July 2015: A Milestone of Cosmic Discovery

Last week, the New Horizons spacecraft went by Pluto, consummating a decade-long reconnaissance of our local cosmic backyard, yielding never-before-seen faces and features, and projecting an image of Pluto in our minds that is based in reality — an image that is of a geologically active world, rather than just a ball of frozen rock and ice.

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Massive heart is now known as “Tombaugh Regio”, after Pluto’s discoverer, Clyde Tombaugh. The whale is a dark region of complicated geologic activity, now dubbed “Cthulhu Regio”. Credit: Discovery Magazine

One of the latest close-up images of Pluto reveals 3,500-metre-high icy mountains near the equator. Located in the darkened whale region and neighboring the lighter heart region, scientists believe that these mountain ranges are only a 100-million years old (quite young relative to a 4.56 billion year old planetary system), pointing to increasing speculation that geologic activity is still ongoing. The image reveals a lot of structures that are roughly 2 kilometres across. But remarkably enough and notwithstanding the fact that Pluto has been hit by objects in the Kuiper belt, a relatively large region of Pluto is crater-less, including the plains above which these mountain ranges lie. This is contrary to earlier predictions. According to scientists, Pluto has a mysterious heat source that is driving such geologic activity and mountain building and that is leading to  crater-less plains by cobbling the surface with fresh material. The source of this energy is not decisively known yet. In fact, Pluto is not gravitationally captured by another giant planet that can provide tidal heating. Furthermore, Pluto and Charon are in tidal equilibrium and are both tidally locked so there are no mutual tidal forces to cause heating and power Pluto’s geologic activity. The geologic activity is essentially Pluto’s own. As such,  it is being speculated that Pluto has radioactive elements in its underlying rock core ( which is about 60 percent of Pluto’s mass) that lead to the activity. Or, it could just be left-over reserve energy from Pluto’s formation (giant impact that lead to Charon). Another possible explanation is thawing and freezing subterranean oceans that store heat.

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Flyover images of mountains and icy surfaces! Credit: NASA
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The 3,500 kilometre high icy mountains. Named The Norgay Montes (in honor of Tenzing Norgay, a Nepalese Sherma mountaineer) taken from 77,000 kilometers away Credit: NASA
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The Norgay Montes located south of the Tombaugh Region ‘heart’ region Credit: NASA

Another latest image reveals significant details of a vast, frozen crater-less plain in the center left of Pluto’s very remarkable heart-shaped region and located north of the icy mountain ranges. According to scientists, this is also a relatively young region, no more than a 100 million years old. The crater-less appearance is also indicative of recent and ongoing geologic activity that repaves the surface with fresh material. But, the surface appears to be broken into frozen cracks and irregularly-shaped polygon regions that are ringed by narrow, meandering troughs, some of which are interlaced with dark material. Clusters of small mounds and smooth hills are also visible, tracing the troughs. Pits are also visible, but probably made by erosion resulting from sublimation, rather than winds as on Earth. The irregularly shaped polygon-like structures could be indicative of convection between the carbon monoxide and the methane and nitrogen atmosphere on the surface and Pluto’s heat on the interior. It is possible that this is a Rayleigh-Benard convection — a type of convection which develops in a layer being heated from below and cooled from above. The upper boundary would be denser than the lower boundary, leading to an eventual convection pattern. Another possible explanation is thermal contraction (a process where cooling materials change dimensions as they cool) cracking those polygon structures onto the plains.

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Pluto’s Sputnik Planum (after Sputnik, the first satellite to be launched), taken by New Horizons’ Long Range Reconnaissance Imager (LORRI) on July 14th from a distance of 77,000 kilometres Credit: NASA
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Possible cause for polygon-like structures on Pluto’s plains? Left: The Sputnik Planum; Right: Benard cells, exhibiting a hexagonal shape

Analysis of the western half of the Pluto’s heart region by the New Horizon’s Ralph instrument reveals a large deposit of frozen Carbon Monoxide ice around the area, specifically in the region outlined in the figure below. The source of such a specific heightened concentration is so far unknown.

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Increased carbon monoxide concentrations in Pluto’s heart. Source: NASA

The New Horizons team was able to construct an atmospheric model of Pluto. It seems to be a predominately pure nitrogen atmosphere. But also methane further down in altitude. In addition, there’s hydrocarbon forming in the atmosphere that then falls off to cover the surface, giving Pluto its red colour. Findings by the New Horizons Solar Wind Around Pluto (SWAP) instrument also reveal that Pluto’s atmosphere is escaping away, leaving a nitorgen-packed tail of plasma, flowing away from the sun. The gas is being lost at a rate of 500 tonnes per hour. The plasma tail is indicative of solar winds ionizing the nitrogen and carrying it away. That is because Pluto’s gravity is quite weak — about one-fifth of Earth’s gravity. So, the atmospheric molecules are ionized by the solar ultraviolet light and have enough energy to billow away and escape Pluto’s weak gravity. The solar wind, which is a continuous supersonic outflow of gas from the sun’s corona, then picks up the ionized molecules past Pluto to form a plasma tail, indicated by blue in the diagram below.

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Methane ice on Pluto, with differing concentrations across the planet. Source: NASA
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Atmospheric ions behind Pluto. Credit: NASA

Pluto’s Moon Charon

Pluto’s moon Charon appears to be surprisingly young and oddly enough, possesses a series of craters, a 6 km deep canyon at the upper right, cliffs, and troughs across it. The ranges of cliffs extend about 970 kilometres across Charon. This is all indicative of internal geologic activity. The largest crater lies at the south pole, at about about 97 kilometres across. Charon also harbors a dark region located near the north pole, at 320 kilometres across. Scientists have dubbed this dark spot “Mordor”. This region might have possibly arisen from material drifting off from Pluto’s atmosphere and onto Charon. But, it’s quite a mystery why Pluto and Charon are quite different indeed. Pluto is predominantly made up of icy water, while Charon harbors immense cliffs and canyons. It’s hypothesized that internal stresses inside Charon could be responsible for these remarkable fractures across Charon’s surface.

First image of Pluto and Charon in the same frame. Taken on July 8th.  Credit: NASA
Composite image of Pluto and Charon in the same frame, showing correct relative sizes, reflectivity, separation, and orientation. Credit: NASA
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First detailed view of Charon, revealing craters and a “mountain in a moat” rising out of a depression on the moon, perhaps caused by an impactor that survived low speed impact on the ice-dominated surface. Credit: NASA
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Charon details. Credit: NASA
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Credit: NASA

Pluto’s Smallest Moon Hydra

Small but still remarkable, the New Horizons team also obtained images of Pluto’s moon Hydra. Although a little grainy, the photo is still quite informative. It indicates that Hydra is a stompy, potato-looking thing and about 43 by 33 kilometres in size.

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Even the grainiest images are still insightful! Hydra, potato-shaped blob moon composed of water ice. Credit: NASA

The Spacecraft

The New Horizons spacecraft has a size that’s comparable to a piano stuck to a satellite dish. It includes propulsion, navigation, communications systems, and payload. It has 16 thrusters for trajectory corrections and altitude control. It has star-tracking navigation cameras to determine the directions in which the spacecraft is pointed at and also the orientation of the instruments. They essentially capture images of stars in the field of view and compare them with an onboard map of 10,000 stars. Onboard are also sensors that detect the angle to the sun and the spin rate. New Horizons operates on a single radioisotope thermoelectric generator that uses pills of plutonium,  and this is a very limited power source. In fact, New Horizons uses the same CPU as the PlayStation 1. It uses fairly high frequency radio waves (via the X band but not the KU-band) to reduce transmission times to Earth.

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The New Horizons spacecraft forms a triangular body, about 0.76 metres in thickness. The spaceship weighs 478 kilograms. Credit: NASA

New Horizons Payload 

a) The Long Range Reconnaissance Imager (LORRI): a high resolution imaging system, with an input aperture 21 cm wide. Exposure times are form 0 milliseconds to about 30,000 milliseconds in 1 millisecond intervals. The images can then be compounded together at a rate of 1 image per second.

b) The Pluto Exploration Remote Sensing Investigation (PERSI): consists of of two instruments: the Ralph and Alice. The Ralph camera combines anchromatic, color imaging, and Infra-red imaging spectroscopy techniques. It is the camera that gave us the currently famous red Pluto photo. There are 16 pixels in LORRI for every pixel in Ralph, but Ralph fills in the colour. The Alice instrument measures the composition and temperature of Pluto’s upper atmosphere, while also performing some atmospheric escape measurements.

c) The plasma and high energy particle spectrometer suite (PAM): PAM consists of two instruments: the SWAP (Solar Wind At Pluto) to measure solar wind velocity and density and The PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation) to measure atmosphere escape rate by determining energy and composition of plasma around the spacecraft and the density of Pluto’s pick-up charged particles.

d) The Radio Science Experiment (REX): Instrument to measure Pluto’s radius, surface pressure, and temperature of lower atmosphere.

And the entire cost for this Pluto mission, 15 years in the making, is just 700 million dollars, far less than what people paid for the last Harry Potter movie, which grossed over 1.3 billion dollars.

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Credit: NASA

Horizons of Discovery Ahead

I have no doubt that there will be a huge wave of discovery sweeping ahead in the times to follow! This fascinating little (or maybe large) world will open up new horizons and deliver a string of cosmic surprises over the next few years! After all, we have just received less than 2 % of the total 50 gigabytes that New Horizons has collected during its flyby. The transfer rate is only 2 kilobytes per second so there’s still a plethora of data to be sent down for the next 16 months. There are also plans by NASA to fly by two potential Kuiper belt targets, 4.8 billion kilometres away from Earth, in early 2019. But, it’s a tantalizing thought that this is the last new planet that we will be able to see in our lifetime. It’s indeed a once-in-a-lifetime opportunity. So let’s relish this moment for as much as we can!

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Our images of Pluto through the years. Credit: NASA

Featured image courtesy of NASA: Most detailed image sent back to Earth before the flyby. Taken at 768,000 kilometres away.

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