Space Sunday: perfectly Pluto

New Horizons (travelling approximately left-to-right) passes Pluto on July 14th, 2015, with Charon beyond, in NASA's Eyes on Pluto simulation
New Horizons (travelling approximately top left to bottom right) passes Pluto on July 14th, 2015, with Charon beyond, in NASA’s Eyes on Pluto simulation

It’s a mission that cost $650 million to mount, took 5 years of planning and building prior to spending 9.5 years in space as one of the fastest man-made objects yet built (and the fastest ever at launch); it has travelled some 4.76 billion kilometres to reach its destination, swinging by and studying Jupiter  (the first time we’ve done so close-up in over decade) in the process. All this for a close encounter which, due to the speed of the vehicle, could be measured in a mere hours.

But what an encounter!

I’m of course referring to NASA’s New Horizons mission which, on July 14th, 2015, after all of the above, flashed by the Pluto-Charon system precisely on target and just 72 seconds ahead of it’s  predicted arrival time of 11:49:59 UTC at its closest point to Pluto.

Encounter trajectory: New Horizons' flight path is shown is red, running right-to-left in 10 minute time increments. The times for the vehicle's closest encounters with Pluto and Charon on July 14th, 2015, are given, together with the times of occultation - when both worldlets would be directly between the spacecraft and Earth
Encounter trajectory: New Horizons’ flight path is shown is red, running right-to-left in 10 minute time increments. The times for the vehicle’s closest encounters with Pluto and Charon on July 14th, 2015, are given, together with the times of occultation – when both worldlets would be directly between the spacecraft and Earth – click for full size

Obviously, the overall encounter has been going on for some time now, as I previewed in my  Space Sunday report of July 12th: what NASA called the “distant encounter phase” started in January 2015, and even now, as New Horizons heads away from Pluto and Charon, observations are still being made. But the mission has always been about the hours immediately either side of that point of closest approach, when New Horizons flashed by Pluto at a speed relative to the planet of 13.77 km/s (8.56 miles per second).

The close approach wasn’t something that could be followed in real-time, the time delay in transmissions from the probe to Earth being some 4.5 hours. This being the case, NASA kept people informed with images and information recorded in the hours leading-up to the period of closest approach, such as a stunning image of Pluto captured by New Horizon’s LORRI and Ralph instruments on July 13th. Since then, they’ve been releasing a steady stream of the initial images that have been returned by the probe.

July 13th: two views of Pluto. On the left is an approximate true-colour image of the surface of Pluto, captured by the LORRI imaging system on New Horizons, and colour-enhanced by data obtained by the Ralph suite of instruments. On the right, a false-colour image indicating the compositional differences comprising Pluto's surface
July 13th: two views of Pluto. On the left is an approximate true-colour image of the surface of Pluto, captured by the LORRI imaging system on New Horizons, and colour-enhanced by data obtained by the Ralph suite of instruments. On the right, a false-colour image indicating the compositional differences comprising Pluto’s surface.

Pluto also appears to be an active planet – more so than had been imagined – with distinct compositional difference across its surface, making understanding of some of its characteristics difficult, so it is going to be some time before a range of questions relating to Pluto’s formation, development, etc., are liable to be answered, as many of them are going to have to wait for the arrival of very high-resolution lossless images from the probe, some of may now be received until well into next year (transmission of all the data recorded by New Horizons will take some 16 months).

In particular, New Horizons focused on a bright region positioned towards the centre of the of Pluto’s sunlit side and initially dubbed “Pluto’s Heart” due to its shape (seen  most clearly in the image above left). Now informally christened “Tombaugh Regio”, after Pluto’s discoverer, Clyde Tombaugh,  the region has been of interest to the science team due to its apparent “youthful” appearance: it is relatively crater-free, suggesting the surface has undergone significant re-working compared to the surface features around it, which are far more heavily cratered.

The mountains of “Pluto’s Heart” are in the south-west of the “Tombaugh Regio” and were imaged on July 14th, 2015, when New Horizons was 77,000 km (48,700 miles) from Pluto, and 90 minutes from its point of closest approach at just 12,800 km (8,000 miles).

The region is home to a series of intriguing features, including the “Norgay Montes”, named after Tenzing Norgay, Edmund Hillary’s companion on the 1953 ascent of Mount Everest. This is a range of mountains rising some 3,300 metres (10,000 feet) above the surrounding plains, and which are estimated to be around 100 million years old, making them one of the youngest surface features seen in the solar system (younger than the Appalachian Mountains in North America, for example). There are believed to be a exposed region of Pluto’s bedrock, itself likely to be heavily comprised of water ice.

Not too far away from “Norgay Montes” lies “Sputnik Planum” (Sputnik Plain – named for Earth’s first artificial satellite), an extensive, crater-less region which is unevenly broken into segments around 12-20 kilometres (7.5 – 12.5 miles) across, and bordered by what appear to be shallow troughs. Some of the latter have darker material within them, while others are traced by clumps of hills that appear to rise above the surrounding terrain.

Intriguing surface features imaged on “Sputnik Planum” by New Horizons from a distance of 77,000 km (48,700 miles) on July 14th, 2015, indicating a range of surface processes possibly at work, such as surface contraction as a result of outgassing, or convection processes from beneath, all of which, when combined with the lack of cratering, suggest the region may be geologically active

Taken together, these features lend weight to “Tombaugh Regio” still being active. This is important, as it tends to upset the scientific apple cart when it comes to theories about how icy worlds work. Until now, it has been thought that the only way for such worlds to retain the kind of internal processes that could give rise to the renewal of surface features, etc., was through the gravitational influences of other large bodies.

In the Jovian system, for example, the four Galilean moons are all influenced by the gravitational presence both Jupiter and one another, which has in particular led to Io’s intense volcanism and Europa having a deep water ocean beneath its icy surface.

However, in Pluto’s case, there isn’t any nearby large body which could have the kind of influence that would generate the necessary internal processes. While Charon is relatively large and relatively close to Pluto, it isn’t sufficiently massive to stir Pluto’s interior. What’s more, the two worlds are tidally locked: Charon always presents the same face to Pluto and vice-versa, which further reduces any tidal influence they might have on one another.

From blur to planet: an animated GIF showing how our view of Pluto has forever changed as a result of New Horizons
From blur to planet: an animated GIF showing how our view of Pluto has forever changed as a result of New Horizons

Thus, Pluto should be too cold to be active; that it’s not already has planetary scientists scratching their heads – and hungering for the lossless images of “Tombaugh Regio” so that they might get even better clues as to what has been / is going on.

A further interesting aspect of the images of “Sputnik Planum” is that it has dark areas scattered across it, trailed by long, narrow streaks, all aligned in the same direction.  It’s not known what the dark material is, or precise what is causing the trails. They are certainly consistent with the idea of material being lifted from deposits and scoured across Pluto’s surface by the wind.

However, the dark spots might equally be geysers actively ejecting material into  Pluto’s atmosphere, where it is being caught by the wind to form long trail-like plumes. If this is the case, not only would it confirm the region is still geologically active, it could indicate the presence of cryovolcanism; one of the activities theorised as occurring on Pluto. Certainly, it is interesting to note that the “Tombaugh Regio” has a disproportionate concentration of carbon monoxide when measured against the rest of the planet, again suggestive of some kind of active outgassing.

False-colour map showing the distribution of methane and nitrogen ice
The LEISA spectrometer reveals an unusual mixing of methane and nitrogen ices on Pluto. For some reason, methane ice (shown in red) is far more concentrated around the equator, and relatively sparse at the poles, which are dominated by nitrogen ice

New Horizons has also confirmed the presence of methane both in Pluto’s atmosphere and on its surface, where it exists as ice. But even here there is mystery; for some reason ice deposits on Pluto have an unusual distribution. Methane ice dominates the equatorial regions in high concentrations, but is markedly less present in the polar regions, which are dominated by nitrogen ice, where the regions between demonstrate a complex mixing of ice types.

And what of Charon? With all of the focus on Pluto, it’s easy to overlook the fact that  New Horizons reached its point of closest approach to Charon, at distance of 28,858 km (17,931 miles), just 14 minutes after passing Pluto, making things extremely tight for the mission to be able to observe and gather data on both.

One of the best images we have of Charon released so far was captured by New Horizons as a part of the imaging sweep which captured the images of Pluto’s “Tombaugh Regio” shown above. In it, we seen a beautiful full-face image of Charon with an inset high-resolution image of surface features captured by LORRI on July 14th, and it reveals the world to be at least as geologically enigmatic and interesting as Pluto.

Charon seen full-full and in close-up, showing some of its striking surface features, including a remarkable “sunken mountain”, seen towards the top of the image

The face of Charon imaged here is intriguing, showing craters, ridge lines and what might be the effects of some kind of faulting or outflow channels. On our own Moon, such lines and scars are associated with lava outflows, and one early theory for Charon is that these lines might be the result of liquid ice outflows (and no, “liquid ice” doesn’t mean “water”! 🙂 ).

An area of particular interest to scientist is at the top of the inset image: a mountain (possibly a volcano) around 20 kilometres in height, but which is situated in a depression about 40 km across, and if the ground on which it stands has imply collapsed under it. Quite what this indicates has left geophysicists at a loss for words, or as Jeff Moore, New Horizons’ Geology and Geophysics lead put it, “stunned and stumped”.  Once again, we’re going to have to wait for the super-high lossless images of Charon to be returned to Earth in order for some educated theories to be put forward as to what it actually is and how it may have formed.

Since passing by Pluto and Charon, New Horizons has continued to observe the pair, and to gather data on them and their environment, including measuring the complex interaction  Pluto’s tenuous atmosphere has with the solar wind.

Since passing Pluto, New Horizons has entered into a region of space affected by the complex interaction between Pluto's atmosphere and the solar wind, which is slowly tearing away the upper layers of Pluto's atmosphere
Since passing Pluto, New Horizons has entered into a region of space affected by the complex interaction between Pluto’s atmosphere and the solar wind, which is slowly tearing away the upper layers of Pluto’s atmosphere

Pluto exhibits a distinctive “bowshock”, caused by the solar wind interacting with a huge “bubble” of ionised particles surrounding the planet, which forms something of a “buffer” between the Pluto’s atmosphere (estimated to be around 1,000 kilometres thick, and space. In streaming around this “bubble”, the solar wind forms an extended shock front which tails around the planet and its atmosphere. As it doe so, it is stripping away the upper layers of that atmosphere at a rate of approximately 500 tonnes an hour – or rough 500 times the rate at which Mars is losing material from its upper atmosphere as a result of a similar mechanism.

Given this rate of loss, there must be processes at work that are renewing Pluto’s atmosphere, which is a layered mix of molecular nitrogen in it upper reaches, combining with methane and other gases lower down, and with larger hydrocarbon molecules close to the surface. One of the strongest contenders for this would be the aforementioned cryovolcanism.

As a mission, New Horizons is far from over; as noted above, a lot of data has yet to be returned, and the vehicle is still observing the Pluto-Charon system (I haven’t even mentioned attempts to image Pluto’s tiny Moons).

Clyde Tombaugh at the Lowell observatory, 1931, where he discovered Pluto in 1930
Clyde Tombaugh at the Lowell observatory, 1931, where he discovered Pluto in 1930 (credit: Associated Press)

However, the probe is now moving deeper into the Kuiper belt, so the question of “what next” inevitably raises its head.

If the opportunity arises, New Horizons will be used to study one or two other large Kuiper belt objects (KBOs). Unfortunately, opportunities for it to do so are limited. The vehicle has now has very limited reserves of hydrazine fuel, required to correctly orient itself and its scientific payload. Two targets for such a study are currently under consideration, and a decision on which, if either of them, might be studied, will be made in August 2015.

If neither of the two primary targets is selected, the main Kuiper Belt phase of the mission will continue through until 2020, and so may offer alternative opportunities. Beyond 2020, however, nothing has been determined; in theory the nuclear RTG powering New Horizons should by able to deliver sufficient wattage to operate the vehicle’s instruments and communications system through to 2026, and this could be extended if the decision is made to turn of some of the instruments. So like the Voyager and Pioneer vehicles before it, New Horizons may be used to help increase our understanding of the outer limits of the solar system as it continues onward into interstellar space.

Clyde Tombaugh,  Pluto’s discoverer, died in January 1997.  One of his final wishes was to have his ashes flown into space. It’s therefore fitting that as New Horizons passed Pluto, and as it continues outward into the depths of space, it carried with it Mr. Tombaugh’s ashes.

All images courtesy of NASA / APL / JHU

2 thoughts on “Space Sunday: perfectly Pluto

  1. One interesting question is what is the gravitational implications of Pluto and Charon being so close together. Would you weigh significantly less on one side of the “planet” than on the other?

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    1. There would be no difference in your “local weight”; Pluto (and Charon) would exert considerably more influence over you than any influence the other could exert over you when standing on the side of the world facing it.

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