Sneaking Up on Pluto (Part 2)
Steve Campbell December 2015 Homepage
I always tend think of the exploration of the Solar System as something everybody knows about and that is probably erroneous for two reasons. First, not everyone was fascinated by the subject at the time, as I was and second, lots of you individuals are not old enough to have been paying attention when it happened. I would assume that some of this is taught in school, but I really can’t say, since it was just beginning to happen when I was at S.P Waltrip High in Houston. Both Shelly Duval and Patrick Swayze had graduated before I got there, in case some fans of either want to know.
One thing that might puzzle the average student might be why we had images of all the Outer planets by the 1970s and 80s and nothing but a dot or smudge for Pluto. That all relates to what was called at the time “The Grand Tour”. As it happened, there was an alignment of the outer planets in the 70’s and 80’s such that it would be possible to use gravity assisted orbital adjustments (“the slingshot effect”) to make it possible for a space probe to visit Jupiter, Saturn, Uranus and Neptune in one long and carefully managed trajectory. That’s another interesting story and I would be happy to tell it later.
Unfortunately, Pluto was not properly aligned to be next in the series of these visits. Why not? One way it was explained to me was: Any trajectory plotted for a probe approaching Neptune to send the probe to Pluto would intersect Neptune itself. So, that is why Pluto remained an unvisited backwater of the Solar System until now.
The alignment of the outer worlds by 2006 was scattered enough that only Jupiter could help send the craft to Pluto and then only in a certain window of time. Missing that window would lengthen then the mission severely or delay the launch by about twelve years until Jupiter came by again. Fortunately, the launch came off well on the first try.
A considerable amount of data was collected in the Jupiter flyby (2). A lot of what was last seen by the Magellan Orbiter was updated and enhanced. Figure AA is a view of the Jovian moon Io which is far more volcanically active than the Earth. That is pronounced with a short “I” by all the Ivory Tower PhD’s and a long “I” by normal people. Major changes in its Geology (Don’t give me a hard time about that word!) were detected.
Many other moons and Jupiter itself were imaged and studied, but we are talking about Pluto, here.
Figure AA: A view of Jupiter’s moon Io as seen from New Horizons during its fly-by of Jupiter Credits: NASA/JHUAPL/SwRI
New Horizons Spacecraft
Figure A is the New Horizons Spacecraft. The main body of the spacecraft is about the size of a grand piano and the whole thing masses as much as a medium sized truck
Figure A: The New Horizons Spacecraft Credits: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
New Horizons (NH) incorporates all that has been learned over the years. The Voyager probes (one of which actually made the complete “Grand Tour”) each had a main antenna that was capable of constant communications with the Earth. This necessitated what is called a “scan platform” that held the instruments that need precise pointing, that moved independently of the antenna. That configuration had proven troublesome on one of the Voyager probes and data were lost. That is because data storage was actually on a ½ inch, 8 track magnetic tape with a total capacity of about ½ Megabyte and a top baud rate of 56 kilobits per second (3). That’s what I said – “stone knives and bear skins!” – so, real-time transmission was required for image data.
The newer probes including NH have fixed instruments that are pointed by turning the entire spacecraft. This in turn means that the probe cannot talk to the Earth and take instrument readings at the same time. What makes it all possible is an enormous memory capacity that is capable of high data rates. This was a luxury that earlier probes could not enjoy. The disagreeable result was that the probe was “radio silent” as it collected the bulk of the science data at Pluto. This is simply because it was busy pointing instrument and taking readings.
That large dish antenna labeled “REX” in the Figure was a lot smaller than they might like but it was also a trade-off to allow more instrumentation. That means that, while the data could be acquired in a big rush during the fly-by, the data return rate was dismally slow by comparison. So much so that the NH is still downloading data, months after the fly-by and will be doing so until November of 2016.
The other instruments are described here as quoted from the New Horizons web site (1):
“The New Horizons team selected instruments that not only would directly measure NASA’s items of interest, but also provide backup to other instruments on the spacecraft should one fail during the mission. The science payload includes seven instruments:
Ralph: Visible and infrared imager/spectrometer; provides color, composition and thermal maps.
Alice: Ultraviolet imaging spectrometer; analyzes composition and structure of Pluto’s atmosphere and looks for atmospheres around Charon and Kuiper Belt Objects (KBOs).
REX: (Radio Science EXperiment) Measures atmospheric composition and temperature; passive radiometer.
LORRI: (Long Range Reconnaissance Imager) telescopic camera; obtains encounter data at long distances, maps Pluto’s farside and provides high resolution geologic data. SWAP: (Solar Wind Around Pluto) Solar wind and plasma spectrometer; measures atmospheric “escape rate” and observes Pluto’s interaction with solar wind.
PEPSSI: (Pluto Energetic Particle Spectrometer Science Investigation) Energetic particle spectrometer; measures the composition and density of plasma (ions) escaping from Pluto’s atmosphere.
SDC: (Student Dust Counter) Built and operated by students; measures the space dust peppering New Horizons during its voyage across the solar system.”
The alert reader will note that the same antenna (REX) that returns data to the Earth is also listed as an instrument. It is used to measure the changes in an Earth-NH transmission as the signal is eclipsed by Pluto’s atmosphere and surface and the same situation was also measured at Charon, thus characterizing the atmosphere of Pluto and of Charon (if any).
A Better View – Like “Way!”
If you are wondering why I have gone on so long about the discovery, naming and early characterization of Pluto, Astute Readers, I will now confess: I wanted to convey – just a bit – that long-delayed anticipation that I felt – literally for years – in awaiting the results of the NH mission. Hence, the title of this article “Sneaking up on Pluto”. That said, I hasten to present a Portrait of the Happy Couple, Pluto and Charon. Please see figure B.
Figure B. Pluto and Charon Credits: NASA/JHUAPL/SwRI
First, I must point out that this graphic is a composite. That is to say that while they are absolutely valid images of Pluto and Charon, they have been cut and pasted into this “Family Album”.
You will no doubt notice some very intriguing and unexpected features of both the planet and its satellite. Far from a near-featureless cratered ice-ball, it is obvious (by lack of craters in some regions) that Pluto has undergone recent changes. There are distinct regions of very different character and color. Charon has a great chasm that spans its diameter and crosses its equator.
The early much interpreted, computer generated images from the scant data received by the Hubble Telescope that indicated differentiated terrain are richly confirmed. The most pronounced feature is the large plain of ice that quickly became known as the “Heart” at this resolution much of it seems featureless and hence craterless. The standard procedure for dating terrain on solar system objects is to count the number of craters of different sizes. When the craters are many and varied, the terrain is obviously very old. When you see an area with no craters, then it is very new, relatively speaking. The Heart was later “officially” named “Tombaugh Regio” in appropriate honor of Pluto’s discoverer.
I should mention that I referred to Tombaugh as a High School Graduate previously, but I feel obliged to point out that he later earned a PhD. My reference to his educational status at the time of the discovery was no slight, but rather was my tribute to the idea that Excellence does not require certification. I have known and worked with many brilliant PhD’s. I have also known and worked with some who were so over-specialized as to be (in my humble opinion) rather shallow and uninteresting people, outside of their rather small zone of competence.
The smooth-looking part (on the left) of Tombaugh Regio is now called “Sputnik Planum”. As we will see in the next images, it is not nearly as featureless as it first appeared.
Pluto in Detail
I will take a leap forward now to some of the most up-to-date images. Figure D is a close-up of the dark region near the Southwest of the Tombaugh Regio. It covers a confluence of three terrain types, the smooth, icy plains at top, the mountains (obviously not associates with craters) in the center and more “conventional” cratered landscape at the bottom.
Figure D: The Dark Area at the Southwest of Tombaugh Regio. Please note the three distinct terrains Credits: NASA/JHUAPL/SwRI
The icy plains are now revealed to have distinct polygonal divisions. The ice in question is actually solid Nitrogen and Methane which, at the ambient temperature of about -230° C behave much like Earth-temperature water ice and flow slowly into valleys as they accumulate. The Mountains at the center of this image are quite clearly not related to craters and probably contain a large fraction of water ice which at Pluto temperatures is as hard and durable as rock. I will cite the good Doctor Shenck (4), again for this insight.
Pluto has been moving farther from the Sun since 1985 and you might expect that the atmosphere could be condensing out to be frozen on the surface. What did puzzle me was the contention that we have no evidence of Pluto’s atmosphere actually freezing out as it moves farther from the Sun. I asked Doctor Shenck if there might be some deposition of atmospheric gasses in the seasonal total-dark areas of the Southern hemisphere and if there might be some data (yet to be downloaded) from instruments that might answer that. He replied positively to both questions.
This image in Figure D seemed to me to give some merit to the idea that some atmospheric “fall out” may have already taken place. The crater at lower left in the image, quite clearly indicates that the ice there accumulated, did not flow from anywhere else, but must have condensed (been deposited) out of the atmosphere. Also, I mentioned that the ice appears to be flowing into valleys and I ask you, how can ice flow if there is not a new supply being deposited on the existing mass of ice? For the record, this image was not available when I spoke to Dr. Schenck.
Just during the writing of this article the answers came from this quote from a NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute Press Release:
“Key to understanding activity on Pluto is the role of the deep layer of solid nitrogen and other volatile ices that fill the left side of Pluto’s ‘heart’ — a vast, 620-mile (1,000-kilometre) -wide basin, informally named Sputnik Planum. New numerical models of thermal convection within this ice layer not only explain the numerous polygonal ice features seen on Sputnik Planum’s surface, but indicate this layer may be up to a few miles thick. Evaporation of this nitrogen and condensation on higher surrounding terrain leads to glacial flow back toward the basin; additional numerical models of nitrogen ice flow show how Pluto’s landscape has been and is still being transformed.”
Figure E is a higher resolution image from the edge of the Tombaugh Regio that shows much more texture to the icy plains and a much better look at the mountains.
Figure E: High resolution color image at the edge of Tombaugh Regio Credits: NASA/JHUAPL/SwRI
Figure F is an even higher resolution of another region of the icy plains. Notice how the ridges that divide the segments are seemingly being covered up with what look like dunes to me. I don’t know what to think of those. But the aforementioned press release indicates that there is a Nitrogen/Methane cycle of evaporation and condensation that drives the glacier-like accumulations. It seems that these gasses play the role of water on Earth, that exists in solid, liquid and gaseous forms on the same planet. Now, there is no evidence (yet) of any liquids on Pluto and I suspect that the cycle is one of sublimation (solid to gaseous) and deposition (gas to solid).
Figure F: Higher resolution image of the icy plains Credits: NASA/JHUAPL/SwRI
Despite the previously mentioned low data rate, the data are accruing at an overwhelming rate and the unmitigated diversity and complexity of this information will no doubt keep Planetary Scientists employed for years to come. I really need to publish this before it gets even further obsolete. But one thing is clear. Pluto is far from the static, frozen, cratered, icy rock it was imagined to be. It is a dynamic and complex world and IMHO, deserves the designation of “Planet” without qualifiers.
Still, it is a tantalizing irritation that the New Horizons probe only provided a “snap shot” of the situation at Pluto and we can only find out what happens next by a similar, massive effort to launch another such probe. It is perhaps a comfort to remember that the technology of the New Horizons probe is about 15 years out-of-date now and the next such probe would be faster, better, cheaper and -especially- less massive. It is not unreasonable to imagine that a Pluto orbiter could be within the realm of possibility. Even if decades later, a new probe could see the changes and the longer the delay, the more obvious those might be.
- New Horizons Spacecraft: http://www.nasa.gov/mission_pages/newhorizons/spacecraft/index.html
- Jupiter Flyby: https://en.wikipedia.org/wiki/New_Horizons#Jupiter_encounter http://pluto.jhuapl.edu/Mission/The-Path-to-Pluto/Jupiter-Encounter.php
- Voyager Data Rate: http://itknowledgeexchange.techtarget.com/storage-disaster-recovery/nasas-voyager-used-8-track-tape-to-go-into-space/
- New Horizons Mission:Dawn and New Horizons MissionsSept. 3, 2015 Lunar and Planetary Institute, Lunar and Planetary Institute September 3, 2015 NASA’s Exploration of Ceres and Pluto: An Update, Dr. Paul Schenk