DPS/EPSC update on New Horizons at the Pluto system and beyond
Last week's Division for Planetary Sciences/European Planetary Science Congress meeting was chock-full of science from all over the solar system. A total of five sessions (one plenary, three oral, and one poster) was devoted to New Horizons at Pluto. It's been a year since the flyby, a year that early science has had a chance to mature. What's changed about our understanding of Pluto in that time?
First of all, an important reminder: New Horizons didn't return its data instantly. We were told it would take 16 months to get all the data down. It took slightly more than that, but the data transmission is now complete, as of last weekend. Hats off to the New Horizons team for not only accomplishing the flyby, but safely returning all the data!
Of course, because data transmission took so long, scientists kept needing to modify analyses to incorporate freshly returned data. It's like an image progressively coming into focus -- the early data gave the team a good sense of what they had, but later data added depth and detail.
I'll give some science summaries below, but first I want to share some news about data release, as well as a pretty picture. In his plenary talk, principal investigator Alan Stern announced that the second delivery of data to the Planetary Data System will come this month. This is going to include a lot of the nicest photos that New Horizons took through both high-resolution LORRI and lower-resolution-but-color MVIC cameras, and it was all downlinked without lossy compression, so it will be an exciting data release. There are two more releases planned for Pluto flyby data, in April and September 2017. Stern also mentioned that NASA has just announced a Data Analysis Program (DAP) for New Horizons, meaning that researchers not on the team can now propose for grant funds to work on the mission's data. Finally, Stern and many other team members shared this absolutely gorgeous color map of Pluto's surface, remarking on how you can see Pluto's color changes strongly with latitude:
NASA / JHUAPL / SwRI
Color map of Pluto
This map contains data from New Horizons' color imager, Ralph MVIC, in a version processed about a year after the Pluto flyby. The color map shows strong variations in Pluto's color with latitude, from its orangish north to its pinkish midlatitudes to its very dark equatorial band, with Sputnik planitia sitting athwart the band.
And now for the science tidbits. Last year, the team reported observing slightly different densities for Pluto and Charon, but weren't yet sure if the two might still have the same composition; Charon, being smaller, might be less compressed. Stern said that more recent work by the mission's geophysicists including Bill McKinnon has determined that Charon really does have a different, lighter composition than Pluto: the rock fraction of Charon is about 59% (with the rest being ice), the rock fraction of Pluto 65 to 67%.
There was much discussion of Sputnik planum, now called Sputnik planitia (more on that shortly), the big bright spot on Pluto that many people call Pluto's "left ventricle" but which would actually be the right ventricle if you were Pluto. Leslie Young talked about how Pluto's climate modelers didn't expect a giant nitrogen-rich basin on Pluto, and that's sent everyone back to the drawing board, understanding how the massive reservoir of volatile materials releases and takes in gas over time. Bill McKinnon showed topographic maps of Sputnik planitia that explained why the name has changed: they now know that Sputnik is actually a deep basin, a massive ice sheet whose top lies 2.5 kilometers below Pluto's mean elevation. That makes it not a plain but a basin, and thus, it's a planitia. In fact, it's most likely an impact basin, an elliptical one 1300 by 900 kilometers in extent, which would have formed from the glancing impact of a 200-kilometer body.
James Tuttle Keane explored the history of that impact a bit more, presenting a hypothesis that the impact and subsequent condensation of a thick layer of nitrogen ice within the basin made it a "positive mass anomaly," like the lunar mascons or the Tharsis plateau on Mars. If you add extra mass to the side of a rotating globe, the globe wants to put that mass on the equator, and the entire outer shell of a world can reorient. It's especially easy to do that if the world has a subsurface liquid layer, as Pluto certainly once had (and might still have). Sputnik planitia is now near Pluto's equator, suggesting that -- like Tharsis -- it's a mass anomaly that has reoriented Pluto's lithosphere. That would have telltale effects in the patterns of tectonic fissures on Pluto's surface.
Oliver White noted that the color of the basin shifts from north to south and that there is more evidence of sublimation pits in the north. Moreover, he pointed out that the northern boundary between the darker and brighter material is coincident with the location of Pluto's northern arctic circle, which is currently bathing in continuous summer sunlight. There may be a net sublimation of nitrogen from the northern part of the heart, leaving behind a darker lag and sublimation pits, and net deposition in the south, giving it a fresher, frostier surface.
John Spencer mentioned Kelsi Singer's work mapping craters on Pluto and Charon, noting that both Pluto and Charon have heavily cratered surfaces that are equally heavily cratered, and both show a relative lack of small craters. Because both worlds are missing small craters in similar proportions, it's probably not due to resurfacing; it's probably a quality of the population of things that hit them, i.e. the Kuiper belt. Spencer also showed a really cool topographic map of Charon showing that it has broad topographic troughs that aren't obvious in visual images. This is work being done by Ross Beyer and Paul Schenk that has yet to be published. Finally, Spencer showed a processed version of a post-flyby Charon image that I don't recall seeing before, in which you can barely see Charon by Plutolight. Spencer said that there was the suggestion of a dark south pole on Charon mirroring its dark north pole, "perhaps."
Early analysis of the stellar and radio occultations they performed with Charon to try to detect its atmosphere resulted in a non-detection. Now the team can quantify that: if Charon does have any atmosphere, it is less dense than our Moon's atmosphere. To put it another way, physically speaking, there have to be some molecules and ions floating around above any solid surface in a vacuum, but there are way fewer of those molecules above a given area of Charon's surface than there even are above the Moon's surface.
Charon's surface is dominated by water ice, but the spectroscopists have located outcrops of ammonium or ammonium hydrates. The geologist in me keeps balking at typing "outcrops of ammonium," but at Pluto's cold temperatures, these materials do function as rocks do on Earth, so "outcrops" they are. Stern also mentioned the intriguing presence of sublimation pits in Charon's surface -- intriguing because nothing would be sublimating there today.
Anne Verbiscer talked about the phase curves of Pluto and Charon. A phase curve is a study of how strongly a surface reflects light depending on the angle at which light travels from the light source, from the surface, to your eye (or your camera). Many surfaces in the solar system display a strong "opposition surge," in which the surface appears to suddenly brighten when you view it with the light source directly behind you. This is a property of rough and/or porous surfaces. From Earth, with Hubble, we can only see Pluto and Charon at phase angles ranging up to 2 degrees, because we're so much closer to the Sun that we almost always see them nearly at opposition. New Horizons got a much wider range of phase angles (though not lowest phase), and Verbiscer showed phase curves, with Charon having a much stronger opposition surge than Pluto. Charon, she said, has a highly porous surface of opaque water ice particles, and Hydra and Nix are similar. Pluto has a more compact surface, with transparent grains of methane and nitrogen. Overall, Pluto's global scattering properties are similar to those of Neptune's moon Triton.
Looking a bit more closely at the way that Pluto's surface reflects light, Bonnie Buratti showed the results of some work she's been doing on quantitative albedo mapping. She reminded the audience that most of the variations in intensity within a spacecraft image are not intrinsic, but are rather due to changes in viewing geometry. So they processed New Horizons' images to make a map showing what the surface would look like if incidence, emission, and phase angles were all zero. She found that Pluto's bright region (Sputnik) is almost as bright as Enceladus, and its darkest regions are under 10% reflective. Of all the other worlds in the outer solar system, only Iapetus has so much contrast; Iapetus is overall darker than Pluto but has a similar factor-of-10 range in its albedo. Buratti went on to speculate that Eris, with its Enceladus-like extraordinarily high albedo, must be active, because a surface so bright is hard to maintain with any amount of contaminants on the surface.
NASA / JHUAPL / SwRI / Emily Lakdawalla
Pluto system family portrait
The four largest bodies in the Pluto system, to scale: Pluto, Charon, Nix, and Hydra. The images were taken at a variety of times, from 16 to 10 hours before New Horizons' closest approach to Pluto. They have been resized to a common scale (how they would appear if New Horizons were 500,000 kilometers away). Not pictured are Kerberos and Styx; images of those moons of comparable quality have not yet been returned by New Horizons.
For the smaller moons, Jason Cook presented evidence that Nix, Hydra, and Kerberos have spectra that are dominated by water ice, which is no surprise. However, on Nix and Hydra, he also showed evidence that there is ammonium and/or ammonium hydrate on the surface. He pointed out an especially odd thing about Nix's spectrum, that the absorption bands due to water ice are very deep, implying that the light has traveled through very large ice grains, a centimeter or more across. Cook speculated a little bit about why the ammonia-related species were easier to spot on the tiny moons than on Charon. Maybe, he said, when micrometeorites impact the small moons, the ejecta escapes, so doesn't cover the surface, so these impacts can expose buried ammonia that gets buried by ejecta on Charon.
Simon Porter presented some results from New Horizons' study of Kuiper belt object 1994 JR1. As its name suggests, it was an early-discovered Kuiper belt object, found during a white dwarf survey, but it hadn't been observed since 2000. Now, thanks to New Horizons, it has a very long arc and thus a very well-determined orbit. They observed it in November 2015 and again in April 2016: the first high-phase photometry of a Kuiper belt object other than things in the Pluto system. The April observations allowed them to get a light curve, a recording of the changing amount of light an object reflects as it rotates. That, in turn, produces a rotation period: 5.69 hours. That's a relatively fast speed that makes it less likely to have a binary companion, and indeed they didn't detect one. One especially cool item that Porter mentioned was that another astronomer, Susan Benecchi, used Hubble to observe JR1 simultaneously -- that is, the same light cone, correcting for the time it took light reflected from JR1 to reach Earth. The Hubble work found it to be redder than 2/3 of objects in similar orbits.
Finally, there's been a little more study of New Horizons' next flyby target. Very little more study, because it's so faint it's accessible pretty much only with Hubble. (New Horizons won't be able to spot it until fairly close to the date of the flyby.) Hubble now tells us it's very red, typical for a classical cold Kuiper belt object, something that's been disturbed very little over the age of the solar system. It's probably 20 to 40 kilometers across, 1000 to 10000 times the mass of comet 67P. Stern said their initial plan for the flyby science includes mapping its composition, photo-mapping its surface, searching for satellites and dust, and searching for a coma.