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Comet Shoemaker-Levy After Crossing Jupiter's Roche Limit
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图注： This panoramic image captured by the Hubble Space Telescope shows fragments of Comet Shoemaker–Levy 9. This comet was discovered in 1993 as the series of fragments you see here. These fragments were orbiting Jupiter. It is thought that at some point in the previous few decades the whole, unfragmented comet had been gravitationally captured by Jupiter. Then in 1992 the comet passed within Jupiter’s Roche limit.
Astronomical objects exert gravitational forces on each other. The closer one is to an object, the larger the force. Astronomical objects have a real physical size and thus the gravitational force exerted on the object will vary across the object, the side of an object closer to another object will feel a stronger gravitational force from that other object than the more distant side. The gravitational stretching distorts the object. This gravitational stretching force is known as the tidal force. When an object is close enough to a large body like Jupiter, the object will feel such a large tidal stretching force that it will overcome the internal gravitational force holding the object together, ripping it to shreds. The distance from the larger body within which this occurs is known as the Roche limit.
When Shoemaker–Levy 9 crossed Jupiter's Roche limit in 1992, the tidal force pulled the comet into separate fragments. Here we see these fragments in a chain as they orbited Jupiter in May 1994. Later in July 1994 the comet fragment plunged into Jupiter’s atmosphere over the course of a week in a spectacular series of impacts. This event provided scientists with a rare opportunity to witness an impact unfolding in real time.
来源： NASA, ESA, and H. Weaver and E. Smith (STScI)
来源链接
词汇表:
洛希极限 , 潮汐力
授权许可： 公共领域 公共领域 图标
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507.00 kB)
Ganymede seen by Juno spacecraft
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图注： Ganymede is the largest and most massive natural moon of Jupiter, and at the same time, the largest moon in the solar system. It also stands out as it is the only moon in the solar system which has an internally generated magnetic field. This is likely created by a liquid iron or iron sulfide core.
With a diameter of over 5,000 kilometers, it exceeds Mercury in size. Although it has more than twice the mass of Earth's Moon, it hosts weaker surface gravity than Mercury, Io, and the Moon due to its lower mean density.
Ganymede's appearance is primarily composed of darker, heavily cratered regions, and brighter, less bombarded ones with deep grooves and ridges indicating geological activity. Whether this might be caused by tidal heating is still subject to further analysis. The brighter regions make up around two-thirds of Ganymede's surface.
The remaining area consists of dark regions, saturated with impact craters, which have been dated back to four billion years. It is assumed that the moon hosts a large saltwater ocean under its icy surface with extreme depths of up to 800 kilometers.
Ganymede likely has a very tenuous oxygen atmosphere, comparable to that found on the Jupiter moon Europa. It cannot be interpreted as a sign of life. Instead, it is thought that hydrogen and oxygen are split up by radiation on the moon's surface.
The first space probe to visit Ganymede was Pioneer 10, performing a high-speed flyby manoeuvre through the Jupiter system. Since then, it has been investigated by several other probes such as Galileo, New Horizons, and Juno. The Jupiter Icy Moons Explorer (JUICE) will be the first to actually enter orbit around Ganymede itself (in around 2034) to further study the moon's magnetic and icy surface. JUICE will impact Ganymede as the last act of its mission.
来源： NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill
来源链接
词汇表:
伽利略卫星 , 卫星
授权许可： 知识共享许可协议 Attribution 2.0 Generic 知识共享许可协议 Attribution 2.0 Generic 图标
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1.85 MB)
