Introduction of Europa and Titan

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Titan (Titan) is a satellite that orbits Saturn. It is Saturn's largest satellite. It was discovered by christiaan huygens, a Dutch physicist, astronomer and mathematician, on March 25th 1655. It is also the first satellite discovered in the solar system after Jupiter and Galileo. Because it is the only satellite in the solar system with a strong atmosphere, it is regarded as a time machine, which helps us understand the earliest situation of the earth and unveil the mystery of how life on earth was born.

name

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Huygens simply called the satellite he discovered "Saturn satellite". Later, Giovanni domenico Cassini will discover four satellites (Titan, Titan, Rhea and IPartos IPartos, respectively named as' Lodia Sidera' ('Louis Star'). Astronomers used to number the five satellites with numbers. Other satellites are called "Huygens Satellite" or "Saturn's Sixth Satellite" (from the known distance to Saturn at that time, Enceladus and Enceladus are at 65438.

Titan's English name "Titan" and seven other Saturn-moons known at that time all came from Sir John Herschel (John Herschel is the son of Sir William Herschel, and William Herschel himself discovered Titan and Enceladus). John Herschel named this new satellite Titan in his book Astronomical Observations on the Cape of Good Hope published in 1847.

physical features

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Titan is Saturn's largest satellite and the second largest satellite in the solar system, larger than the planet Mercury (though not as big as Mercury) and second only to the largest Jupiter satellite Ganymede in the solar system. But recent observations also show that its dense atmosphere may make people overestimate its diameter. Like many other satellites, Titan is larger in mass and volume than the asteroid 134340 (formerly Pluto).

Titan has an average radius of 2575km, mass 1.345× 1023kg, and average density 1.880× 103kg/m3. The half-length diameter of Titan's orbit around Saturn is 122 1850 km, the eccentricity is 0.0292, the included angle between the orbital plane and Saturn's equatorial plane is 0.33, and the period of revolution is 15, 22: 00, 4 1: 24. Titan's rotation period is the same as that of period of revolution and similar to that of the moon. Titan has a dense atmosphere, mainly composed of nitrogen. The surface air pressure is 1.5× 105 Pascal and the surface temperature is-178℃.

Titan's mass is similar to that of Ganymede, Callisto, Triton and asteroid 134340. Titan is half water ice and half solid matter. There is a solid core in different crystalline ice layers at a depth of 3400 kilometers. The inside of the core should still be hot. Although Titan is similar to other Saturn moons, its core density is higher, which is caused by gravity compressing its interior due to its huge volume.

atmospheric condition

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Titan is a satellite known to have a real atmosphere, and other satellites only have tracer gas at most. Gerard P. Kuiper first discovered the existence of the atmosphere in 1944 by using a spectral telescope. He found that the local methane force in Titan's atmosphere reached 100 millibar. Later, the observation of Voyager spacecraft also confirmed the existence of the atmosphere on Titan. In fact, Titan's atmosphere is a little bigger than that of the earth, and the force on the planet surface is 1.5 times that of our earth. The dense clouds on Titan's surface cover its surface topography. It is generally believed that the surface of Titan is solid or liquid ethane. Recently, from the radar survey of the earth, it is found that there is no large-scale ethane ocean, but there may still be small ethane lakes. Later, scientists studied the recent photos sent back by Cassini spacecraft. Researchers observed Titan through ground-based telescopes. At that time, they thought there were various signs that there might be a liquid ocean on this Saturn satellite. However, scientists still have doubts about the current conclusion, because previous observations show that there is indeed a flickering liquid reflection on Titan's surface, especially the results observed by a large radio telescope a few years ago, which proves that there is a liquid ocean very likely.

Titan's atmosphere is 94% nitrogen-the only planet in the solar system that is rich in nitrogen except Earth-and there are a lot of different kinds of hydrocarbon residues (including methane, ethane, diacetylene, methylacetylene, propionitrile, acetylene and propane, as well as carbon dioxide, cyanide, hydrogen cyanide and helium). These hydrocarbons are thought to come from methane in Titan's upper atmosphere. When methane reacts with solar radiation, it will produce dense orange-red smoke clouds. The organic deposits on Titan's surface seem to be covered with tar, which is called tholin. Titan has no magnetic field protection, so when it sometimes moves outside Saturn's magnetosphere, it will be directly exposed to the solar wind. This leads to ionization of the atmosphere and releases some molecules in the upper atmosphere.

Near the surface, the temperature of Titan is about 94K. Water ice will sublimate at this temperature, so there will be a small amount of water vapor in the atmosphere. There are different clouds on Titan's surface besides the fog that covers the whole world. Clouds may consist of methane, ethane or simple organic matter. Other rare and complex chemicals are the reasons why Titan looks orange in space.

In June 2004, Cassini flew over the bright and cloudy South Pole in Titan's photo, but no expected methane was found. This puzzles scientists, and the related research on the composition of clouds is still in progress, so our past knowledge about Titan's atmosphere may need to be rewritten.

In 2004, Cassini observed the atmosphere and found that Titan's atmosphere was "super-rotating". Like Venus, its atmosphere rotates much faster than the surface.

Surface characteristics

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By 2004, people's understanding of Titan's surface map was still very lacking. In any case, humans used the Hubble telescope and Cassini Huygens' infrared rays to capture a high-brightness picture of an area the size of Australia. The unofficial name of this area is ‘Xanadu Regio'. Nobody knows what it's like there. Similar Hubble Space Telescope, Keck Telescope and Very Large Telescope have also observed another dark area of similar size on Titan. It is speculated that it may be a liquid methane or ethane ocean, but the data observed by Cassini may be other substances. Cassini also sent back a large number of high-resolution geomorphological images of Titan, including mysterious linear stripes, which some scientists think may be caused by crustal tectonic movement.

An observation of +065438+ flying over Titan on June 26th, 2004 found that there were almost no craters on Titan's smooth surface, and these craters had a strong contrast between light and shade under the action of light, which was probably caused by frequent crustal reconstruction caused by Titan's hydrocarbon rain or snow falling into the craters or active volcanic eruption. Optical splitter of the detector found that the sunlight emitted from the bright area and the dark area has the same wavelength. This means that they may be composed of (or at least covered by) the same substance. As for what substance it is, people don't know yet. Hydrocarbon lakes or seas that people had hoped to find by observing the light reflected by objects or liquids with detectors have not been detected so far. This makes scientists suspect that Titan's surface may be completely ice or muddy.

In order to better understand the surface topography, Cassini spacecraft used radar remote sensing mapping technology when flying near Titan. The first photo sent back shows that the surface is a complex, rugged and flat area. This landform seems to be caused by volcanoes. Volcanoes may spew water and ammonia. In addition, some striped landforms similar to wind erosion have been found. Some impact craters seem to be filled. Wherein the liquid can be liquid hydrocarbon. I'm not sure if there is anything in the lake yet. The signals returned from other areas seem to be solid or liquid, but other explanations still exist. Titan looks really smooth, and there is no landform more than 50 meters on the surface.

explore

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Titan is the only satellite in the solar system with an atmosphere. Huygens photographed thick clouds about 19 km from the surface of Titan. Scientists point out that the main component of this cloud is probably methane. After landing, "Huygens" also found that the surface material of Titan is constantly evaporating, producing more methane. It is speculated that there were a lot of methane-like hydrocarbons on the early earth.

Scientists were ecstatic when they smelled this gas similar to the early earth. Dr. Andrew Bauer, who participated in the project, was as excited as a child and shouted, "Oh, my God! This is incredible. After analyzing the photos and data sent back, this misty orange planet is made up of 95% nitrogen, and the remaining gases are methane and other hydrocarbons. These atmospheres are very similar to the early earth. This means that our dreams have really come true. " At the same time, there are traces of carbon monoxide and carbon dioxide in Titan's atmosphere, which remind scientists of the earth 4.5 billion years ago. Some astronomers say that Titan is the best place to find alien life in the solar system.

There may be clay and water on the surface.

In the first published color photo of Titan, the planet is full of orange, and its surface is as porous and elastic as a sponge. Scientists believe that the surface material that looks most similar to Titan's photo is the moist soil and clay on the earth. The brightness of Titan's surface is also much darker than previously expected. Scientists also found traces of erosion from the bottom of the objects in the photo, which indicates that they may have been washed by rivers before.

Marty Tomasco, an expert at the Moon and Earth Laboratory of the University of Arizona, said: "We have speculated that there is fluid on Titan, but we have been unable to prove it." Now, according to Huygens' bird's-eye view photos taken over Titan 16 km, there is no doubt that this is certain. He said that light-colored mountains and hills can be clearly distinguished from the photos, and black linear fluid passes through them, which may be lava or ocean. From the photos, this ocean consists of islands of different sizes and coastlines wrapped in mist. John Janica, the person in charge of exploring the surface of Titan, said: "After research, we think that the black part of the photo is the ocean and the white part is the island." In addition, astronomers speculate that there may be water on the surface of Titan, only in the form of ice, and it is not excluded that there may be liquid water deep on the surface.

There is a wind blowing on Titan.

In addition to sending back precious data and color photos, the microphone on Huygens also captured the loud noise that roared by. Scientists have not yet reached a conclusion, but the probe has detected that there is wind blowing on Titan, and the wind speed is about 24 kilometers per hour. Marcello Fulchignoni, a scientist from the European Space Agency, played an audio that was converted from radar measurements when Huygens passed through Titan's atmosphere. As Huygens gets closer to the surface of Titan, the radar signal becomes faster and sharper.

Voyager 1 and Voyager 2 have visited Titan. Voyager 1 trying to get as close as possible to Titan. Unfortunately, Voyager 1 didn't have any instruments to penetrate the fog on Titan, because it didn't know there were clouds on it. Many years later, Voyager 1 filmed it with an orange filter. Although the causes of light and dark landforms such as "Xanadu" and "Sickle" have not been explained, these areas have been observed by the Hubble Space Telescope with infrared rays since then. Voyager 2 only made a cursory inspection of Titan. Voyager 2' s team had to choose between adjusting Voyager 2' s orbit to examine Titan in detail and using another orbit to visit Uranus and Neptune. Because voyager 65438+,

Cassini-Huygens arrived in Saturn on July 1 2004 and began to measure the surface topography of Titan with radar. Cassini probe flew over Titan on June 26th, 2004 +065438+, and took many high-resolution images of Titan's surface, showing bright and dark spots that human eyes have never seen before. Cassini released Huygens on Christmas Day on June 25th, 2004, and Huygens was released on June 25th, 2005 at 65438+1October 25th.

Huygens probe landed on Titan in June 65438+1October 65438+April 2005.

The secret of the great goddess

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It has been more than 300 years since Huygens discovered Titan, and Titan is still a mystery to be solved. If you want to have a deeper understanding of Titan, you need human beings to continue to explore.

Outstanding "talents"

Why do astronomers pay special attention to Titan? Titan is favored and valued by astronomers because of its outstanding talent. Titan's unique "talent" is manifested in the following aspects:

First of all, Titan is 4828 kilometers in diameter, ranking second in the satellite world, much bigger than Pluto and almost as big as Mercury. Its mass is 1.8 times that of the moon, its average density is 1.9 grams per cubic centimeter, which is about 1/3 of that of the earth, and its gravity is 14% of that of the earth.

The average distance between Titan and Saturn is 6.5438+0.22 million kilometers, and it orbits Saturn in a nearly circular orbit. Like the moon, it always faces its planet Saturn in the same direction. In other words, if you look at Titan from Saturn, you will always see only the same half of Titan. Its orbit is basically on Saturn's equatorial plane. You can think about it. It is hard to imagine that a celestial body as big as Titan runs in a nearly perfect orbit with a radius of about 654.38+0.22 million kilometers. It's not easy to draw such a circle. It shows the natural wonders in the evolution of celestial bodies.

Secondly, in 1944, the Dutch-American astronomer kuiper made a systematic spectral observation of Titan, and found that there was methane gas on Titan, thus confirming the existence of a dense atmosphere on Titan. Until now, Titan is still the only satellite with an atmosphere among more than 60 satellites known in the solar system. How can this not be particularly favored by astronomers?

Thirdly, according to the movement characteristics, physical condition and chemical composition of Titan, astronomers determined that Titan evolved with Saturn, belonging to a stable satellite, and could not be a small celestial body captured by Saturn later. At one time, some astronomers compared Titan's celestial characteristics and celestial environment with that of the Earth, such as mass, volume, surface gravity, surface temperature, atmospheric composition, water and ice content, rotation and revolution, in order to obtain clues about the evolution of early life materials.

Is there life on other celestial bodies? This question has been haunting astronomers' minds. Huygens, the discoverer of Titan, wrote in his book "Wonders of Celestial Bodies, Speculation on Residents, Plants and Their Worlds on Other Planets": If we think that there is nothing on these celestial bodies except endless desolation, …

It is very unreasonable to even think that there can be no advanced creatures there, so we belittle them. It is true that judging whether there is life on any celestial body is a very serious scientific problem.

At present, I am afraid it is unrealistic to be too optimistic, but it is also unfounded to be too pessimistic. Practice is the only criterion for testing truth. As for the life information on Titan, it is still a mystery that is not optimistic, but it will be answered in the practice of continuous exploration.

Seen from the earth, Titan is an 8.4-magnitude star. You can't see it directly with your eyes. With a good telescope, you can only see a little red dot-shaped disk. Why this color? Some people think that this may be because there are complex organic molecules on Titan. Of course, relying entirely on ground observation can't solve such problems, and it can only be "an armchair strategist".

With the rapid development of space industry, interplanetary probes have made unprecedented achievements. At present, two interstellar spacecraft have personally detected Titan. They are the pioneer of American launch 1 1 and Voyager 1.

September 1979, 1, Pioneer 1 1 flew over Saturn and visited Titan. However, when Pioneer 1 1 visited Titan, a strong solar wind was catching up, which seriously affected the information sent back. The ground control center only received five high-resolution photos taken at a distance of 350,000 kilometers. In the photo, Titan appears beautiful orange-red, just like a ripe orange. Voyager 1 flew to Titan in June1980+065438+1October1. When it was closest to Titan, it was only 4000 kilometers away from the cloud top, and the exploration was a complete success. It was this time that the diameter of Titan was measured as 4,828 kilometers instead of 5,550 kilometers as previously thought.

Voyager 1 Investigation on Titan shows that Titan does have a thick atmosphere, about 2700 kilometers thick, which is higher than the density of the earth's atmosphere. The main components of the atmosphere are nitrogen, accounting for 98%, methane accounting for 1%, and a small amount of ethane and hydrogen. There is also nitrogen in the atmosphere of Venus, Earth and Mars, but not as much as Titan.

Voyager 1 also found that Titan's atmosphere is foggy. The dense fog layer prevented sunlight from reaching the surface of Titan, which affected the observation of the surface of Titan by travelers 1. At the same time, some scientists believe that Titan's atmosphere is full of methane according to the observation data of Voyager 1.

In order to further study the relationship between Titan's atmosphere and life, carl sagan, a planetary physicist at Cornell University in the United States, conducted a simulation experiment on Titan's atmosphere. Researchers believe that Titan's atmosphere contains a lot of nitrogen, which produces a variety of pre-life chemicals. Sagan pointed out: "A similar process may have happened on the early earth. However, the pre-life chemical process that happened on Titan is probably lifeless, because the temperature there is far below the freezing point of water. "

Speaking of which, have you ever wondered: Why is Titan the only satellite with such a rich atmosphere? This has always been a question that planetary physicists are thinking about. Some people think that this may be because Titan's surface temperature is high enough to maintain a considerable amount of methane and ammonia to keep the ice on its surface in equilibrium. It is also possible that the ice on Titan contains methane and ammonia, and it is easy to form an atmosphere at Saturn's temperature. The third possibility is that Titan's atmosphere will not escape like Jupiter's strong magnetic field. The fourth possibility is that Titan is massive and can differentiate internally. The differentiated ice is concentrated on the surface, and its gravity is enough to prevent most of the gas from escaping.

So far, only three probes, Pioneer 1 1, Voyager 1 and No.2, have flown to Saturn to explore Saturn. 1 September 19791day, Pioneer 1 1 became the first probe to visit Saturn after six and a half years of space travel. It flew 20,200 kilometers from the cloud top of Saturn, explored Saturn 10 and sent back the first photos of Saturn. Pioneer 1 1 not only discovered two new Saturn probes, but also sent back the first photos of Saturn. Pioneer 1 1 not only discovered two new Saturn rings and Saturn's1/satellite, but also confirmed that Saturn's magnetic field is 600 times stronger than that of the earth. On September 2, it crossed the plane of Saturn's ring for the second time, and turned to Titan by Saturn's gravity, thus detecting this planet that may be pregnant with life.

1 980165438+1October 12, Voyager1flew to the place where it was 0/2600 kilometers away from Saturn/kloc-0, and sent back more than 10000 color photos. This exploration not only confirmed the existence of Enceladus 10,1and 12, but also discovered three new Saturn satellites. When it flew less than 5000 kilometers from Titan, it first detected and analyzed the atmosphere of Saturn's largest satellite. It is found that there is neither enough water vapor in Titan's atmosphere nor enough liquid water on its surface.

198 1 On August 25th, Voyager 2 flew over the top of Saturn Cloud 10 100 km, and returned more than18,000 photos of Saturn. It is found that the surface of Saturn is cold and windy, and there are strong and stable storms at high latitudes in the northern hemisphere, even more violent than those on Jupiter. Saturn also has a great red spot, 8000 kilometers long and 6000 kilometers wide, which may be caused by the disturbance and rotation caused by the updraft in Saturn's atmosphere falling into the clouds again. Lightning passes through Saturn's rings from time to time, and its power is tens of thousands or even hundreds of thousands of times that of lightning on earth. It confirms once again that Saturn has seven rings. Saturn's rings are composed of particles and gravel with a diameter of several centimeters to several meters. The particles in the inner ring are smaller and the particles in the outer ring are larger. Due to the different particle densities, the halo presents different colors. Each ring can be subdivided into thousands of small rings. Even if it is considered empty, there are several small rings in the Cassini seam. In the high-resolution photos, we can see that the F-ring has five small intertwined rings. The overall shape of Saturn's rings is similar to a huge compact record, extending from Saturn's cloud top to 320 thousand kilometers away.

Voyager 2 discovered 13 Saturn's new satellites, increasing the number of Saturn's satellites to 23. It inspected nine of the satellites and found that there was a large crater on the surface of Titan, with a diameter of 400 kilometers, and the bottom was domed upward, and there was a huge crack around the satellite for almost three quarters. One hemisphere of Partos is dark and the other is bright; Titan's rotation period is only 9~ 10 hour, which is far from 550 days in period of revolution. Titan's actual diameter is 4828 kilometers, not 5800 kilometers as originally thought. It is the second largest satellite in planets of the solar system. It has a dark and cold surface, a sea of liquid nitrogen, a dark red sky, and occasionally nitrogen rain mixed with hydrocarbons. This is an ideal place for human beings to understand the origin of life and various chemical reactions.

In order to further explore Saturn and uncover the mystery of Titan's life, the United States and ESA jointly developed the priceless Cassini Saturn probe. 1997101October 15, the probe was launched and began a long journey of seven years. It is expected to fly into nearby space in 2004, explore Saturn for four years, and land on Saturn's largest satellite Titan for the first time. Cassini is about 2.7 meters in diameter and has a total weight of 6 tons. It consists of an orbital probe and a lander. Its orbital probe is called Cassini, and it is equipped with 12 detection instruments. The lander named Huygens is equipped with six scientific instruments. In order to speed up the flight to Saturn, Cassini flew over Venus in April 1998 and gained its first acceleration. Then it made a revolution around the sun, and flew over Venus again in June 1999, and got the second acceleration. In August of the same year, he flew close to the earth and gained the third acceleration.

After that, the Cassini probe will fly over Jupiter in June 5438+February 2000 to get the last acceleration. It plans to fly to its destination to meet Saturn in July 2004 and enter the orbit around Saturn. In June165438+1October of the same year, the Huygens lander will leave the Cassini probe and fly to Titan, pass through its clouds, make a soft landing on Titan, and then send the detected data back to Earth through the Cassini orbiter orbiting the Earth. After entering the orbit around Saturn, Cassini's mission is to fly around Saturn 74 times, investigate the dynamics of Saturn's atmosphere and atmospheric circulation on the spot, fly to Saturn's satellites many times, including 45 times near Titan, and draw the surface structure map of Titan through its clouds and gas layers with radar. It is estimated that 500,000 images of Saturn, Saturn's rings and Titan's family can be sent back at close range. Huygens will be the first probe to land on a satellite of a big planet. During the 2.5-hour landing, it will analyze the atmospheric composition of Titan, measure the wind speed and detect the suspended particles in the atmosphere, and keep the working state 1 hour after landing, revealing whether there is an ocean frozen by water and ice on Titan and whether there is some form of life. The data it collects and the images it takes are sent back to Earth by Cassini detector.

What is the surface of this fascinating Titan? It should be said that there is no intuitive information so far. Scientists have made many possible speculations, and the description of Titan by science fiction writers is even more wonderful. However, everything should respect science.

According to the nitrogen content in Titan's atmosphere, the surface temperature of Titan is much lower than that of the Earth, ranging from -20 1 to-190℃, and the volume and mass of Titan. Some scientists speculate on its internal physical conditions and surface characteristics, and first look for the proportional relationship between rocks and ice on Titan. It is estimated that the rock material on Titan accounts for about 55% of its total mass, and the rest is ice. Titan's surface is an icy liquid ocean. The ocean consists of 70% ethane, 25% methane and 5% dissolved nitrogen. The entire liquid ocean is about 1 km thick and surrounds Titan. 1on June 4-5, 989, radar detection was carried out from the earth to Titan, and the results showed that there might be land areas on Titan.

Voyager 1 also found that the light and shade of Titan's northern hemisphere are different: the southern hemisphere is bright and the northern hemisphere is dark. What caused this? It may be caused by the different seasons on Titan. When Voyager 1 visited, Titan's northern hemisphere was just in beginning of spring. However, some people think that this may be the influence of Saturn's magnetosphere on Titan. Anyway, it's still unclear. Titan's atmosphere has a strong light absorption capacity, which can absorb about 80% of the sunlight falling on it. Most of this heat is absorbed by fog particles and methane gas in the atmosphere, and perhaps only 5% ~ 10% of solar energy reaches the surface of Titan.

Since Huygens discovered Titan, there seem to be more and more mysteries about Titan for more than 300 years. In fact, this is not surprising, which shows that our understanding is getting deeper and deeper. Copernicus, a great Polish astronomer, famously said, "Man's bounden duty is to be brave in exploration."

Orbit long radius (astronomical distance unit) 9.539

Track long radius (ten million kilometers) 1427.0

Constant Week of Revolution (Japan) 10759.5

The rendezvous period of revolution (days) 378

Track eccentricity 0.056

Track inclination (degree) 2.5

Ascending meridian (degree) 1 13.3

Perihelion longitude (degree) 92.3

Average track speed (km) 9.64

Equatorial radius (km) 60330

Flatness 0. 102

Mass (Earth mass = 1) 95. 159