The interstellar asteroid 1I/2017 U1 (otherwise known as 1I/’Oumuamua) is fast receding into the distance, towards the constellation of Pegasus (1). The existence of this rocky visitor from the stars was announced last October (1). Its trajectory was too fast for it to be a solar system comet – even one from the furthest reaches of the Oort Cloud. That was an exciting discovery, because that meant that 1I/2017 U1 was the first confirmed observation of an object arriving in the solar system from deep space.
Although 1I/2017 U1 was initially considered to be an interstellar comet, that thinking changed when it failed to emit any gases as it performed its perihelion transit around the Sun (3). This barren rock, confirmed as an interstellar asteroid (4), is now speeding away from the Sun. It spent a relatively short time in the observation zone of professional telescopes, thanks to its great speed, but this was enough to reveal more weirdness (5). It is an elongated object spinning head over tip, doing cartwheels through the solar system. Some wondered whether it might be artificial, given the lack of coma as it traversed past the Sun. But attempts to pick up signals from the object came up blank (6). Still, its shape is nothing like any known body in our Solar System. If solar system asteroids resemble rocky potatoes, then 1I/2017 U1 is more like an interstellar carrot, spinning haphazardly through our system. To remain intact under these conditions, its internal structure must be robust (7).
The colour of our interstellar carrot is neutral with a reddish hue. The colouration may be patchy across its surface. Solar system minor bodies (asteroids, Kuiper Belt Objects, Trojans) vary in colour, often dependent upon which population group any particular object belongs to. Continuing my daft vegetable analogy, solar system potatoes come in different varieties. Many are neutral in colour, some are reddish, others distinctly red. Like comparing a Maris Piper to a King Edward. If we compare 1I/’Oumuamua’s colouration to those of various classes of solar system objects, then it seems to most resemble those of the dynamically excited populations of Kuiper Belt Objects. However, it is less red than the scattered Trans-Neptunian objects whose orbits extend beyond the heliopause (7).
I’ve often discussed the origin of various elements and compounds on Earth – most notably the isotopic ratio of water, and what that might tell us about the origin of terrestrial water (1). Data about this can help provide evidence for the Earth’s early history, and often the data is inconsistent with the general theories of oceanic origin, like the ‘late veneer theory’, for instance, where the bulk of terrestrial waters were supposed to have been supplied by comets. It turns out that the water was on this planet all along (2,3), raising questions about why the Sun’s heat had not driven this relatively volatile resource away from the primordial Earth during the early history of the solar system.
Despite such evidence, the ‘late veneer theory’ continues to hold ground for many scientists, and tends to go unchallenged within the science media. This is apparent within the following excerpt about a new paper on the mysterious presence of a particular isotope of the noble gas xenon found in ancient terrestrial water encased in rock:
“The scientists have been analysing tiny samples of ancient air trapped in water bubbles found in the mineral, quartz, which dates back more than three billion years. The team found that the air in the rocks is partly made up of an extremely rare form of the chemical element, xenon. It is known as U-Xe and what makes it so rare is that it isn’t usually found on Earth. The component is not present in the Earth’s mantle, nor is it found in meteorites.
“Therefore, the team believe that the U-Xe must have been added to the Earth after a primordial atmosphere had developed. Simply put, comets are the best candidates for carrying the U-Xe to the planet. Co-author, Prof Ray Burgess, from Manchester’s School of Earth and Environmental Sciences explains: “The Earth formed too close to the Sun for volatile elements, such as U-Xe, to easily condense and they would have rapidly boiled off the surface and been lost to space.
“”The reason that oceans and an atmosphere exist at all is because volatiles were still being added after the Earth formed. The puzzle is in identifying where the volatiles came from and what objects carried them to the early Earth. The difficulty is that many of the different volatile ingredients that were originally added have been thoroughly mixed together by geological processes during Earth’s long geological history.”” (4)
It turns out that xenon, in general, is mostly absent from the Earth’s atmosphere, particularly compared to other noble gases like argon. No one knows why. Perhaps the missing xenon is encapsulated within rocks buried deep within the Earth. Or perhaps, conversely, it has been driven off the Earth because it is not easily captured by rocks like perovskite (5). Xenon is missing from Mars, too, which may allude to its propensity for loss from a weak atmosphere.