The adventures of the Voyager 2 spacecraft continue as it passes the rings of Uranus. Scientists suspect that violent events in the early history of the planet may have shaped Uranus and its strange collection of moons.
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00:00August 20, 1977. A Titan rocket is launched. It carries the spacecraft Voyager 2, bound for the planets Jupiter and Saturn.
00:24NASA scientists decide to push the aging spacecraft onto distant Uranus.
00:29Even the most powerful telescopes on Earth can barely see Uranus.
00:33Scientists know only that it has rings and several small moons,
00:37and that sometime in its ancient past it was violently tilted by unknown forces so that Uranus spins sideways to the sun.
00:46What accident in space caused this bizarre event? Do the moons perhaps hold the key?
00:53Nine years after launch, NASA scientists await the first pictures from Uranus,
00:59and hope to solve the mystery of the planet that got knocked on its side.
01:04Next on NOVA.
01:06Major funding for NOVA is provided by this station and other public television stations nationwide.
01:17Additional funding was provided by the Johnson & Johnson family of companies, supplying health care products worldwide.
01:25And by Allied Signal, a technology leader in aerospace, electronics, automotive products, and engineered materials.
01:56The night sky has always drawn human vision beyond the bounds of Earth.
02:022,000 stars, occasional comets, and five planets are visible to the naked eye.
02:08These have defined the universe since before recorded history.
02:12No other planets were known until amateur astronomer William Herschel built a powerful telescope in the basement of his house in Bath, England.
02:23In March of 1781, while studying the skies from his backyard, he observed a previously unknown object in the heavens.
02:31It was a landmark in astronomy.
02:34Herschel had discovered a new planet. We now call it Uranus.
02:41200 years later, we still know almost nothing about it.
02:45It is nearly 2 billion miles from Earth and tipped on its side.
02:49Five dark moons circle the planet.
02:52The smallest inner moon was only discovered in 1948.
02:59In 1984, this picture was made through a powerful telescope and then enhanced by computer.
03:05It confirmed the 1977 discovery that Uranus has a system of faint, very narrow rings, seen here girdling the planet.
03:14Uranus might have remained unknown and mysterious, except for Voyager 2.
03:20This unmanned little spacecraft would fly within 50,000 miles of the planet in January 1986.
03:27It would send back images and information that were absolutely unobtainable from Earth.
03:32This historic encounter was made possible by a rare coincidence of interplanetary geometry.
03:39The nine planets of our solar system revolve around the sun at vastly different rates.
03:44But every 175 years, the orbits of the outer planets, Jupiter, Saturn, Uranus, and Neptune, line up,
03:51so that, theoretically at least, a spacecraft from Earth can visit each one in turn.
03:56This rare alignment occurred in 1977.
04:00After five and a half years of preparation, NASA's Jet Propulsion Laboratory planned to launch two identical spacecraft,
04:07Voyager 1 and Voyager 2.
04:10Both would fly toward Jupiter, then on to Saturn.
04:17At Saturn, Voyager 1 would be flung upwards out of the solar system.
04:22But if it were possible, if the spacecraft could still function,
04:26Voyager 2 would fly on alone until it reached Uranus.
04:30Finally, Voyager 2 would continue on to its last planetary rendezvous with Neptune, and on to interstellar space.
04:40The Titan Centaur rocket that carried Voyager 2 was launched from Cape Canaveral.
04:45Its three million pounds of thrust set Voyager, barely heavier than an automobile,
04:50on course at 45,000 miles per hour toward the outer reaches of the solar system.
04:58Voyager, a marvel of engineering design, reached its first objective, Jupiter, in 1979.
05:04It relayed to JPL thousands of surprising pictures of the immense planet.
05:09Then, like a slingshot, Jupiter's enormous gravity sent the spacecraft on toward its next goal, Saturn.
05:16Voyager sent back more than 15,000 images of the majestic ringed planet.
05:21The sturdy spacecraft was then headed on toward Uranus.
05:24But there were fundamental problems that JPL scientists and engineers needed to solve
05:29before the scheduled encounter four years later.
05:32Voyager's computers have limited memory.
05:35The Uranus encounter would severely tax their capacity.
05:38Uranus is so far from the light of the sun that photographing there would be like
05:42taking pictures in a ballpark lit by a birthday candle.
05:46And because Voyager would be nearly two billion miles from Earth,
05:49it's already think radio signal would be very difficult to receive.
05:55So from Pasadena, the JPL team drastically reprogrammed Voyager
05:59until it was effectively a different spacecraft.
06:03First, they activated one of Voyager's three backup computers.
06:06It reduced the amount of information that needed to be sent back.
06:10Then, to cope with the darkness, they developed a system of picture making
06:14called image motion compensation.
06:16It eliminated the blur that would have resulted from the long exposures needed at gloomy Uranus.
06:22Voyager's cameras remained fixed while the spacecraft swung gently
06:25to hold the image in focus as it sped by.
06:29Finally, to capture Voyager's think radio signal,
06:32JPL electronically linked its radio antennas in Madrid,
06:36in the Mojave Desert at Goldstone, California,
06:39and in Canberra, Australia.
06:41They made, in effect, a single great ear
06:44listening to Voyager's 20-watt signal two billion miles away.
06:48Australia's Great Parks radio telescope was added to triple NASA's receiving capabilities.
06:55By November 1985, the newly reprogrammed Voyager
06:58was sending sharp, clear pictures of Uranus back to JPL's Pasadena complex.
07:04The vast Voyager team of 300 scientists and support staff,
07:08some of whom had been working together on Voyager for 14 years,
07:12geared up in anticipation of the January 86 flyby of the mysterious blue planet.
07:18Then, only four days before Voyager reached its closest encounter with Uranus,
07:23the images coming back from the spacecraft deteriorated in quality.
07:28They were degraded by a pattern of horizontal bars.
07:32Voyager's imaging team knew that all the close-up pictures of Uranus and its moons
07:36would be compromised unless the problem was found and repaired immediately.
07:40Engineers hoped it was a problem with the receiving equipment,
07:43but they discovered it was in the spacecraft itself,
07:46a physical failure of the computer memory.
07:48Oh, when I got home from Disneyland, where I'd been with the grandkids,
07:52my wife said somebody called and there was a problem on the spacecraft.
07:56So I came in about 10 o'clock.
07:59They had concluded pretty firmly that it wasn't a ground problem.
08:04In other words, ground software or ground communications.
08:09Even if they could find the problem, it would still take 2 hours and 45 minutes for corrections,
08:14traveling at the speed of light, to reach Voyager, 2 billion miles away.
08:19Time was running out.
08:21These two computer programmers hunted all night through the volumes of information
08:25generated by Voyager's onboard computers, trying to find the exact location of the failure.
08:31453. 453. We found it.
08:34When it was built, Voyager was the most advanced spacecraft ever.
08:38But now its computers are aging and they suffer from an electronic form of dementia
08:43where single cells can fail and cause instructions to be misread or scrambled.
08:48That's what had happened.
08:50Programmers sent up a new sequence of commands,
08:52which simply detoured information around the faulty location.
08:56And the new instructions got there before the most critical time, closest approach.
09:01JPL scientists learned more about Uranus in that 8-hour period
09:05than had been learned in the 205 years since its discovery.
09:09Voyager sent back over 7,000 images.
09:12Some yielded up their information immediately.
09:15Others may take 5 or 10 years to analyze.
09:18Here is what Voyager saw.
09:20The planet remained mysterious, even at close range.
09:24An enormous, hazy blue ball.
09:29Time-lapse pictures shown here at 40,000 times normal speed
09:33showed how Uranus looked with its moons orbiting around it.
09:38Voyager's cameras used filters of red, green, and blue to enhance the planet's color and detail.
09:45But even enhanced, Voyager's pictures showed that Uranus is remarkably featureless.
09:50A 5,000-mile-thick gaseous atmosphere enshrouds the planet.
09:54Below it is a 7,000-mile layer of superheated water at 8,000 degrees Fahrenheit.
10:01At the very center, there may be a molten rocky core.
10:04Uranus is a giant gas ball in space,
10:07made up of helium, hydrogen, methane, water, and ammonia.
10:11Nearly four Earths could fit across its diameter.
10:14Yet there is no solid surface at all, no place to stand.
10:18And Uranus's orientation to the sun is truly bizarre.
10:22Let me use this orange to tell you about Uranus.
10:25Uranus is different from the other planets because it's tipped on its side.
10:30Earth spins more or less straight up and down,
10:34which means that the sunlight is more or less falling on the equator and less on the pole.
10:39But Uranus is spinning on its side,
10:42which means that the sunlight is directly overhead at the pole
10:46and is exclusively falling on this one hemisphere.
10:50Of course, the year on Uranus is about 80 Earth years,
10:54and so in about 20 years, the orientation will change
10:59and the sunlight will be more or less overhead on the equator.
11:02This schematic animation shows Voyager's point of view
11:05as it approached Uranus head on to the pole.
11:08Due to this angle of approach, JPL's meteorologists were hoping to monitor continuously
11:13features that would reveal the nature of the Uranian weather system,
11:16its wind and cloud patterns the length of its days.
11:19One possibility was that we'd see a pattern in the clouds that we'd never seen before.
11:25I was actually betting, and a lot of other people were betting,
11:29that the pattern of circulation would be around the pole
11:34in spite of the pole being tipped over.
11:37In other words, it would be just a tipped version of Jupiter and Saturn.
11:42In any case, we were very anxious to see what the first Voyager images showed.
11:48Well, here they are.
11:52Basically, the first Voyager images showed nothing.
11:55We couldn't see clouds, patterns. It was featureless.
12:01On the other hand, there was a possibility that there were cloud features in these images,
12:06but they were too low contrast to be able to see them here.
12:11The solution was to stretch the contrast and see if there was something there.
12:18In fact, we stretched the contrast and played a sequence of images together
12:24so it would be a movie, and if anything was there, we'd see it move.
12:30Now, here's the result of our first attempt to make a movie of the cloud motions.
12:39Now, what you see here is not cloud features on Uranus,
12:43but mostly just the noise, the blemishes in the camera.
12:48These blemishes are actually dust specks on the lens,
12:52and we've never been bothered by them before
12:54because the things we were trying to look at were always so prominent.
13:00We, in fact, put together another movie when the spacecraft was closer,
13:07and finally we saw something.
13:12You have to understand the perspective here.
13:15The spacecraft is approaching from over the pole.
13:19The planet is rotating counterclockwise, and the sun is more or less behind the spacecraft.
13:27And you see two significant things.
13:29First of all, faint bands so that, in fact,
13:34Uranus is more or less a tipped-over version of Jupiter and Saturn.
13:39And secondly, you see small white features going around,
13:45and we can, in fact, clock these white features and measure wind speeds.
13:51These features are going around about every 16 or 17 hours,
13:56which is the length of a Uranian day.
13:59Color enhancement of the pictures made the overall pattern of the clouds and winds more visible.
14:04The belts and zones of atmospheric circulation appear as concentric circles.
14:09Uranus looks like an eerie eyeball in space.
14:13Because of the dense gaseous atmosphere that enshrouds the planet,
14:17Voyager's cameras concentrated on other major objects in the Uranian system,
14:21its rings and its moons.
14:23This is a view from Voyager, intensely computer-enhanced,
14:27of the outermost ring of Uranus called Epsilon.
14:30Nine narrow rings of Uranus were discovered by Earth-based astronomy in 1977.
14:37Starting with the outside Epsilon ring,
14:40they are called Delta, Gamma, Eta, Beta, Alpha, 4, 5, and 6.
14:56The rings, which are enhanced to look white here, are actually pitch black.
15:02To see them at all, the contrast of these pictures has been exaggerated
15:06to a level that makes the featureless background appear mottled like an asphalt road.
15:12The rings themselves are several thousand miles apart and only a few miles wide.
15:20As this animation shows, planetary rings are not solid,
15:24but are made up of myriads of independently orbiting boulders, chunks of ice, and pebbles.
15:30All planetary rings are unstable and have a natural tendency to spread apart.
15:36They diffuse away out into space or are pulled in toward the planet.
15:42Well-defined rings, like those at Saturn and Uranus, have something keeping them in place.
15:48Based on what they had discovered at Saturn,
15:50JPL scientists expected to find 18 small shepherd moons at Uranus.
15:55Shepherds circle the planet within the ring plane,
15:58combining or deflecting material into rings and keeping it from escaping into space.
16:03But they only found two of them.
16:06This is one, just outside the Epsilon ring,
16:09and this is the other that keeps the inner edge of Epsilon from drifting down into the planet's atmosphere.
16:15Voyager's pictures do not show what keeps the other rings in place.
16:18There may be shepherds too small to see. JPL scientists are still baffled.
16:24Voyager was programmed to photograph the ring plane not only on approach but also from the side
16:29and then looking back toward the sun, after it had passed the planet.
16:33It was these views that produced the most startling discoveries.
16:37This picture is of particular interest because in it Voyager made its first discovery
16:42of a ring that had not been observed from the ground.
16:46Just as the nine known rings of Uranus were very narrow, so was this newly discovered ring.
16:51And it exists between the orbits of the two outermost rings, the Delta ring and the broad Epsilon ring.
17:01And you can see it here as a faint feature.
17:05In this very same geometry, when the spacecraft had imaged closer to the body of the planet than it did here,
17:13we discovered yet another ring, a much broader ring.
17:16This picture has a substantial amount of smear in it
17:19because of the fact that the spacecraft is basically plummeting through the ring plane at this point.
17:24But we see an unmistakable broad ring of material that exists in fact interior to the known rings.
17:32And this ring of material is about 2,000 kilometers wide.
17:39Really, our view of the Uranian ring system had not substantially changed by this point.
17:44We had expected to discover diffuse rings.
17:47We had expected to discover shepherding satellites.
17:52We were totally surprised by the next picture that you're about to see.
17:59In this remarkable photo, Voyager is in the shadow of the planet looking in the direction of the sun.
18:06And we see in this picture a level of detail and structure
18:10that nobody expected to see in the Iranian ring system.
18:16Again, the clearly defined rings are seen with the sun behind the spacecraft camera on approach.
18:24Looking back into the sun, tiny specks of dust gleam in the light,
18:28revealing a vast sheet of material, an entire ring plane.
18:33So what we're seeing when we look at this remarkable picture
18:38is a sheet of material, a very fine material, about a million times smaller or more
18:44than the largest particles which exist to the rings,
18:47and material which is easily victimized by magnetic field forces or collisions with neutral particles,
18:55and material which is falling inwards towards the planet.
18:59The rings are probably being eroded away right before our eyes.
19:05By shining a radio beam through the rings back to Earth,
19:08Voyager discovered that the boulders that make up the rings are about a yard across.
19:13Tiny specks of dust are being ground off these rocks
19:16and orbit the planet between the well-defined rings as they spiral down into the planet itself.
19:22In a few hundred million years, these rings will have worn away to nothing.
19:27The narrow rings themselves contain patterns revealed for the first time by Voyager.
19:33The outer epsilon ring was photographed as light from a distant star flickered through it.
19:40Its intricate structure is very like the fine grooves of a phonograph record.
19:46Another Voyager discovery was completely unexpected.
19:51Orbiting just outside the rings are many previously unknown small moons.
19:56They are provisionally named after the year of their discovery.
20:00So, for example, the first moon of Uranus discovered in 1986 is called 1986 U1.
20:06Ten new moons have been discovered from the spacecraft images,
20:09bringing the total number of known moons to 15.
20:12Before Voyager's arrival at Uranus, there were five large satellites known from Earth-based observations.
20:18Voyager presented a unique opportunity to search for small satellites.
20:22And in this frame we can see the planet, the ring system,
20:27and several other small objects, some of which we know are stars,
20:33and one of the previously discovered small satellites.
20:38And this small image here is the current discovery.
20:44But it could be a camera blemish.
20:46Now, a camera blemish is something that repeats from frame to frame.
20:49It's an imperfection on the camera surface or a spot of dust.
20:53But we can catalog those because we have many frames.
20:56We can create a blemish catalog, and I can display that
21:00and in fact show that the candidate is in fact not a blemish,
21:05so therefore it remains a valid candidate.
21:08Now, the other thing I can do with my computer program here
21:11is to mark that position so that I can predict,
21:16if it is in fact in orbit, I can predict where it should move to in other frames.
21:21And now having marked that location in this one frame,
21:24I can predict to another frame,
21:27taken slightly later in time than the first frame,
21:30in which I calculated basically an orbit for that object,
21:34and predict where it should be just as I predict where other objects in the field should be.
21:38At that point we have a scientific discovery of a new satellite of Uranus.
21:44Only one of these newly discovered small satellites or moons,
21:471985 U1, was photographed close up.
21:51It is about 106 miles across and only barely round.
21:55At least one impact crater is visible,
21:57and its surface, though it gleams here, is very dark and somber.
22:02The moons of Uranus are named after sprites and spirits in English literature.
22:08Miranda, Ariel, Umbriel, Titania, and Oberon.
22:16Voyager was precisely programmed to photograph exact frames of each of the five minor moons.
22:32This computer animation shows the journey that Voyager took through their domain.
22:53All the moons are dark and unimaginably cold.
22:58At 300 degrees below zero, they are the coldest worlds yet seen by mankind.
23:08Even the largest of them, Titania, has no atmosphere, no life at all.
23:13But Voyager scientists hope to garner evidence from these desolate moons about how they formed and when.
23:43Perhaps from the moons would come clues about how Uranus itself became the strange tilted planet we see today.
24:14Even after Voyager scientists had seen images of the planet and its rings,
24:18they had no idea what the moons would really look like.
24:30This dark, remote, and lonely realm would prove to be more baffling and perplexing than anyone could have expected.
24:38And Voyager's images of the moons, flawlessly executed, may raise more questions than they answer.
24:45This is Oberon, the outermost moon of Uranus, as it was actually photographed by Voyager.
24:51It is about 960 miles in diameter.
24:56It is the largest moon in the solar system.
25:01The moon is about 1,000 times the size of the sun.
25:06The moon is about 1,000 times the size of the sun.
25:11It is about 960 miles in diameter, only about half the size of Earth's moon.
25:17Like everything in the Uranian system, it is dark, gray, dingy, and gloomy.
25:22It has an enormous mountain-like object on the side, a half mile high.
25:26This may be the central peak of a giant crater several hundred miles across.
25:31The rest of Oberon's surface is scarred with impacts.
25:35What might have caused the impact craters?
25:37And what can be learned from these pictures about the history of Uranus?
25:41The type of thing that we're looking for in a picture like this is evidence for what the structure of the planet is like.
25:48What caused it to be like this?
25:50And what we see are craters which have bright rays running out from them across a darker surface,
25:56and then rather darker areas in the center of craters.
25:59What type of thing could create that?
26:01One of the simplest models that we have, and probably a correct one for this moon,
26:06is that it results from a layered structure.
26:08We have dirty, very rock-rich material deep within the planet,
26:13and then a cleaner ice layer outside that, and then on top of that, a dirty debris layer, if you will.
26:19And so what we're seeing over most of the moon's surface is this dirty debris layer,
26:23and when a large crater punches through that, it spreads brighter ice out over the surface.
26:27That creates the bright rays.
26:30Voyager found that all the moons of Uranus are made up of rocks and ordinary ice.
26:35Over time, the heavier rock has coalesced into the interior.
26:39Most of the ice that covers it is quite clean, and the thin surface layer is sooty and dark.
26:48In Oberon's ancient past, it might often have been struck by comets or meteors,
26:53common events in the outer solar system.
26:55Such impacts would break through the outer crust of dark material.
26:59Deep craters would be gouged out,
27:01and then the clean white ice below would spread out onto the surface.
27:05And then subsequent to that, we believe that we must have had some period of, if you will,
27:11volcanic filling in of the bottom of these craters
27:14with an even darker slurry of dark, possibly organic-rich material.
27:18This is what you might call an archetypical Uranian moon.
27:22It's more or less what we expected to see from these things made of ice and rock.
27:27Moving in toward Uranus, the next moon is Titania.
27:31The early low-resolution images of this moon look very much like Oberon.
27:35It, too, is an impact-scarred ball of ice and rock about a thousand miles in diameter.
27:44But the close-up view showed something quite extraordinary.
27:47Geologically, Titania is very different from Oberon.
27:52There are gigantic canyons and rifts visible on the surface.
27:56At some time in the past, even at temperatures of 300 degrees below zero,
28:01the surface of Titania expanded,
28:04perhaps when the moon swelled as the ices within it melted and froze again.
28:14An interesting hypothesis was that these bodies had a fair amount of methane ice
28:20and ammonia ice incorporated in them, in addition to ordinary water ice.
28:24This is reasonable for the outer solar system because the temperatures are so cold there
28:28that things that we think of as normal gases at Earth conditions
28:32become ices at these very, very low temperatures.
28:36The presence of something like methane ice along with the water ice
28:40has two important possible consequences for these satellites.
28:43One is that its melting temperature is much lower than for pure ice,
28:49and that means that we may easily heat these things up to the temperatures
28:53where we get interesting geology going on.
28:55The other interesting point about methane ice is that when one takes them in the laboratory
29:00and irradiates them with charged particles, electrons, ultraviolet radiation,
29:04they become dark.
29:06You produce dark red hydrocarbon deposits from irradiating this ice.
29:10And we were more or less looking for this type of material when we went to the Uranian system.
29:16One of the ways in which we can test that hypothesis is by looking at the color of these objects.
29:21This is a colored picture of titania.
29:23It doesn't look very different from the black-and-white picture.
29:25We can quantify that with the computer by looking at a profile or intensity trace
29:32across this picture, which shows us the intensities in the three different colored bands
29:38that went into making up this picture.
29:40And what that shows is that indeed,
29:42titania shows very little difference between the three different colors.
29:46So we're left with a problem here.
29:48Either the hypothesis is not right at all, or it needs to be modified.
29:53The hypothesis was that titania's dark surface was methane ice,
29:58darkened by the intense radiation of the zone in which it orbits.
30:02That radiation can actually be heard on a recording made by the spacecraft.
30:07But analysis of Voyager's images of titania proved it wasn't methane.
30:14Another plausible theory is that titania's inky surface is of truly ancient origin
30:20and is a dusting of leftover carbon from the creation of the planets.
30:24If it is carbon, then it is possible to simulate what the surface of titania actually looks like.
30:31We do have samples of rocks that come in from beyond the Earth,
30:36extraterrestrial materials, which are very dark.
30:39This is a sample of the meteorite Allende.
30:42And it's dark because it does have a great degree of carbon in this dark matrix material here.
30:48Now, we can take in the laboratory and extract these carbon materials chemically,
30:53and that's been done for another meteorite.
30:57It's some of the darkest material known to man,
31:00and it comes from out in the solar system where we're looking at these things.
31:04Now, we can't actually fool around with this material because it's very precious for further studies,
31:10but we do, it turns out, have a good analog, ordinary lamp black,
31:15whose physical and chemical properties are quite similar to the meteorite extract.
31:21So we can perform a little experiment and see what we would expect to be going on
31:26if we, on the surface of one of these satellites,
31:29if we took ordinary ice and mixed it with some of this material for darkening it, for instance.
31:35Here we have a bucket of water ice at somewhat higher temperatures
31:39than we have on the Saturnian and Uranian satellite systems.
31:43We can just sprinkle a little bit of this lamp black in on top of it,
31:47and it doesn't take very much to make a surface exceedingly dark.
31:53We mix that up, and what we see is a mixture of material
31:59that's not dissimilar to what perhaps might be actually on the surface of one of these things.
32:07This, in all probability, is what icy titania would look like to a person standing on its surface,
32:12if such a thing were possible.
32:18Inside Titania's orbit is Umbriel.
32:22It is smaller than the two outer moons, only about 740 miles in diameter.
32:27Umbriel is the darkest of the five major moons.
32:31An enigmatic white donut shape is visible at the top of this picture on Umbriel's equator.
32:36Dark Umbriel appears to be scarred all over with impact craters,
32:41which suggests that it has changed little since its formation.
32:45The craters themselves may enable the Voyager scientists to find out how old the surface is.
32:54The solar system, including the Earth, Uranus, and all its moons,
32:58was made about four and a half billion years ago,
33:01out of an enormous ring of material circling the sun.
33:05Gradually, this swirling ring condensed into planets and moons,
33:09which formed by sweeping up the smaller objects around them.
33:15The oldest surfaces in the solar system still show the scars from this process.
33:21They are covered with impact craters from this continuous bombardment.
33:28Even the Earth has a few craters.
33:31This one, Meteor Crater in Arizona, is about one mile across,
33:35and the impact happened only around 22,000 years ago.
33:39But weathering and other geological processes have wiped out most of the Earth's older craters.
33:49Voyager's images of Umbriel, however, show that its surface is saturated with craters.
33:54This indicates that the moon has undergone very little geological change since it was formed.
33:59So its surface is very old, older perhaps than that of any of the other moons of Uranus.
34:05The puzzling donut is probably a much more recent crater,
34:08which has not yet darkened to the color of the rest of Umbriel.
34:15Further in towards Uranus orbits the fourth large moon, Ariel.
34:19It is about the same size as Umbriel.
34:22Its surface is relatively bright, almost twice as bright as dark Umbriel.
34:26Ariel is scarred by a complicated pattern of fractures and rift valleys.
34:33Everywhere else, as these images show,
34:37Ariel is marked with evidence of geological activity that is difficult to explain
34:41according to any current theory of planetary geology.
34:48We see fault systems that cross the surface
34:52and organized rectangular or linear patterns that transect one another.
35:00They look very much like grobbin, we call them on the Earth,
35:04in which a sliver of the crust breaks on a pair of planes
35:10and falls down into the Earth very much like a doorstop going into a slot.
35:18The process that causes that is extension, stretching of a planet.
35:24And the planet is trying to increase its surface area.
35:27So one way is simply to pull those faults apart
35:31and the wedge drops down, increasing the area.
35:35The valleys of Ariel have smooth floors,
35:38unmarked by the fault lines that made the valleys themselves.
35:41To understand how curious this is, it is necessary to go back into Ariel's past.
35:47After the great bombardment, Ariel's surface expanded,
35:50perhaps as the ice froze, and it cracked in a crisscross pattern.
35:58Then some material, maybe ice or liquid slush,
36:03flowed into these valleys, filled them up,
36:06and gave them the appearance they have today.
36:09The process is very similar to some volcanic eruptions on the Earth.
36:13But where did the ice or slush come from?
36:20If we trace back up these canyons,
36:24this common floor appears to be connected to another region
36:31up on the higher terrains on Ariel.
36:34And we can see a smooth region up in here.
36:40If we look a little farther, we see that it appears
36:43that this valley in here was breached.
36:46Material flowed out, partially buried an impact crater.
36:51We can still see half of it.
36:53It's fairly viscous or thick flow because it had to stand
36:56and retain a fairly sizable flow front right in this region.
37:01It flowed over the edge of this cliff on the edge of this canyon,
37:05muted the topography there, flowed on down through another notch,
37:09and then flooded the floors of these valleys.
37:12So some sort of volcanic activity had to occur on even Ariel.
37:18Here the temperatures are 70 degrees above absolute zero.
37:22Things like water ice and just about any other material
37:26that we find commonly would be solid beyond comprehension.
37:34Ariel has another mystery.
37:36Snaking along the middle of some of the smooth-floored valleys
37:39are sinuous canyons.
37:41When the imaging team first saw these,
37:43some thought they might be river valleys,
37:45rather like this dramatic canyon on Earth.
38:01Here we are at the Grand Falls of the Little Colorado River.
38:06It comes in here and goes on down in a sinuous path.
38:11And this sinuous valley looks much like the sinuous valleys
38:15we see on Ariel, one of the moons of Uranus.
38:19What's happened here is a complex story
38:21because we think this right angle is made by early faults.
38:26And then the river was carved,
38:29and because the rocks were broken by the fault,
38:32it made this right angle bend and followed the fault trace.
38:36The sinuosities are due to the meandering of the river.
38:42We have a complex intertwining of faulting, stream valley cutting,
38:47and lava flows that make this complicated geologic story.
38:56When we first saw the sinuous valleys on Ariel,
39:00the question was, could water have flowed on the surface there?
39:05But that's not possible on Ariel.
39:07It's much too cold for surface water to flow,
39:10and there's no atmosphere.
39:14Here we are at a different kind of sinuous valley.
39:19This one is very different because, as you can see,
39:24we're at the foot of a volcanic mountain.
39:27And this was built by a fire fountain
39:30depositing little fragments of lava that chilled in the air
39:34and fell down.
39:35And then later, a lava flow broke out at the base of it
39:39and flowed out in this direction.
39:42And as it flowed, it chilled on the margins,
39:45building these marginal levees.
39:47And the sinuous channel went down
39:49until the lava flow chilled and stopped flowing.
39:53The question is, what's the sinuous valley on Ariel?
39:57Is it cut by flowing water?
39:59We don't think so because there are no tributaries
40:02or the other fine details that running water flowing makes.
40:06Is it a lava channel?
40:08It doesn't have these marginal levees like we see here.
40:12The only one left is fault valleys.
40:15That is, tectonism breaks the crust,
40:18allows material to drop down, and forms the sinuous valley.
40:24The big question is, how do we decide this kind of thing
40:27when you see pictures of a planetary surface for the first time?
40:31First, you have to look,
40:33not allow your prejudices of what you hope to see there
40:37blind you to what's really there.
40:39And second, to sort through
40:41all of the geologic processes that you're aware of
40:44because each planet will be slightly different.
40:47And then to carefully compare what you see in the picture
40:50with what your memory is
40:52of all the other geologic processes that you've seen
40:56and then try to come to the proper conclusion.
41:01Even after the puzzling and mysterious pictures
41:04of the other four moons,
41:06no one at JPL was prepared for Miranda.
41:09It is the smallest of the major moons of Uranus,
41:12barely 300 miles across,
41:14and it is the closest to the planet itself.
41:17It is unlike any moon ever seen before
41:20and is one of the most enigmatic objects in the solar system.
41:24Even from a distance,
41:26scientists could see this odd right-angled shape.
41:29An unexpectedly complicated and exotic object in space.
41:34As the highest-resolution pictures show,
41:37some of the surface is an ancient cratered landscape,
41:40much like the surface of Umbriel.
41:48Other parts are scored with gigantic swaths and grooves.
41:52Miranda seems to include
41:54all the landforms in the solar system rolled into one.
41:57One NASA scientist described it as a moon designed by a committee.
42:08Strange Miranda is not made of the same stuff all over.
42:16Here, a crater has exposed white material.
42:19In other places, craters have broken through
42:22to more of the same gray material that dusts the surface.
42:26This is the right angle that was seen earlier.
42:29Once they saw it up close, the imaging team called it the chevron.
42:33It is about 100 miles long.
42:38Most extraordinary of all,
42:40Voyager photographed a palisade of gigantic white cliffs of ice,
42:44the biggest ever seen.
42:47It shows a fault which has broken the crust
42:51on what looks like a scissors fault,
42:54in which it's hinged back here and it breaks open like this.
42:58And the relief on that fault is of the order of 20 kilometers.
43:06These cliffs on Earth,
43:08as they are called,
43:10is of the order of 20 kilometers.
43:14These cliffs on Earth,
43:16the Vermilion Cliffs of the Grand Canyon,
43:19are 1 kilometer high.
43:21Their scale can be judged
43:23from the size of the trees growing in the forest at the top.
43:29The cliffs on Miranda are over 24 times higher than these,
43:33twice the height of Mount Everest.
43:36This is a mosaic of Miranda,
43:38elongated because it is made up of all the pictures
43:41taken by Voyager as it sped by.
43:44The mosaic shows that the strange landforms on Miranda
43:47are confined only to certain places
43:50and seem to be superimposed on the much older cratered surface.
43:56The first step was to try to classify things
44:00into groups that could be handled conceptually
44:05or we could try to unravel
44:08the story of one particular type.
44:11And as an example,
44:13these three terrains look very different.
44:16One we refer to as the Flapjack
44:18and another one as the Chevron,
44:21and then one we refer to as Circus Maximus
44:24because of this racetrack-looking affair that runs around the outside.
44:28First reaction, you might think those are very different.
44:31We did.
44:33But if you start to look carefully at the three,
44:37you realize that they have some very, very common aspects.
44:42Well, first let's start out with the outer two, the large ones.
44:46And I'll go here to a map.
44:50This is a map that's a composite of this mosaic plus earlier images.
44:55And here you see the Circus Maximus region
44:59and the Flapjack region
45:02have about the same overall shape and size.
45:08They're both about 250 kilometers end-to-end,
45:12and they both possess this quasi-rectangular shape
45:16with the rounded corners.
45:18They both possess an outer margin
45:21which runs around this rectangular pattern
45:26with the curved edges, you can see here and here.
45:30Also, looking at the Chevron region,
45:34we can see some aspects that are very much like the other two.
45:39These three are the same basic geologic phenomenon
45:45that is operated to different degrees.
45:49The mystery that we want to solve for Miranda
45:52is how these extraordinary features are found just in some places
45:56and not all over the satellite.
45:59The secret to that may lie in the impact history
46:03of all of the satellites, of Miranda in particular.
46:06We can imagine that Miranda, at one point in time,
46:10was warm enough that the rocky parts and the icy parts were separated,
46:16the rock sinking to the middle and the ice to the outside.
46:20Let's imagine this beaker represents a portion of Miranda,
46:23the whole satellite being a round feature like this,
46:26with the ice having risen to the top
46:29and the rocks having settled to the center
46:31when Miranda was warm enough at one time
46:33for this separation to take place.
46:36Now, we know from study of the satellite surfaces
46:39and a calculation of the rates of cratering
46:43at different places within the satellite system
46:46that Miranda has probably been broken up perhaps a dozen times
46:50by very large impacts in the same time
46:53that the craters that we see on Oberon were formed.
46:57Now, if we imagine that the last time this happened,
47:00Miranda was smashed with a projectile, a comet nucleus,
47:05big enough to break the whole satellite apart,
47:08this nicely sorted arrangement of ice and rock
47:11would be distributed as debris that would go into orbit
47:15and would be mixed up in the orbit
47:18and spread out following the orbit of Miranda.
47:21Now, that debris won't remain there very long.
47:24It will want to collect back again into a satellite,
47:27rather quickly as a matter of fact.
47:29But when it collects back in,
47:31the pieces that will fall into the satellite
47:34will come in different orders, icy chunks, rocky chunks.
47:38And so when we finish up,
47:40the final satellite will then be all mixed up and scrambled,
47:46looking quite different than it did at the outset.
47:50So we end up with a satellite that will look something like this.
47:54Now, of course, the satellite is not very happy in that state.
47:58The rocks do want to get back to the middle
48:01and the ice wants to get to the outside.
48:04But it has to get warm enough for that to happen.
48:08The heat generated by the tidal forces on the moon melted the ice.
48:13As the melting ice rose,
48:15it started cracking the surface with patterns of grooves.
48:19This happened only in certain places,
48:21probably above deep masses of ice
48:23or where rock was resting near the surface
48:26and sank into the depths of the reassembled moon,
48:29causing the unusual patterns we see.
48:36It's likely that all of these objects were blasted apart
48:40and reassembled multiple times during their accretion.
48:44So if some of these fault patterns
48:46were induced by those kinds of catastrophic events,
48:50it would have been a tremendous earth-shaking,
48:55Miranda-shaking event to be there.
49:00But the major catastrophic event of the Uranian system
49:04is the one that knocked it sideways to the sun.
49:07Almost certainly, Uranus once orbited upright,
49:10as the other planets do.
49:12Like a spinning gyroscope,
49:14a planetary system has its own stability.
49:18Its revolutions continue even if it is tipped over.
49:24It now seems that ferocious collisions
49:26between Uranus and one or more Earth-sized objects occurred
49:30and knocked the planet on its side.
49:32Even so, it continues to spin,
49:34but its pole, not its equator, faces the sun.
49:38Where did these impacting bodies come from
49:41that pummeled the satellites of Uranus
49:44and probably tipped Uranus itself over on its side?
49:47To answer that question,
49:49we have to go back to the origin of the planets.
49:52And here we have a diagram
49:54showing the planetary regions we see it today
49:57with the orbit of Neptune, Uranus, Saturn, Jupiter, Mars,
50:03and the orbit of the Earth and the close planets and the sun itself
50:07just in this tiny region in the center.
50:09And all of these dots in this region out here
50:12represent a swarm of small bodies of ice and rock
50:16out of which Uranus and Neptune accumulated.
50:21These bodies were perturbed by the growing planets.
50:25As they were accumulated into the planets,
50:27most of the objects actually were thrown out of the solar system,
50:31and only about 5% fell onto the planets themselves.
50:37One of these big impacts probably produced a spray off of Uranus
50:42and formed the satellites that we see.
50:45But the swarm of small bodies, which we call planetesimals,
50:49continued to buzz around the planets in this region
50:53for about a half a billion years.
50:56And so long after the planets were formed,
50:59there was still an intense bombardment.
51:02According to this theory,
51:04Uranus and its moons revolved upright as the other planets do.
51:08But early on in its history,
51:10a massive planetesimal struck the planet and violently tipped it over.
51:16Tidal forces disrupted the orbits of the moons,
51:18which destroyed each other in collisions.
51:25The debris settled into a wide new ring,
51:28which condensed into a fresh collection of moons
51:31orbiting to match the tilted planet.
51:34Later on, other planetesimals from the same swarm
51:37reappeared periodically to smash the moons.
51:51The apparently random differences between the moons
51:54would be explained by their luck
51:56in escaping the shattering impacts of the planetesimals.
52:00Oberon, and especially Umbriel,
52:02seemed from their old cratered surfaces
52:05to have escaped recent collisions.
52:12Ariel has a younger surface
52:14that shows signs of geological activity.
52:17It and Miranda were probably broken apart
52:19and reassembled many times over
52:21as chunks of primordial matter periodically smashed into them.
52:26What became of the planetesimals
52:29once they had bombarded these moons?
52:32About 5% of these objects
52:34were actually accumulated to form the planets,
52:36and as they grew, they perturbed the rest of these small bodies
52:40into larger and larger orbits,
52:42and finally most of them were actually
52:44thrown completely out of the solar system.
52:46Somewhat less than 10% remain today
52:49in a huge cloud surrounding the sun,
52:52and as passing stars go through or near this cloud,
52:55they perturb a few of them,
52:57which fall down toward the sun
52:59to the neighborhood of the Earth,
53:01and we see them as comets.
53:03The realm of the planet Uranus
53:05is a world we now know something about
53:07thanks to Voyager and NASA's team of scientists.
53:107,000 images from the Uranus encounter
53:12answered many basic questions
53:14about the distant seventh planet,
53:16the length of its day, its weather,
53:18its rings and moons.
53:20Voyager's success is a remarkable technological achievement,
53:23and its importance to space science
53:25will continue for many years.
53:27Even though Voyager 2 is now an old spacecraft,
53:30it performed essentially flawlessly at Uranus.
53:32In fact, it performed better at Uranus
53:34than it had at Saturn because of some design changes
53:36that we've made along the way,
53:38and we expect that the spacecraft will perform equally well
53:41when we arrive at Neptune in August of 1989.
53:44Neptune is 3 billion miles from Earth,
53:46so we know even less about Neptune
53:48than we knew about Uranus.
53:50We do know that it's a planet about the same size as Uranus,
53:53and we, in fact, want to study the weather
53:55in that planetary atmosphere.
53:57We'd like to study the fragments of rings
54:00which have been discovered in orbit around Neptune,
54:03and we'd like to measure and detect
54:05whatever kind of a magnetic field Neptune might have.
54:08And finally, there is a major moon in orbit around Neptune
54:11called Triton, which is very intriguing
54:13because from Earth there is evidence
54:15that there may be methane,
54:17that is, natural gas in a very tenuous atmosphere.
54:20There may well be lakes of liquid nitrogen on the surface.
54:23If so, Triton would be the most interesting object
54:26yet visited in the solar system.
54:30This computer animation shows the Neptune encounter
54:33that will take place on the 24th of August, 1989,
54:38and the Triton flyby the next day.
54:43Voyager's encounter with Neptune
54:45will propel it out of the solar system
54:47and into deep space like its twin, Voyager 1.
55:03At JPL, the engineers are now preparing
55:06for the Neptune encounter.
55:09Perhaps by the time Voyager reaches it,
55:12scientists will have worked through the full legacy
55:15of its exploration of Uranus.
55:18Pointing its camera backward toward the sun,
55:21Voyager sees mysterious Uranus recede into the distance.
56:12NASA Jet Propulsion Laboratory, California Institute of Technology