught up with events. Decided to review the properties of
    Pluto’s moons, the other five beside Charon, the principal one.
    It was noted that Pluto is among the reddest objects in the solar system, but its moons are rather gray. It has been suggested that the “whack” which created them might have wiped away their volatiles such as nitrogen….

    Well, that poses a problem for an exo-Pluto orign for “Oumuamua too.
    If there were some sort of energetic collistion that caused a block of nitrogen to flail off into deep space, then a lot of N2 would be “begone”.

    Another problem I sense, despite the plausible nature of cold N2 evaporation without spectral or reflective evidence, is how come local comets don’t seem to behave that way first time coming in? I presume that Kuiper Belt and Oort Cloud objects fall out now and then. Initially
    they might not be “periodic”, but definitely highly eccentric and anticipated to return, whether in 100 or 1000 years. Granted that some have faint tails or disappointment, ( e.g., Kahoutek), but do they really resemble ‘Oumuamua that closely?

  • ljk April 19, 2021, 10:14

    When Stars Get Too Close to Each Other, They Cast Out Interstellar Comets and Asteroids


    In October 2017, humanity caught its first-ever glimpse of an interstellar object – a visitor from beyond our solar system – passing nearby the Sun. We named it Oumuamua, and its unusual properties fascinated and confounded astronomers. Less than two years later, amateur astronomer Gennady Borisov found a second interstellar object: a comet-like body that began to disintegrate as it passed within 2 AU of the Sun (1 AU equals the distance from Earth to the Sun).

    Where do these interstellar objects come from? How common are they? With a sample size of just two, it’s difficult to make any generalizations just yet. On the other hand, given what we know about star formation, we can begin to make some inferences about the likely origins of these objects, and what we are likely to see of them in the future.

    One of the most likely culprits for ejecting asteroids and comets into interstellar space is close encounters between stars. Four researchers studying this question – Susanne Pfalzner, Luis Aizpuru Vargas, Asmita Bhandare, and Dimitri Veras – released a paper last week examining this process.

    Full article here:


    The paper here:


  • ljk May 25, 2021, 10:25

    So even if Oumuamua IS an alien lightsail, it still isn’t good enough for some humans…


  • ljk June 21, 2021, 15:00

    Oumuamua’s Star Trek: Potential Origin in a Giant Molecular Cloud?

    by Pratik Gandhi | Jun 16, 2021 | Daily Paper Summaries

    Title: Evidence Suggesting that ‘Oumuamua is the ~ 30 Myr-old product of a Molecular Cloud

    Authors: Cheng-Han Hsieh, Gregory Laughlin, Hector G. Arce

    First Author’s Institution: Department of Astronomy, Yale University, New Haven, CT 06511, USA

    Status: available as an arXiv preprint; submitted to The Astrophysical Journal [open access]


    Ever since it was first observed in October 2017, the interstellar interloper 1I/‘Oumuamua has sparked numerous debates in both the astronomical community and in the eyes of the public: on its nature and composition, on where it came from, and possibly most importantly, on whether it had a natural origin.

    Although a majority of researchers agree that ‘Oumuamua is most likely not an alien artifact, the question of its origin remains one of the biggest mysteries and hotly discussed topics in astrophysics today. The issue was made even more compelling by the discovery in August 2019 of a second interstellar visitor, comet 2I/Borisov, and astronomers have been grappling with a number of questions since then:

    How many freely orbiting interstellar objects are there?
    Where did ‘Oumuamua and Borisov originate?
    Could we predict the arrival of future interstellar interlopers?

    The authors of today’s paper focus on ‘Oumuamua, and investigate the hypothesis that it might have originated in a nearby Giant Molecular Cloud (GMC). GMCs are the sites of star formation in galaxies as the birthplace of star clusters or stellar associations. To test the GMC-origin theory for ‘Oumuamua, the authors compare its orbital dynamics to that of known stellar associations.

    Full article here: