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00:00Mercury is the planet nearest to the sun in our solar system. Mercury's surface on its
00:13day side is hot enough to melt lead. The planet has a diameter about 40% larger than that
00:25of the Earth's moon. During the second episode of our 13-part series, Journey Through the
00:35Solar System, we will examine the planet Mercury in detail.
00:55I'm your host, Larry Ross, Director of Space Programs at NASA's Lewis Research Center,
01:25in Cleveland, Ohio. Before we begin our examination of Mercury, we will preview the
01:31planetary lineup for our following shows so that you can see how Mercury fits into the picture.
01:37A NASA film called Our Solar System was made to aid young students. I think you'll enjoy
01:45seeing the short film, which introduced youngsters to the order of the planets in our solar system.
01:55My very educated mother just served us nine pizza pies. My very educated mother just served us nine
02:21pizza pies. Let's see. There are nine. The nine first letters of those nine words. And there are
02:29nine planets with Mercury closest to the sun and Pluto furthest away. And in between, my very educated
02:38mother just served us nine pizza pies. Mercury is the smallest. Venus is about the same size we are,
02:47but the resemblance quits there. Its atmosphere is ten times as dense as ours, shrouding the surface
02:53completely. And the average temperature is the boiling point of lead. And Earth? Earth? Well,
03:00that's us. My very educated m. Mars. How about Mars? Through Earth-bound telescopes, we can watch
03:11Mars turn greener by season. Watch what might be polar ice caps advance and recede. Does that
03:17mean Mars has a summer and winter? Earth telescopes develop questions. Photographs from spacecrafts
03:23like Mariner are beginning to supply answers. But man, do we still have a lot to learn. Hey, NASA!
03:30My very educated mother, J. J is for Jupiter the giant, 300 times the size of Earth. Its atmosphere is a
03:43lethal smog of ammonia and methane and hydrogen. Likely, there are fantastic lightning bolts
03:50flashing all through it. But you know, that's almost the same combination our Earth had a few
03:56billion years ago. At about the same time, early forms of life appeared. Did you know that Jupiter
04:02has a big red spot on it? It's oval-shaped as wide as Earth and three times as long. And it shifts
04:09position on the planet in a mysterious way. Why? Questions. Questions. My very educated mother. Saturn has those rings, and we've measured them. They're more than 37,000 miles wide, but only about six miles thick. Hmm. Then comes Uranus and Neptune. Jupiter, Saturn, Uranus, and Neptune are called the four giant
04:39planets, but Pluto is puny and cold. So, that makes nine planets spinning around our sun, one of a hundred billion suns in our single galaxy. In turn, one of 10 billion other galaxies. My very educated mother just served us nine pizza pies.
05:09The word planet is derived from a Greek word meaning wanderer. Planets were called wanderers
05:15because they were observed to change their positions relative to the stars.
05:21The planets of our solar system are divided into two main kinds.
05:25Gas giants are the planets Jupiter, Saturn, Uranus, and Neptune. They are composed mostly of gases.
05:32The other category of planet is the terrestrial type. Earth is in this category. This does not mean, of course, that the other planets in this category closely resemble the Earth. The terrestrials are those planets of the inner solar system which are small and rocky.
05:57Current theory suggests to us that the sun and planets formed from the collapse of a big cloud of dust and gas about five billion years ago. The center of the cloud compressed to make the sun. Planets formed in orbits around the sun.
06:18The theory also suggests that the inner planets are relatively small and rocky because they originated from a hotter part of the dust cloud where much of the gas was lost. Mercury, named for the Roman messenger god, is denser than the moon and seems to have a large iron core.
06:38Mercury was only a blur in Earth-bound optical telescopes until NASA sent the Mariner 10 probe on three flybys of the planet in 1974 and 1975. Incidentally, Mariner 10 was launched by the Atlas-Centaur rocket managed by Lewis. Now, here are excerpts from a NASA film entitled Mercury, Exploration of a Planet.
07:08Mercury is a small planet so near the sun that it is scarcely visible in the bright glare of our star. Like an elusive ghost, it is best observed from Earth in the twilight hours.
07:38When the sun is gone from the sky, the ancient Romans gave it its name, Mercury. So little was known about the planet that scientists could only guess at its origin and evolution.
07:57Like the planet Mars, it could have had violent volcanoes, deep canyons scarred by violent winds.
08:09Or like our own moon, its surface might be pitted with craters, a record of ancient events written on a planet's surface.
08:19The best photographs of Mercury added little to our knowledge. But when man learned to travel in space, the electronic cameras of a spacecraft called Mariner 10 revealed the planet in startling detail.
08:37In June of 1975, scientists from five nations assembled at the California Institute of Technology to exchange the information they had gained from the exploration of the planet Mercury by the spacecraft Mariner 10.
08:58Included in the group were scientists representing many techniques of exploration, including the detection of various forms of radiation, the study of gases in a planet's atmosphere, and the powerful scientific tool, photography of the surface of a planet.
09:16Welcome to Pasadena. We are convened.
09:24The photographic exploration of Mercury was led by Dr. Bruce Murray of the California Institute of Technology.
09:32The objective I have is to present a relatively simple interpretation of the history of Mercury and to draw some conclusions about the other terrestrial planets.
09:45The work that Dr. Murray and his colleagues are engrossed in goes beyond the exploration of a single planet, Mercury.
09:53They are seeking insights into the history of our entire inner solar system.
09:59In March of 1974, Mariner 10 flew past Mercury. During the next year, it would perform two more flybys.
10:09Mariner 10 returned almost 3,000 pictures of the surface of Mercury.
10:15These pictures reveal craters and other features which were formed very early in the history of Mercury and may refer to events that affected the entire inner solar system, Mars, Earth, Moon, and Venus, as well as Mercury.
10:32The pictures of Mercury reveal craters and many other features.
10:42And to the geologist, these features and these pictures are like the pages of a history book.
10:48And the history book for Mercury may go back almost to the formation of the solar system itself.
10:58Five million years ago, a vast cloud of gas and dust floated through the galaxy we call the Milky Way, the ghostly remains of a great star that had died in a gigantic explosion.
11:19The gas and dust is twisted and shaped by magnetic forces, electrical currents, and the subtle pull of gravity.
11:29As it swirls around a thickening core, the huge cloud gradually flattens into a disk.
11:39Dust and gas gravitate slowly inwards, and eddies begin to form in the cloud.
11:48Particles of matter cluster into solid bodies.
11:58The larger bodies continue to grow, sweeping up particles and dust as they orbit the condensing core of the disk and begin to heat under the increasing gravitational pressures.
12:25In the central core of the disk, incredible heat is being generated, hotter and hotter, until it reaches critical temperatures.
12:37And then, billions of years ago, a nuclear reaction occurred, and our sun was ignited.
12:51Its intense radiation repels the surrounding gas and dust.
12:57The increasing heat of the inner planets has formed them into molten spheres.
13:03Mercury, like the other inner planets, is continuously bombarded by debris that form craters on its surface.
13:13In the final stages of its formation, Mercury glowed from internal heat.
13:21But lava was forced to a surface being torn by collisions with masses of rock that were shaping the planet.
13:30The same heat that triggered the lava flows also melted rock and metals in the interior of Mercury.
13:39Heavy iron concentrated at Mercury's center to form a dense core,
13:44contained by a thin shell of lighter material brought to the surface by lava flows.
13:50Again, the surface was gouged and pitted by a great series of impacts, leaving huge craters on the surface.
13:59The Mariner 10 photographs have revealed these early craters to scientists for the first time.
14:08Music
14:20One of these craters was formed by a massive rock, perhaps a small planet, that crashed into Mercury.
14:29In the Mariner 10 photographs, half of this huge crater was hidden on the night side of the planet.
14:38Today, this huge crater is called Caloris.
14:43The impact spewed millions of tons of debris across the surface of Mercury, creating a ring of mountain ranges over a mile high.
14:54The floor of this basin was split by great surface cracks, wide and deep.
15:03As this surface continued to form, deep inside Mercury, its heavy iron core began to contract.
15:12The surface buckled, cracked, and great sections hundreds of miles wide were split open,
15:19leaving immense, mile-high walls of rock called scarps.
15:29Again, there was a great flow of lava covering many of the large craters and leaving smooth, flat plains.
15:39This set the stage for the final episode in the history of Mercury, a period of light cratering of the Great Plains.
15:51In this time, Mercury died.
15:56The internal heat that triggered many of the events in the planet's history turned off.
16:03And for three and a half billion years, Mercury has remained as we see it today.
16:11Before Mariner 10, there were a multitude of possible Mercuries.
16:25Now there's only one, the Mercury of craters and basins and plains.
16:30And this illustrates how photography can be an exploratory tool.
16:35It displays features which were perhaps never even anticipated
16:40and sometimes could not even be imagined before the experiment was flown.
16:45In a sense, it provides answers to questions that were not even asked.
16:50This was true of Mars when Mariner 4 returned pictures of what appeared to be a moon-like surface.
16:56It had been expected that Mars might be very much like the Earth,
17:00with folded mountain ranges and many other Earth-like features.
17:05Instead, it looked like the moon, and that was a big shock.
17:09And it was an even bigger shock when Mariner 9 in 1971 visited Mars again
17:15and found huge volcanoes and even some peculiar sinuous channels
17:21that looked like they might have been formed by flowing water at some point in Mars' history.
17:27Now Viking has transported man's vision to the actual surface.
17:33The fantasy of science fiction has been replaced by the strange, lonely reality of the Martian landscape.
17:40When Mariner 10 flew by Venus, it discovered an unexpectedly well-organized set of cloud motions.
17:50And the unexpected was certainly encountered in the case of Mercury,
17:55which turned out to exhibit not only moonlight features on its surface,
18:01but a sequence of events there that seemed to be very similar to those of the moon.
18:06And that's extraordinary, because Mercury is very different inside compared to the moon.
18:13And that means it must have had a different history of heating and modification of the surface from the inside.
18:22And furthermore, it lives in a very different part of the solar system, much closer to the sun.
18:28Therefore, it must have experienced a different exterior history as well.
18:33And yet, the pictures show that Mercury has a surface history very similar to that of the moon.
18:40And to me, that's extraordinary.
18:43A comparison of the craters on the moon, Mercury, and Mars
18:47shows that these inner planets were subjected to cratering at the same time.
18:53For Mariner 10, it was believed that the source of cratering of the inner solar system
18:57was the asteroid belt orbiting beyond Mars.
19:01Because of the difference in distance from the belt,
19:04it was assumed that no two planets would be cratered the same.
19:09Those planets farther from the asteroid belt, like Mercury, would have fewer craters.
19:15But a careful count of the craters on Mercury,
19:18and a comparison with those on the moon and Mars,
19:22showed them all to be roughly equal.
19:25If the crater count is equal, then the source could not be the asteroid belt,
19:30but must have come from elsewhere in the solar system.
19:34One strong possibility is the planet Jupiter,
19:37with a gravitational attraction second only to the sun.
19:41It is possible that on two occasions,
19:44Jupiter literally hurled millions of tons of rock inwards towards the sun
19:49to impact the planets of the inner solar system, including Mercury.
19:54This cratered record on Mercury was read three times because of a fortunate coincidence.
20:01Mercury's orbit around the sun is 88 Earth days.
20:06Mariner 10's orbit was twice as long, or 176 days.
20:11These synchronous orbits allowed Mariner 10 to fly past Mercury a second and third time.
20:19The path of the first flyby was on the dark side of Mercury.
20:31The second encounter sent the spacecraft past the light side of the planet,
20:36specifically for photography.
20:39After another six-month journey around the sun,
20:43Mariner 10 made its third and final encounter
20:46to look again at a major discovery made during the first flyby,
20:50the presence of a magnetic field around Mercury, similar to Earth.
20:57A bow shockwave blocks the solar wind from the sun
21:02and deflects it around the planet,
21:05creating an environment similar to the magnetic field around Earth.
21:09This discovery was a surprise,
21:13because Mercury's slow rotation about once every 57 Earth days
21:17had led scientists to believe that the planet could not generate a magnetic field.
21:23It is believed that Earth's magnetic field is generated by an interaction
21:27between the planet's faster rotation and its molten core.
21:33Mariner 10's discovery may change theories on how these fields are formed,
21:38and has given scientists their first opportunity
21:41to compare two magnetic fields in the inner solar system.
21:48There were other measurements made at Mercury by Mariner 10.
21:53Infrared showed that Mercury's surface ranges in temperature
21:57from 700 degrees Fahrenheit to 300 degrees below zero,
22:01the widest temperature range of any planet.
22:06The ultraviolet experiment revealed that Mercury has a very thin atmosphere of helium.
22:14Sensors measured the invisible cosmic rays which flood our solar system at tremendous speeds,
22:20penetrate any surface, and are unaffected by gravitational forces.
22:25Their source is unknown.
22:30Exploration is an adaptive process.
22:34Each new piece of information adds to the value of others that were accumulated earlier.
22:40We had the results of the Apollo program on the Moon to help us understand the time scales
22:45and to some extent the processes we were seeing when we looked at Mars with Mariner 9.
22:54Now that Mariner 10 has looked at Mercury, we can begin to compare it with the Moon and Mars,
23:00and begin to realize that there is a common solar system history
23:03which has been recorded on these planets,
23:06whose early surfaces have not been erased by erosion and other atmospheric processes.
23:12That same common history affected the Earth, too.
23:16And so by looking at these surfaces, reading those records,
23:21we're in fact looking back into the Earth's history,
23:25into a heretofore unexplored domain of time, our own history on the Earth.
23:31Now we can begin to compare and contrast, to look for similarities and differences,
23:37and try to recognize our family relationships among the terrestrial planets.
23:44Are we cousins or brothers?
23:47Or are all of us bizarre strangers that happen to inhabit the same portion of the solar system?
23:52That's the task of comparative planetology.
23:56That's where we go from here.
23:58What we know about Mercury comes, for the most part, from the Mariner 10 mission,
24:02which we just saw described in the film.
24:05Mercury looks much like the Moon.
24:08However, on Mercury, there do not seem to be dark deposits of lava,
24:12which covered the Moon 3 to 4 billion years ago.
24:17About half the planet was photographed by Mariner 10 during its three flybys.
24:29Before 1965, scientists thought Mercury always kept one side facing the Sun.
24:36But after radar measurements of the planet done by astronomers at Arecibo Observatory in Puerto Rico,
24:42Mercury was found to rotate once every 59 Earth days.
24:47The rotation speed at its equator is only 6 miles per hour.
24:52Earth's rotation speed at the equator is about 1,000 miles per hour.
24:57Mercury's orbit is an egg-shaped ellipse compared to the nearly circular orbit of Earth.
25:04When Mercury is at its nearest approach to the Sun, the orbital speed increases.
25:10This change in orbital speed causes Mercury to keep one face toward the Sun
25:15during this close-in approach.
25:18Later, when Mercury moves farther from the Sun,
25:21the planet's rotation changes the face that sees the Sun.
25:25Because Mercury gets as close as 28.5 million miles to the Sun,
25:30the planet becomes very hot on the day side, as much as 950 degrees Fahrenheit.
25:37At the same time the day side is hot,
25:40the night side of Mercury is as much as 346 degrees Fahrenheit below zero.
25:47Mercury's gravity is about a third of Earth's.
25:50The gravity of the planet Venus was used to slingshot the Mariner 10 spacecraft to Mercury in 1974.
25:58Mariner 10's primary mission was to Venus, the second planet from the Sun.
26:04Using the gravity of Venus to deflect the spacecraft to Mercury was the first trial of this method.
26:11As we will see in a later program,
26:13the Pioneer and Voyager spacecraft also used a gravity assist
26:18to fly by several planets in the outer solar system in a grand tour.
26:24Jupiter and Saturn have already been flown by,
26:27and a Voyager craft is on the way to Uranus and perhaps Neptune.
26:42Spacecraft have explored more than 20 planets and moons.
26:46There is a planned progression to the study of Mars.
26:50There is a planned progression to the study of our solar system.
26:54The first step is what we have seen in this program, an unmanned flyby of the planet.
27:03Next comes unmanned spacecraft which orbit and land on the worlds.
27:13The last stage is manned landings and the return of samples from the planet to Earth.
27:20This last step has, of course, as yet happened only on Earth's moon.
27:34This is Larry Ross, saying goodbye from the NASA Lewis Research Center in Cleveland, Ohio.
27:40I hope you can join us for the next program entitled, The Veil of Venus.
27:50Music
28:20Music