Tuesday, July 21, 2015

Four Fabulous Dwarfs

Figure 1. Small Solar System objects photographed by the Dawn and New Horizons missions, shown at their relative sizes with radii in kilometers (km). Ceres is the largest object in the Asteroid Belt; Pluto is the largest object in the Kuiper Belt. Both are currently designated dwarf planets, although even Pluto is notably smaller than the Earth’s Moon (radius 1737 km). Now that Vesta has been imaged as a battered, non-spherical remnant, it no longer seems as planet-like as it used to.
Dawn and New Horizons – dutiful space robots launched years ago into realms beyond Earth – have fulfilled their missions over the past few months by returning spectacular images of Ceres and Pluto, the principal dwarf planets of our Solar System. These missions have also illuminated two of their companions: Vesta, which shares a similar orbital space with Ceres, and Charon, which can be described as Pluto’s moon, but is really one constituent of a unique “binary system” otherwise unattested in planetary science. Until very recently, these four worlds had been imaged only as blurry dots. Now astronomers are using the latest photos to make high-resolution maps of their surface features.

Ceres, Vesta, and Pluto were all thought to be planets when they were discovered, respectively in 1801, 1807, and 1930. In each case, however, research by later generations established that these objects are not only much smaller than originally estimated, but also accompanied by thousands of other similar, smaller objects in the same orbital space. 

Figure 2. Terrestrial planets, dwarf planets, and moons

This figure represents 3 of the 4 terrestrial planets, 2 dwarf planets, and 12 of the 19 spheroidal moons in our Solar System. All four terrestrial planets and two moons (Io and Luna) are rocky. All the other small spheroids in our system, which have orbits wider than Mars, are a combination of rock and ice.
We now know that Ceres and Vesta are respectively the largest and third-largest objects in the Asteroid Belt, a swarm of debris orbiting mostly between 2 and 3.5 astronomical units (AU). Both were demoted from planetary status in the mid-nineteenth century. We also have brand-new evidence that Pluto is the largest object in the Kuiper Belt, another swarm of debris orbiting mostly between 30 and 50 AU. Pluto was demoted to dwarfdom only in 2006, after more than a decade of new evidence revolutionized our understanding of the Kuiper Belt.

I still run into people who resent Pluto’s demotion. I kind of understand their feelings, since I was a child once myself, fond of underdogs and eager to back contrary viewpoints. Nevertheless, between Dawn and New Horizons, I was much more excited by the former, given my interest in asteroids and their role in the Solar System’s evolution. That inclination changed dramatically just the other day, when those amazing photos of Pluto and Charon were beamed back to Earth (e.g., Figure 1). Vesta and Ceres are truly fascinating, but Pluto and Charon are something else again! 

Figure 3. Two views of Vesta

These photos of Vesta’s south pole, taken from different distances at slightly different angles, hint at the object’s original spheroidal shape.
The asteroid known as 4 Vesta was evidently once a rocky spheroid, more than four billion years ago when our system was young. Although 2 Pallas is slightly larger, Vesta is second in mass only to Ceres among the asteroids. Radionuclide analyses suggest that Vesta was born just 3 million years after the Sun, making it one of the oldest offspring of our parent star. As such, Vesta is probably a remnant protoplanet – a rare survivor of the ancestral swarm from which all the planets and moons in our system coalesced. Over the aeons, collisions with other objects have blasted away big chunks of Vesta’s mantle and left a multitude of surface scars in the form of craters and chasms. The result is a peculiar object resembling a slightly squashed pumpkin that some knife-wielding maniac has carved at random.

This battered world orbits the Sun at a semimajor axis of 2.36 AU in a period of 1325 days. Its orbit is as steeply inclined to the plane of the Solar System as that of Mercury (7 degrees) and as eccentric as that of Mars (0.09). 

Figure 4. Comparison view of Ceres and our Moon

The topography of Ceres looks even flatter and gentler than the outward-facing hemisphere of the Moon, which is remarkable for its blandness in comparison with the Earth-facing side. Note that these images are shown at different scales; the diameter of the Moon is 3.7 times that of Ceres.
Well-resolved photos and other data on Ceres began arriving only in January, so astronomers have had less time to digest this information than they’ve had for Vesta, which Dawn visited in 2011-2012. The photos reveal a slightly oblate sphere with a thoroughly cratered surface. Some craters feature bright white spots that have incited considerable scientific interest, but so far no robust explanations (icy patches? salts?). Overall, the landscape looks rather Lunar, except that Ceres has neither dark areas nor mountain ranges. Although much the same description applies to the far side of our Moon, the Lunar far side still looks more rugged than Ceres (Figure 4).

According to a widely accepted view, Ceres has a rocky core covered by an icy mantle accounting for about 50% of its volume and 25% of its mass. This is similar to the structure proposed for the spheroidal moons of Jupiter and Saturn. This model, along with recent observations of water vapor escaping from discrete areas on the surface of Ceres, led some people (e.g., me) to entertain the possibility that Ceres would resemble Europa or Enceladus: flat frozen plains with markings suggesting the activity of subsurface seas. As it turns out, Ceres bears a much closer visual resemblance to Rhea and Tethys, two of Saturn’s icy moons (Figure 5). Its crust might be an amalgam of ices and clays.

Figure 5. Ceres and Tethys at their relative sizes

Ceres orbits the Sun in an orbit wider than Vesta’s, with a semimajor axis of 2.77 AU and a period of 1682 days. Its inclination (almost 10 degrees) and eccentricity (0.12) are also larger than Vesta’s. Nevertheless, the eccentricity of Mercury – the most eccentric of the eight planets – is even higher, at 0.21. Both the orbital elements and the morphology of Ceres and Vesta should eventually shed light on the origin of the Asteroid Belt. For now, though, we have no compelling hypotheses that are also consistent with the widely endorsed Grand Tack model of system history. 

Pluto and Charon 
If we compare asteroids to Kuiper Belt objects (hereafter Kuiperians), we can’t avoid noticing that size matters. Pluto and Charon are significantly larger than the largest asteroids, providing scope for much more varied, planet-like topographies. But environment matters, too: Vesta and Ceres both orbit solo, while Pluto and Charon constitute a binary system, with each member of the pair tidally locked to the other.

Figure 6. The Pluto System

The two Kuiperians zip around their common center of mass – which falls outside the volume of Pluto – in a period of only 6.4 days, maintaining a mutual separation of 19,570 km (just 5% of the 384,400 km separating Earth from our Moon). Four other objects orbit the central binary – tiny, irregular moons christened Nix, Hydra, Kerberos, and Styx, all with circular orbits that are coplanar with the motion of Pluto and Charon (Figure 6). Remarkably, all four were discovered in the past 10 years, and thanks to New Horizons, we already have photos of two of them.

Figure 7. Nix and Hydra, moons of Pluto

Contrary to expectations, the surface morphology of the two big Kuiperians at the center of this knot of satellites reflects recent geologic activity. Charon has few visible craters, Pluto even fewer. The New Horizons team estimates that parts of Pluto’s surface assumed their current form only within the past 100,000 million years, unlike the truly primeval topographies of Mercury, Vesta, and the Moon. 

Pluto is thought to be composed of a rocky core with an icy mantle. Its surface consists largely of frozen nitrogen with traces of methane, and it also supports a tenuous atmosphere containing nitrogen, methane, and carbon monoxide. These atmospheric gases evidently arise from evaporation of surface deposits during the long Plutonian summer, which happens when Pluto is near perihelion (i.e., its closest approach to the Sun). The gases create a haze that floats more than 100 km above the surface. New Horizons observed a large volume of nitrogen escaping into space, creating a comet-like tail. As summer ends, the gases likely freeze again and rain onto the surface, creating the variegated landscapes we see.

Figure 8. Ice mountains in Tombaugh Regio on Pluto

Above, the Norgay Mountains amid a craterless plain. Below, icy mountains in the southwest, adjacent to the black, cratered terrain of Chthulhu.
To my great surprise, the Plutonian landscape is colorful, ranging from charcoal black to orange to snow white. Recent telescopic observations suggest that the landscape’s colors are changing over time, becoming brighter and redder; these changes might be seasonal. A few striking geographical features on Pluto have already been named. The big white heart-shaped area visible in Figure 1 is now called Tombaugh Regio, after Clyde Tombaugh, Pluto’s discoverer. The New Horizons team reports that this area is covered with frozen carbon monoxide – not exactly the most romantic chemical compound! Tall, icy mountain ranges have also been imaged in this area (Figure 8), which appears to be devoid of impact craters. Adjacent to Tombaugh is a dark region named Chthulhu, after the unspeakably dreadful entity who haunts the tales of H.P. Lovecraft (Figure 9).

According to the latest news (July 24), Pluto's surface deposits of frozen methane, nitrogen, and carbon monoxide qualify as glaciers, flowing across the landscape much like glaciers on Earth.

Figure 9. Two hemispheres of Pluto
These images were recorded as New Horizons approached Pluto, explaining the difference in resolution.
Charon looks just as interesting as its binary partner. At the north pole is a very dark crater-like formation nicknamed Mordor, after the land of the Shadow (AKA Sauron) in the tales of J.R.R. Tolkien. Elsewhere the new photos reveal smaller craters, chasms, white and dark patches, and broad plains. Recent publications indicate that Charon’s composition includes a larger proportion of ices than Pluto’s, and that water ice in particular is common on Charon. These characteristics, combined with its size and surface features, lend Charon a distinct resemblance to the four largest moons of Uranus (Figure 10).

Pluto and Charon orbit the Sun in a period estimated at 248 years, so astronomers have observed only about one-third of a Plutonian year so far. With an eccentricity of 0.249, Pluto’s projected orbit is more elongated than that of any full-size planet. Neptune, the nearest planet to Pluto and the outermost of the canonical eight, follows an almost perfectly circular orbit with a period of 164 years. As a result, the Pluto-Charon system regularly crosses Neptune’s orbit. Yet this configuration appears to be stable on billion-year time scales, thanks to its machine-like precision. Neptune and Pluto are engaged in a 3:2 resonance, such that Neptune completes three orbits for every two orbits of Pluto. This “time-sharing” arrangement guarantees that the two objects are always far apart during their regular orbit crossings, preventing collisions or perturbations. 

Figure 10. Charon alongside the four largest moons of Uranus, shown to scale


The intricacy of this configuration suggests a sort of Baroque timepiece worthy of Kepler or Leibniz. It certainly injects an exciting counterpoint into the grand symphony of the spheres. For those of you who like music along with your astronomy, I recommend two Plutonian excursions: “Pluto Drive,” a spooky tune by The Creatures, recorded in 1989, and “Orfeo, vincesti,” a lyrical bass aria sung by Pluto in the opera Orfeo, composed by Antonio Sartorio in 1672 and performed by Harry Van Der Kamp in a recording from 1999 (unfortunately not on Youtube). As Sartorio intuited and New Horizons has proven, even Pluto reveals an unexpected beauty - and a surprisingly big heart! - when observed in the right light. 

Figure 11. Pluto's atmosphere back-lit by the Sun

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