Centauri Dreams

One reason for catching up with recent planetary science here in the Solar System is the upcoming arrival of Juno, which enters into polar orbit around Jupiter on July 4. Juno’s arrival is a reminder that the past year has been packed with interesting news from places like Pluto/Charon (New Horizons), Comet 67P/Churyumov-Gerasimenko (Rosetta), and the topic of today’s post, the intriguing dwarf planet Ceres, as studied by the orbiting Dawn spacecraft.

But the recent Ceres news hasn’t just involved Dawn. Paolo Molaro (INAF-Trieste Astronomical Observatory) had led a study looking at the bright spots Dawn found upon approaching Ceres last year. The data Molaro and team drew on came from the European Southern Observatory’s 3.6-meter instrument at La Silla and its HARPS spectrograph, which have shown us not only the motion of the bright spots as Ceres rotates but also variations that indicate volatile material within them. The suggestion is that this material evaporates when exposed to sunlight.

Ceres’ nine-hour rotation produces a small but measurable Doppler effect, with the bright spots expected to affect the spectrum of the reflected light, producing what shows up as a radial velocity variation within the overall Doppler rotational measurement. But the resulting measurements were more complex than expected, indicating a change in the reflectivity of the bright features in Occator crater. Says co-author Antonino Lanza (INAF-Catania Astrophysical Observatory), a co-author of the study:

“The result was a surprise. We did find the expected changes to the spectrum from the rotation of Ceres, but with considerable other variations from night to night.”

Image: The bright spots on Ceres as imaged by the Dawn spacecraft at an altitude of approximately 1500 kilometers. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

The changes in albedo vary from night to night, showing up as random patterns on short and long timescales. If, as has been suggested, the reflective material in the bright spots is fresh water ice or hydrated magnesium sulfates, then these changes could be caused by evaporation, forming highly reflective plumes. Still unknown is the energy source that seems to be driving the leakage of material from deeper below.

The work plays interestingly off other recent observations, as the paper notes:

Perna et al. (2015) found variations in the slope of visible spectra at the level of 2-3 percent over 1000 Angstrom with a variation in the relative reflectivity of about 10 percent in the region between 500 and 800 nm. Herschel detected water vapor plumes erupting off the surface of Ceres, which may come from volcano-like ice geysers (Küppers et al. 2014). The recent Dawn observations suggest that the bright spots could provide some atmosphere in this particular region of Ceres confirming Herschel’s water vapor detection (Witze 2015). It has been noted that the spots appear bright at dawn on Ceres while they seem to fade by dusk. That could mean that sunlight plays an important role, for instance by heating up ice just beneath the surface and causing it to blast off some kind of plume or other feature.

This description would account for plumes quickly losing their reflectivity as sunlight conditions change, only to form again in a cycle of evaporation and freezing that is also reflected in the radial velocity measurements from HARPS. Subsequent analysis of data from Dawn has shown bright localized areas that are consistent with hydrated magnesium sulfates, including a bright pit on the floor of the Occator crater that shows probable sublimation of water ice. Haze clouds are thus produced, appearing and disappearing in a regular daily cycle.

This ‘diffuse haze,’ as the authors describe it, fills the floor of Occator and then disappears almost completely at dusk, providing a possible counterpart to the radial velocity variability that Molaro and team have detected. That’s useful, because if both are produced by the same cause, then we have a way to continue to monitor daily activity on Ceres even after the Dawn mission ends. Further radial velocity observations and analysis of the Dawn imagery could confirm this possibility.

The paper is Molaro et al., “Daily variability of Ceres’ Albedo detected by means of
radial velocities changes of the reflected sunlight,” published online by Monthly Notices of the Royal Astronomical Society 7 February 2016 (abstract / preprint).

Friday’s look at the possible composition of Pluto’s Sputnik Planum took me into a deep enough dive on the two papers — Pluto gets my full attention! — that I ran out of time. I had planned to include the images below in that post, but we can do that this morning as a reminder that New Horizons shows no signs of running out of data. What caught my eye here was the possible presence of a cloud, which you can see at the top right of the left image, and in the top inset image.

The wispy structure is tens of kilometers across (the entire inset measures about 230 kilometers) and if it is a cloud, it’s the only one we’ve yet picked out of the New Horizons imagery. But if you consider the rest of the image, it would make sense that we could see a cloud here — notice how the haze layers are brightened by the sunlight that grazes Pluto’s surface at a low angle. Also in the top right inset, the southern parts of Sputnik Planum’s nitrogen ice fields show up (click the image to enlarge), along with peaks of the Norgay Montes.

Image: Looking back at Pluto with images like this gives New Horizons scientists information about Pluto’s hazes and surface properties that they can’t get from images taken on approach. The image was obtained by New Horizons’ Ralph/Multispectral Visual Imaging Camera (MVIC) approximately 21,550 kilometers from Pluto, about 19 minutes after New Horizons’ closest approach. The image has a resolution of 430 meters per pixel. Pluto’s diameter is 2,374 kilometers. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Here we’re at a high phase angle, meaning the Sun is on the other side of the dwarf planet as New Horizons leaves the system. The Planetary Society’s Emily Lakdawalla, who is an expert on spacecraft imagery, explains phase angle in typically clear and concise terms:

“Phase angle” means the angle from the Sun, to the target being observed, to the observer — it’s basically a number applied to the descriptive terms we use for lunar phases like crescent, half, gibbous, and full.”

Which means that a full moon is the equivalent of zero phase, while a half moon is a phase angle of 90 degrees. A high phase angle marks a new moon. Back in 2009, Emily wrote an outstanding post on the appearance of Saturn’s moons at different phase angles, as viewed of course by the Cassini spacecraft. This next bit is also worth quoting:

Cassini has returned well over 200,000 images from the Saturn system, but it hasn’t surveyed every moon at every latitude and longitude at every possible phase angle; we do the best we can with what we have, and always hope to get more data. If you’ve ever wondered what the point is in Cassini getting more and more images of the moons, this is why — Cassini will never fully sample every possible combination of latitude, longitude, and phase; more images will fill in gaps and make a more complete picture of the moon’s photometric behavior, which tells us what its surface is made of and how it varies from place to place.

How much more strongly those words apply to New Horizons, which while giving us a treasure trove of data, was unable in its fast flyby to depict large portions of Pluto’s surface in any detail. I know we’ll get back out there again, but as I mentioned at the end of Friday’s post, I’m impatient. I want to get to Eris, and Haumea, and Makemake, and make the kind of studies that will surely yield still more surprises among the ice worlds. Sputnik Planum is yet one more incentive: Possible bergs of water ice floating amidst a millennially elastic nitrogen sea.

Have another look at the Pluto imagery at the top of the post, where the inset at bottom right brings out further detail on the night side. The high phase angle in this case helps us tease out valleys and peaks that were not nearly as apparent in the days before New Horizons’ closest approach. The inset shows us a scene approximately 750 kilometers wide, a twilight view that only becomes apparent because of the relative positions of spacecraft, dwarf planet and Sun. And below is the view without insets, added here to provide a bit more of the ‘wow’ factor.

Image: The view without inset imagery. An uninterrupted look at Pluto at high phase angle shows sunlight filtering through the dwarf world’s atmospheric hazes. All of this is a reminder of how much good science we can do after closest approach. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

It’s been a week spent catching up with space mission news, focusing on Rosetta, Juno and today, New Horizons. Usually I ponder what I’m going to write each day on Centauri Dreams while I’m having breakfast, a quiet time to reflect on recent events. And if Jay Melosh (Purdue University) is to be believed, I might have taken inspiration from the dish of oatmeal sitting in front of me when it comes to Pluto. Because Melosh and grad student Alex Trowbridge led recent research that may explain what we see at Sputnik Planum.

A bit of background before I return to that bowl of oatmeal. We’ve seen that Sputnik Planum has an unusual appearance, visible in the photo below, that shows patterned polygons. One way of explaining this is to invoke icebergs floating on a sea of nitrogen ice. Melosh and Trowbridge believe the polygons could be what are called Rayleigh-Bénard convection cells, which flag convection that occurs in a fluid that is being heated from below. Says Melosh:

“Imagine oatmeal boiling on the stove; it doesn’t produce one bubble for the entire pot as the heated oatmeal rises to the surface and the cooler oatmeal is pushed down into the depths, this happens in small sections across the pot, creating a quilted pattern on the surface similar to what we see on Pluto. Of course, on Pluto this is not a fast process; the overturn within each unit happens at a rate of maybe 2 centimeters per year.”

Image: Like a cosmic lava lamp, a large section of Pluto’s icy surface is being constantly renewed by a process called convection that replaces older surface ices with fresher material. Scientists from NASA’s New Horizons mission used state-of-the-art computer simulations to show that the surface of Pluto’s informally named Sputnik Planum is covered with churning ice “cells” that are geologically young and turning over due to a process called convection. The scene above, which is about 400 kilometers across, uses data from the New Horizons Ralph/Multispectral Visible Imaging Camera (MVIC), gathered July 14, 2015. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

We’re looking at a surface, then, that is under continual renewal as older surface ice is replaced by new material coming up from below. The nitrogen ice of Sputnik Planum appears in a basin on a planetary surface primarily composed of water ice. Structurally weak, nitrogen ice has a low enough viscosity to flow in an area where water ice, of much higher viscosity, forms hard mountains. Thus we see mountains of water ice at the edges of the polygon regions. The researchers believe these have floated as icebergs within the convection current.

Analysis of the New Horizons data shows that the polygons are between 16 and 48 kilometers across. Melosh and Trowbridge calculate that the nitrogen ice must be at least five kilometers deep for the water icebergs to appear where they do, and analysis of the width-to-depth ratio of the nitrogen convection cells suggests the figure is closer to 10 kilometers, quite a bit deeper than the minimum needed for convection to occur. Over millions of years, the moving nitrogen ‘blobs’ can merge, with ridges marking places where cooler nitrogen ice sinks back down.

Thus we have a way to explain a Sputnik Planum surface that appears to undergo frequent renewal, a surface that investigators have thought to be less than ten million years old. The new work drops that number to about one million years. That’s a reminder that on icy dwarf planets in the Kuiper Belt, materials we normally think of as gases can produce surface change, making them active in complex ways. Considering we’re dealing with a surface that averages -229° C (44 Kelvin), a surface this active emphasizes the need for dedicated missions to the outer Solar System. Only actual data from the mission target can refine or overturn earlier models.

A second paper appearing in the same issue of Nature reinforces the convection explanation, although it may take a probe on the surface to actually confirm it. This work comes from William McKinnon and colleagues (Washington University, St. Louis). Says McKinnon:

“For the first time, we can really determine what these strange welts of the icy surface of Pluto really are. We found evidence that even on a distant cold planet billions of miles from Earth, there is sufficient energy for vigorous geological activity, as long as you have ‘the right stuff,’ meaning something as soft and pliable as solid nitrogen.”

McKinnon’s computer modeling shows solid nitrogen being warmed by internal heat and rising before cooling sufficiently to sink again, beginning the cycle all over again. The work explains X- or Y-shaped features that appear at the junctions where multiple convection cells once met. Pluto’s atmosphere may well be renewed by this activity. Principal New Horizons investigator Alan Stern calls Sputnik Planum, an area bigger than Texas and Oklahoma combined, ‘one of the most amazing geological discoveries in fifty-plus years of planetary exploration.’

Will we find similar processes at work at similar timescales — a few centimeters a year — on other dwarf planets? The next target for New Horizons isn’t a dwarf planet, but 2014 MU69 may still come up with some surprises of its own, assuming NASA approves funding for the 2019 flyby as part of an extended mission. But wouldn’t it be fascinating to one day compare the surfaces of places like Eris and Makemake with what we’ve found at Pluto’s heart?

The papers are Trowbridge et al., “Vigorous convection as the explanation for Pluto’s polygonal terrain,” Nature 534 (2 June 2016), pp. 79-81 (abstract) and McKinnon et al., “Convection in a volatile nitrogen-ice-rich layer drives Pluto’s geological vigour,” Nature 534 (2 June 2016), pp. 82-85 (abstract).

Interesting news about Jupiter this morning even as the Juno spacecraft crosses into the realm of Jupiter’s gravity. It was six days ago that Juno made the transition into Jupiter space, where the gravitational influence of Jupiter now dominates over all other celestial bodies. And it will be on July 4 of this year that Juno performs a 35-minute burn of its main engine, imparting a 542 meters per second mean change in velocity to the spacecraft for orbital insertion.

The spacecraft’s 37 flybys will close to within 5000 kilometers of the cloud tops. I only wish Poul Anderson could be alive to see some of the imagery. I always think of him in relation to Jupiter because of his stunning 1957 story “Call Me Joe,” describing the exploration of the planet by remote-controlled life forms (available in Anderson’s collection The Dark Between the Stars as well as various science fiction anthologies).

Image: Launched in 2011, the Juno spacecraft will arrive at Jupiter in 2016 to study the giant planet from an elliptical, polar orbit. Juno will repeatedly dive between the planet and its intense belts of charged particle radiation, traveling from pole to pole in about an hour, and coming within 5,000 kilometers of the cloud tops at closest approach. Credit: NASA/JPL-Caltech.

Our view of Jupiter has changed a lot since 1957, and Anderson’s low temperature, high pressure surface conditions have been ruled out, but the tale still carries quite a punch. As to Jupiter itself, today we get news that data from the Very Large Array (New Mexico) have been used to create the most detailed radio map ever made of its atmosphere. The work allows researchers to probe about 100 kilometers below the cloud tops using radio emissions at wavelengths where the clouds themselves are transparent.

Recent upgrades to the VLA have improved the array’s sensitivity by a factor of 10, a fact made apparent by the new Jupiter maps. Working the entire frequency range between 4 and 18 gigahertz, the team from UC-Berkeley supplements the Juno mission, anticipating its arrival to create a map that can put the spacecraft’s findings into context. Because the thermal radio emissions are partially absorbed by ammonia, it’s possible to track flows of the gas that define cloud-top features like bands and spots at various depths within the atmosphere.

We’re learning how the interactions between internal heat sources and the atmosphere produce the global circulation and cloud formation we see in Jupiter and other gas giant planets. The three-dimensional view shows ammonium hydrosulfide clouds rising into the upper cloud layers along with ammonia ice clouds in colder regions, while ammonia-poor air sinks into the planet amidst ‘hotspots’ (bright in radio and thermal infrared) that are low in ammonia and circle the planet just north of its equator.

“With radio, we can peer through the clouds and see that those hotspots are interleaved with plumes of ammonia rising from deep in the planet, tracing the vertical undulations of an equatorial wave system,” said UC Berkeley research astronomer Michael Wong.

Image: The VLA radio map of the region around the Great Red Spot in Jupiter’s atmosphere shows complex upwellings and downwellings of ammonia gas (upper map), that shape the colorful cloud layers seen in the approximately true-color Hubble map (lower map). Two radio wavelengths are shown in blue (2 cm) and gold (3 cm), probing depths of 30-90 kilometers below the clouds. Credit: Radio: Michael H. Wong, Imke de Pater (UC Berkeley), Robert J. Sault (Univ. Melbourne). Optical: NASA, ESA, A.A. Simon (GSFC), M.H. Wong (UC Berkeley), and G.S. Orton (JPL-Caltech).

Fine structure becomes visible in this work, especially in the areas near the Great Red Spot. The resolution is about 1300 kilometers, considered to be the best spatial resolution ever achieved in a radio map. “We now see high ammonia levels like those detected by Galileo from over 100 kilometers deep, where the pressure is about eight times Earth’s atmospheric pressure, all the way up to the cloud condensation levels,” says principal author Imke de Pater (UC-Berkeley). The work is reported in the June 3 issue of Science.

Image: In this animated gif, optical images of the surface clouds encircling Jupiter’s equator –including the famous Great Red Spot — alternate with new detailed radio images of the deep atmosphere (up to 30 kilometers below the clouds). The radio map shows ammonia-rich gases rising to the surface (dark) intermixed with descending, ammonia-poor gases (bright). In the cold temperatures of the upper atmosphere (160 to 200 Kelvin, or -170 to -100 degrees Fahrenheit), the rising ammonia condenses into clouds, which are invisible in the radio region. Credit: Radio: Robert J. Sault (Univ. Melbourne), Imke de Pater and Michael H. Wong (UC Berkeley). Optical: Marco Vedovato, Christopher Go, Manos Kardasis, Ian Sharp, Imke de Pater.

Earlier VLA measurements of ammonia levels in Jupiter’s atmosphere had shown much less ammonia than what the Galileo probe found when it plunged into the atmosphere in 1995. The new work resolves the issue by applying a technique to remove the blurring in radio maps that occurs because of Jupiter’s fast rotation. The UC-Berkeley team reports that it can clearly distinguish upwelling and downwelling ammonia flows using the new methods, preventing the confusion between the two that had led to the earlier mis-estimates of ammonia levels.

The paper is de Pater et al., “Peering through Jupiter’s Clouds with Radio Spectral Imaging,” Science 3 June 2016 (abstract).

Yesterday’s look at organic compounds on Comet 67P/Churyumov-Gerasimenko needs to be augmented today by a just released study of the comet with implications for how all comets evolve. But first, a renewed pointer to the Kickstarter campaign for KIC 8462852, the unusual star whose light curves continue to baffle astronomers. Please consider contributing to the project, which would raise enough money ($100,000) to support a year of observations.

We’re about halfway through the campaign but not yet at the halfway point in funds. Have a look at the information provided on the Kickstarter page, or in my essay A Kickstarter Campaign for KIC 8462852, which also has the relevant links. We know the light curves of ‘Tabby’s Star’ are not periodic, so we need continuous monitoring to gain more data on what may be happening there. If we can raise the funds, the Las Cumbres Observatory Global Telescope Network, already supporting the project, can give us the multi-wavelength observations we need.

A Comet’s Evolution

The rubber-duck shape of Comet 67P/Churyumov-Gerasimenko has long been noted. The ‘neck’ of the comet is what connects the two larger lobes, as is obvious in the image below. As a new study led by Masatoshi Hirabayashi (Purdue) and Daniel Scheeres (University of Colorado) points out, two large cracks appear on the neck connecting the two larger lobes. The team simulated rotation rates for the twin-lobed assembly different from its actual 12-hour spin.

The result: Two cracks similar enough to those on 67P to show just how much stress is imparted. The rotation rate is variable in an object like this one because flybys of the Sun or of Jupiter can produce a gravitational torque. And as also appears in the photo, cometary outgassing is a factor, with compounds like carbon dioxide and ammonia sublimating from the surface. A fast enough spin produced by these factors can cause the two lobes to separate. Seven hours per rotation is what it takes for the head of the ‘duck’ to break off.

Image: Comet 67P’s distinctive shape tells us much about its history. Credit: ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0.

The researchers used numerical models that examined 1000 instances of 67P ‘clones’ under varying conditions over a 5000 year period. What Hirabayashi and Scheeres have learned is that the breakup and reassembly is an ongoing process as comets respond to these stresses. It’s also one that could last the lifetime of the comet. Says Scheeres:

“The head and body aren’t going to be able to escape from each other. They will begin orbiting each other, and in weeks, days or even hours they will come together again during a slow collision, creating a new comet nucleus configuration.”

As strange as it looks, Comet 67P may not be all that unusual. So far we have imaged seven comets at high resolution, five of which are bi-lobed. The researchers have learned that all of the bi-lobed comets have similar volume ratios between each lobe, an indication that the same cycle of disassembly and reassembly is happening in them as well. In some, there are similarities to what we find in a certain kind of asteroid. From the paper:

…bilobate nuclei observed by spacecraft encounters or ground-based radar have component volume ratios consistent with their nuclei being trapped in a similar cycle to that of 67P’s nucleus. For bilobate nuclei with a volume ratio between their lobes larger than about 0.2, the total energy of these systems will be negative after fission. This means that they are bounded in a similar way to some rubble pile asteroids; however additional sublimation effects could further erode or spin up the individual lobes before re-impact.

The process may be a major factor in cometary evolution, giving us insights into how these objects change over time:

Taking material density to be constant, we computed the volume ratios of the imaged bilobate nuclei of comets 1P/Halley, 8P/Tuttle, 19P/Borrelly, 67P and 100P/Hartley 2; we found that all of these nuclei had a volume ratio higher than 0.2… Observed nuclei with a single component might either be primordial, or have been part of a multi-component object, from which smaller parts are more easily shed.

Window into the Late Heavy Bombardment?

67P/Churyumov-Gerasimenko is a Jupiter-family comet orbiting the Sun every 6.5 years; such periodic comets are thought to originate in the Kuiper Belt, far beyond Neptune’s orbit. We learn that chaotic spin rate changes and the subsequent breaking into parts and reassembling probably caused the breakup of many ancient periodic comets originating at similar distances from the Sun. Enough erosion would have been produced by the continuing reconfiguration of their nuclei to reduce their ability to survive migration into the inner Solar System.

This could explain why comets were not a strong factor in the late heavy bombardment some four billion years ago, when numerous asteroids collided with the early terrestrial planets — two recent papers have made this case. “The reconfiguration cycles of short-period cometary nuclei,” the paper adds, “constitute a new evolutionary process that could affect their ability to survive during migration into the inner solar system.”

ESA’s mission to Comet 67P may, in other words, be giving us insights into the primordial bombardment that reshaped terrestrial worlds. The paper is Scheeres et al., “Fission and reconfiguration of bilobate comets as revealed by 67P/Churyumov-Gerasimenko,” Nature 1 June 2016 (abstract).