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Headshot of Emily Lakdawalla

Spectacular New Horizons photo of Pluto's hazes and mountains: How it was made

Posted By Emily Lakdawalla

17-09-2015 16:04 CDT

Topics: trans-neptunian objects, New Horizons, pretty pictures, Pluto, explaining technology, dwarf planets beyond Neptune, explaining image processing

Before I write any more words, just gaze upon this image for a while. Make sure you enlarge it to its full resolution.

Pluto’s majestic mountains, frozen plains and foggy hazes

NASA / JHUAPL / SwRI

Pluto’s majestic mountains, frozen plains and foggy hazes
Just 15 minutes after its closest approach to Pluto on July 14, 2015, NASA’s New Horizons spacecraft looked back toward the sun and captured this near-sunset view of the rugged, icy mountains and flat ice plains extending to Pluto’s horizon. The smooth expanse of the informally named icy plain Sputnik Planum (right) is flanked to the west (left) by rugged mountains up to 3,500 meters high, including the informally named Norgay Montes in the foreground and Hillary Montes on the skyline. To the right, east of Sputnik, rougher terrain is cut by apparent glaciers. The backlighting highlights over a dozen layers of haze in Pluto’s tenuous but distended atmosphere. The image was taken from a distance of 18,000 kilometers to Pluto; the scene is 1,250 kilometers wide.

There are so many things to see. Look at all those haze layers! Or maybe it's an optical illusion, fewer layers with vertical waves that make them seem alternately thicker and thinner. Look at the pointy peaky mountains and how we can see into their shadows because of twilight. Look at the boundaries between rugged mountains and smooth plains. Look at the flow fields in the plains. Look at the mountains poised atop Pluto's curvature. You never see a view like that on Earth because Earth is so much bigger than its mountains; even the Moon is not so rugged. Look at the mountains casting shadows into Pluto's night. And just look, look, look at how much detail there is.

The NASA release accompanying this image does a great job explaining some of the features you can see in it, so I won't rehash that here -- go read it for yourself! Instead, I'll tell you more about how this picture was made, and why it's different from previous Pluto images you've seen.

The detail is there because this is a very large image. Nearly all of the Pluto photos we've seen so far are from New Horizons' Long Range Reconnaissance Imager, LORRI. The photo above is our first high-resolution view of Pluto from New Horizons' other camera, Ralph. Ralph itself is two different cameras; the one used for this photo is the Multispectral Visible Imaging Camera, or MVIC. I usually refer to MVIC as New Horizons' color camera, but that's a bit sloppy of me. Color isn't all that MVIC does, and it doesn't always do color.

LORRI is a simple camera to understand -- it's point and shoot. It is black-and-white, with a square detector, 1024 pixels square, over an extremely narrow field of view of 0.29 degrees. It has special optics designed to prevent distortion in the image. Choose how long you want the exposure to be (1 to 10,000 milliseconds), then point and shoot, and you have an image. Sometimes, to keep file sizes small and improve signal in low light situations, you can bin LORRI images 4x4, so the resulting pictures are 256 pixels square. That's pretty much it for LORRI.

MVIC is quite different from LORRI. Like many space cameras (including HiRISE and CTX on Mars Reconnaissance Orbiter and LROC on Lunar Reconnaissance Orbiter), it is a pushbroom imager, meaning that instead of pointing and shooting, you sweep a linear detector array across a surface, building up a long, skinny image swath over time. It's sort of the same way that a photocopier or scanner works. The detector is 5024 pixels wide, of which 5000 are actually photoactive, so its images are 5000 pixels wide by an arbitrary number of pixels long.

The 5000 pixels are spread across a 5.7-degree field of view. If you do the math you'll find that individual MVIC pixels are almost exactly four times wider than LORRI pixels, and MVIC images cover a field that is nearly 20 times wider than a LORRI image. So that big mosaic of Pluto that I posted on Tuesday, which took a 4-by-4 array of LORRI images to make, would occupy less than one-fifth of the width of the MVIC field of view. Here's a diagram that shows how all the fields of view overlap on the sky.

New Horizons instrument fields of view (FOVs)

Figure 3 from Weaver et al. 2008, "Overview of the New Horizons Science Payload"

New Horizons instrument fields of view (FOVs)
The fields of view (FOVs) of the Ralph MVIC (blue and yellow), Ralph LEISA (red), Alice airglow (green), and LORRI (purple) instruments are projected onto the sky plane; the listed boresights are measured in-flight values. The angular extent of each instrument’s FOV is also listed. The spacecraft +X direction is out of the page, the +Y direction is up, and the +Z direction is to the left. The LORRI field FOV overlaps the narrow portion of the Alice airglow channel, and the MVIC FOV overlaps the wide portion. The LEISA FOV overlaps the MVIC FOV.

Typically, pushbroom imagers don't actually have any moving parts to sweep the detector across the surface. The Mars and Moon imagers I mentioned earlier use the spacecraft's orbital motion (which is very fast and happens at a nearly constant speed because of the circular orbits) to sweep the detector across the surface. For New Horizons, they actually rotate the whole spacecraft to sweep the MVIC detector across the target of interest.

Using spacecraft motion to build up an image introduces a problem. If your spacecraft is moving very fast, you don't get very much time to expose the detector to the scene. And if the scene is dark, there won't be many photons around to hit the detector in the limited time that you have. New Horizons has to move fast, and there is not much light at Pluto. One line of pixels just is not enough to gather enough light fast enough to make a well-exposed image.

Time-delay integration comes to the rescue. MVIC's panchromatic detector -- the black-and-white detector that was used to capture the image in this post -- is 5000 pixels wide and also 32 pixels tall. When Ralph starts taking an image, it exposes the whole 32-pixel-tall detector. The spacecraft tells Ralph how fast it thinks it's rotating, and Ralph uses that information to determine how long it takes one row of pixels to move along the target. As spacecraft rotation carries the detector one pixel's worth of distance along the scene, MVIC dumps the charge accumulated from one row into the next one, and continues building up signal from the same spot on that next row. This repeats 30 more times before the accumulated charge is dumped into another CCD, called a readout register. The end result is that they can expose the image 32 times longer than they would be able to without time-delay integration.

Time-Delay Integration (TDI)

Figure 5 from D. Reuter et al., 2008: "Ralph: A Visible/Infrared Imager for the New Horizons Pluto/Kuiper Belt Mission"

Time-Delay Integration (TDI)
Illustration of TDI in operation. In step 0, a target is about to enter the field of view of a TDI array. In step 1, the image of the target has moved a single pixel width into the first row of the array. In step 2, the charge is transferred to the second row of the array and integration continues. A new target has moved into row 1. The next charge transfer occurs when the image of the target has moved another row. In step 32, the target has moved 32 rows, with the charge being transferred each time the image moves another row. At the next charge transfer, the charge in row 32 is transferred to a serial column for pixel readout.

MVIC actually has six of these 5024-by-32-pixel detectors. The one I described already makes up MVIC's  "panchromatic," channel, which sees the full spectrum of light that the MVIC detector is capable of detecting, with wavelengths of 400 to 975 nanometers. There is a second panchromatic channel for redundancy (insurance against the failure of either one of them, or of either one of two redundant sets of Ralph's electronics). The other four detectors have filters in front of them, limiting the light that reaches them only to certain wavelengths. One has a blue filter; one has a red filter; one has a near-infrared filter; and one has a methane filter. Because they see less light (it's filtered), they may rotate the spacecraft more slowly to take color MVIC imgaes, allowing longer integration time on each pixel. Finally, Ralph has one more channel, the framing channel, which is 5000 by 128 pixels in size and operates like a more traditional framing camera. Its main purpose is to be available for optical navigation in the event that LORRI fails. (You need a lot of redundancy for a long-duration mission like New Horizons.)

Armed with all this information, I can now answer some questions about the image:

Question: If MVIC is New Horizons' color camera, why is this image black-and-white?

Answer: Either because only one channel was used to take this image, or because they have only received one channel's worth of data on the ground. To make a single color image, you have to downlink three photos, one each from three different channels; in other words, color images take three times as much data as black-and-white ones. As a matter of fact, New Horizons team member John Spencer has confirmed that only the panchromatic channel was used for this photo, because of the limited time and demands on the spacecraft during closest approach. If they were shooting in color, they wouldn't also be able to do as much with LORRI at the same time. I can't wait to see the LORRI images that were taken around the same time as this one!

Question: What are the short streaks in the sky above Pluto?

Answer: Those are stars. New Horizons was tracking Pluto while taking this image, but Pluto was also moving at the same time, so things in the background -- the stars -- got streaked.

Streaks and bands in the New Horizons MVIC image of Pluto's limb

Question: What is the banding in the dark areas of the image?

Answer: it's a periodic noise that comes from Ralph's electronics, which happens during readout -- something that also affects Cassini images. I'm not sure why it hasn't been removed in this image.

One curious thing about the banding is that it's not horizontal. That's a clue that the image has been rotated from its original orientation. Another clue that we're looking at just part of the original image is its size: 3420 pixels wide, less than the MVIC full width of 5000 pixels. So it's either been downsized or cropped. I've confirmed that it hasn't been downsized, only rotated and cropped. To give you a sense of how much more Pluto there would be in the original image, I've rotated the image to make the banding horizontal, and widened it to the full 5000-pixel width of the MVIC detector. The image may also be taller than this; there's no way to know how many rows it started out with. There's more Pluto yet to see!

Cropped MVIC image of Pluto

NASA / JHUAPL / SwRI / Emily Lakdawalla

Cropped MVIC image of Pluto
The dark gray area represents the full width of the MVIC detector, 5000 pixels.

Unfortunately, we won't be seeing the full image on the New Horizons raw image site on Friday. That's because only LORRI images are being rapidly released as raw data on the New Horizons website; MVIC images are not going to be released in the same way. So we will have to wait until the proprietary period is over to see the whole thing. I've been trying to understand when that proprietary period ends by reading the New Horizons Data Management and Archiving Plan (PDF) that was approved before launch. There are two planned releases of Pluto flyby data: the highest-priority ("Group 1") data come out early, and the lower-priority (Group 2 and 3) data come later. It looks like the archiving plan requires the first release of Group 1 Pluto flyby data two months after all of it is downlinked. In the original document that was estimated to be November 2015. Data from Groups 2 and 3 are supposed to come out in a second release 13 months after the flyby or 11 months after the end of the Group 1 downlink -- which would be July or August of next year. I'll see if I can get some clarity on this schedule from the mission.

Am I sad that I can't get my grubby fingers on these images right away? Maybe a teeny bit, but not very much. I can hardly keep up with the awesomeness of these images at the rate they're coming out. I'm glad that the fun of discovering new things on Pluto is going to be spread out over the next year. As long as the New Horizons science team doesn't tease me about what they can see that I can't, I'm happy to be patient.

Seriously, New Horizons folks, stop teasing.

Argh!

 
See other posts from September 2015

 

Read more blog entries about: trans-neptunian objects, New Horizons, pretty pictures, Pluto, explaining technology, dwarf planets beyond Neptune, explaining image processing

Comments:

stfletch: 09/17/2015 05:41 CDT

I think this may be the single most breathtakingly beautiful planetary image I have ever seen. New horizons and Pluto have been an absolute marvel. Can't wait to see the rest of the pics.

ethanol: 09/17/2015 06:03 CDT

Fantastic explanation Emily, these sorts of detailed instrumental descriptions are one of things that make your blog indispensable. As to the image: imagine what the view must be from the peak of one of those mountains… The curvature would be apparent, and the mountain range would fall away beneath the horizon on either side, in a thin atmosphere like that you could see hundreds of miles, though I imagine the haze would still give a sense of visual depth. If only I knew the colors!

David: 09/17/2015 09:56 CDT

I'd like to be on the shoreline of one of those nitrogen seas, watching the 'ice' sublimating into the atmosphere. . .

Kim: 09/17/2015 10:46 CDT

Banding-when not rotated, you will see it's horizontal. And yes its from the electronics. This is our first ever MVIC image in that mode and we did apply a FFT filter but when too aggressive it modified real features. So what is here is a work-in progress to understand the banding. No usable data in this mode was taken at Jupiter. There are cosmic rays that were removed as well. But that's mainly all we did since the image looks beautiful. During a second pass at calibration we're looking at that striping.

Chris: 09/18/2015 10:20 CDT

Mindblowing!! It feels like a compilation of every breathtaking image ever taken in our solar system. Looking at all the detail makes me wonder if a picture taken from the same vantage point in 1930 or one taken 85 years in the future would look the same as far as ice flows and positions of various features. Maybe in 85 years we'll have another picture to compare to.

Josh: 09/18/2015 06:29 CDT

Fantastic pics! Never would have expected to (A) get an airplane-esque view of Pluto prior to one of Mars, (B) have arguably the most gorgeous picture in the solar system come from Pluto, or (C) to see a 'Nitrogen' cycle on a kuiper belt object. Crazy speculation...might Tombaugh Regio have LIQUID nitrogen deep in it?

That60sKid: 09/18/2015 08:41 CDT

Thank you very , very much for the detailed and very accurate and understandable description of this instrument! Now let's see if you can do as well explaining the LEISA data cube !

That60sKid: 09/18/2015 08:51 CDT

Josh, I asked a couple of my favorite planatery scientist whether the mountains on Pluto meant it had a molton core. They tell me that the geography is very indicative of a liquid core or liquid layers. Of course calling them "molton" is probably not a good word choice .

Josh: 09/19/2015 03:48 CDT

@ That60'sKid: I too believe that Pluto is a geologically bustling, differentiated, partially 'molten' :) world...just curiously wondering whether there was liquid nitrogen near the surface under Tombaugh Regio, a bit like, say, lake vostok on Earth. ANY amount of internal heat getting out from under regio would likely liquify some of the nitrogen, though I'm not sure how we would test whether there actually is any

Arbitrary: 09/20/2015 03:55 CDT

Methink it looks like Norway. So I am disappointed at NASA for not having found anything more exotic even out there! :)

Vance: 09/22/2015 01:29 CDT

Looking at Sputnik Planum and the Eastern lobe of Tombaugh Regio, they are theorizing that that area is covered by nitrogen snowfall condensing from nitrogen that had evaporated from the ice sheet on Sputnik Planum. In this, I recognize a local climate process that is similar to the lake effect that takes place here near where I live. Here Sputnik Planum plays the climate role of an ocean or sea, providing a source of "moisture" that then condenses in orographic precipitation as it is pushed over the higher terrain of the eastern lobe of the heart. I think we can see this process actively occurring at the time this image was taken. We see the haze layers of the atmosphere. These are mostly consistent across the elevation of each layer. However, if you look to the right of the image just above the horizon, we see a haze layer that is patchy. I think we may actually be looking at clouds over the eastern lobe that are forming as the air is forced up over the higher elevation there. These clouds are thin, but may be able to produce a "diamond dust" snowfall similar to that which commonly occurs over the South Pole of Earth with water. This may be occurring under these clouds even as the image was being taken.

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