Jupiter Rings

The rings of Jupiter are a system of planetary rings that surround said planet. It was the third ring system discovered in the solar system, after the ring systems of Saturn and Uranus. Jupiter's rings were first observed by the space probe Voyager 1, and have been extensively investigated during the 1990s and early 20th century XXI using the Galileo, Cassini and New Horizons probes. They have also been observed from ground-based observatories and the Hubble Space Telescope over the past 25 years. Observations from the Earth's surface require the most powerful telescopes available.

The Jovian rings are weak and composed primarily of dust. They consist of four structures: inside, a thick torus of particles known as the halo or halo ring, a relatively bright but exceptionally thin main ring, and two wide, thick, faint rings called the Thebe diffuse ring and the Amalthea diffuse ring. by the names of the satellites of whose material they are formed.

The main ring and halo consist of dust ejected from the Metis and Adrastea satellites, and other unobserved bodies, as a result of high-speed meteorite impacts. High-resolution images obtained in February 2007 by the New Horizons revealed a rich, fine structure in the main ring.

In the visible light band and in the near infrared, the rings show a reddish color, except for the halo, which has a neutral or bluish color. Applying photometric models to the various observations available from both space probes and telescopes in Earth's surface, it is inferred that the size of the particles is 15 μm in radius in all the rings except the halo, although the model results are closer to observations when non-spherical particles are considered than when spherical ones are considered.. The halo is probably composed of submicroscopic dust.

The total mass of the ring system, including the unobserved bodies that generate ring material, is not exactly determined, but is likely to be in the range of 10¹¹ to 10¹⁶ kg. The age of the ring system is not known, but they may have existed since the formation of the planet.

Discovery and exploration

The existence of Jupiter's rings was inferred by observations of the radiation belts made during the flyby of Jupiter by the space probe Pioneer 10 in 1974, in which a decrease in the count of high-energy particles in the belts between 50,000 and 55,000 km above the planet's surface.

In 1979 the Voyager 1 probe obtained the first image, through overexposure, of the ring system. A greater number of images were obtained by the Voyager 2, which which allowed us to make a first description of the structure of the rings. The planet Jupiter has been visited on many other occasions. The Galileo orbiter obtained higher quality images between 1995 and 2003, which greatly increased knowledge about the Jovian rings. In 2000 the Cassini probe, en route to Saturn, its final destination, he made extensive observations of the entire ring system. And finally, images transmitted by the New Horizons probe in February and March 2007 allowed us to observe the structure of the ring in detail for the first time. main. Jupiter's ring system is one of the objectives of the Juno mission.

In addition, observations from the Earth's surface by the Keck telescope between 1997 and 2002, and by the Hubble Space Telescope in 1999 revealed rich structure in backlit images.

Structure

Jupiter's ring system comprises four main structures: a thick torus of particles known as the halo or the halo ring, a main ring b> relatively bright, but very thin, and two wide, very thin and faint outer rings named for the satellites of whose material they are composed, fuzzy ring of Amalthea and fuzzy ring of Thebe b>. The main characteristics of the rings are specified in the following table:

Main ring

Appearance and structure

The narrow and relatively thin main ring is the brightest part of Jupiter's ring system. Its outer edge is located about 129,000 km from the center of the planet, that is, 1,806 Jovian equatorial radii (R_J=71,398 km), and coincides with the orbit of the smallest of Jupiter's inner moons, Adrastea. Its inner edge is not marked by any satellite and is located at 122,500 km or 1.72 RJ.

The width of the main ring is approximately 6500 km. The appearance of the main ring depends on the illumination geometry of the rings. With forward illumination the ring's brightness begins to decrease greatly at 128,600 km, just inside Adrastea's orbit, and reaches the background level at 129,300 km, just outside Adrastea's orbit, indicating that it clearly serves as a shepherd satellite for the ring. The brightness increases in the direction of Jupiter and has a maximum near the center of the ring at 126,000 km, although there is a pronounced gap near the orbit of Metis at 128,000 km. The interior of the main ring, on the other hand, slowly blurs, blending with the ring halo. In front lighting all of Jupiter's rings are especially bright.

With backlighting or backlighting the situation is different. The outer edge of the main ring, located 129,100 km, slightly beyond Adrastea's orbit, is clearly delimited. The satellite's orbit is marked with a gap in the ring so there is a thin ringlet just outside the ring. orbit. There is another ringlet just inside the orbit of Adrastea followed by a gap of unknown origin located 128,500 km away. A third ringlet is located on the inner side of the gap produced by the orbit of the Metis satellite. The brightness of the ring drops sharply just outside it, thus delimiting the gap. Inside the orbit of said satellite the brightness of the ring increases much less than in frontal illumination.

Thus when backlit the main ring appears to consist of two different parts, a narrow outer part that extends from 128,000 to 129,000 km and includes three small rings separated by gaps, and a weaker inner part that extends from 122,500 to 128,000 km and lacks visible, front-lit structures. The Metis Gap serves as their respective boundaries. The main ring structure was discovered by the Galileo orbiter and is clearly visible in back-illuminated images obtained by the New Horizons probe in February-March 2007. However, observations made by the Hubble Space Telescope, the Keck Telescope, and the Cassini probe did not detect it, possibly due to a lack of spatial resolution.

Observed in back illumination the main ring appears to be very thin, extending in a vertical direction no more than 30 km. With lateral illumination the thickness of the ring is between 80 and 160 km, increasing somewhat in the direction of Jupiter. The The ring appears to be much thicker in front illumination, around 300 km. One of the Galileo orbiter's discoveries was a faint and relatively thick cloud of material in the main ring (around 600 km).), which surrounds its interior part. The cloud grows in thickness towards the inner edge of the main ring at the place of transition to the halo ring.

A detailed analysis of the Galileo images revealed longitudinal variations in the brightness of the main ring not connected to the observed structure. The images from said probe also showed clusters of material in the 500 to 1000 km scale rings.

In February and March 2007, the New Horizons probe carried out an exhaustive search for new satellites within the main ring. Although no satellites larger than 0.5 km were discovered, the probe's cameras detected seven small particle masses. They orbit just inside Adrastea's orbit within a dense, small ring. The conclusion is that they are clumps and not small satellites based on their azimuthally extended appearance. They extend between 0.1º and 0.3º along the ring, which corresponds to between 1000 and 3000 km. The accumulations are divided into two groups of five and two members respectively. Their nature is unclear, but their orbits are close to an orbital resonance of 115:116 and 114:115 with the Metis satellite, so they may be structures caused by this interaction.

Particle size distribution and spectra

The spectra of the main ring obtained by the Hubble space telescope, the Keck telescope and by the Galileo and Cassini probes have shown that the particles that form it They form are red, with a higher albedo at longer wavelengths. The existing spectra cover the range from 0.5 to 2.5 μm. No spectral characteristics have been found that have allowed the identification of specific chemical compounds, although the Cassini observations showed evidence in the absorption band near 0.8 μm and 2.2 μm. The spectra of the main ring are very similar to those of the satellites Adrastea and Amalthea.

The properties of the main ring can be explained by the hypothesis that they contain significant amounts of dust sized from 0.1 to 10 μm. This would explain the greater brightness of the front-illuminated images than those illuminated from the back. In any case, it is necessary that larger bodies exist to explain the brightness obtained in the backlit images and the complex structure in the bright outer part of the ring.

The analysis of the available spectral and phase data leads to the conclusion that the particle size distribution of the main ring responds to the power law:

${displaystyle n(r)=Atimes r^{-q},}$

where n(r) dr is the number of particles with radio between r and r + dr and ${displaystyle A}$ is a normalizing parameter chosen to match the total flow of light from the ring. The parameter q is 2.0 ± 0.2 for particles with r less than 15 ± 0.3 μm, and 5.0 ± 1.0 particles with r greater than 15 ± 0.3 μm.

The distribution of large bodies in the range from meters to kilometers is currently not determined. Illumination in this model is determined by particles with r around 15 μm.

The above-mentioned law allows the estimation of optical depth, ${displaystyle scriptstyle tau }$the main ring: ${displaystyle scriptstyle tau }$_l = 4.7 × 10^-6 for large bodies and ${displaystyle scriptstyle tau }$_s = 1.3 × 10^-6 for dust. This optical depth means that the total section of all particles in a ring section is 5000 km2. The main ring particles are supposed to have a spherical shape. Total powder mass is estimated at 10⁷ and 10⁹kg. The mass of large bodies, excluding Metis and Adrastea satellites, between 10¹¹ and 10¹⁶kg, depending on its maximum size. The upper value corresponds to a diameter of approximately 1 km. These can be compared to those of Adrastea, which is 2 × 10¹⁵Amaltea, 2 × 10¹⁸kg and Moon, 7,4 × 10²²kg.

The presence of two types of particles in the main ring would explain why its appearance depends on the direction of illumination. The dust diffuses light preferably in a frontal direction and forms a relatively thick and homogeneous ring surrounded by the orbit of Adrastea. In contrast, larger bodies, which spread more light rearward, are confined within the region between the orbits of Metis and Adrastea in various small rings.

Origin and age

Dust is constantly removed from the main ring by a combination of the Poynting-Robertson drag effect and the electromagnetic forces of the Jovian magnetosphere. Volatile materials, such as ice, evaporate quickly. The half-life of the dust particles in the ring varies from 100 to 1000 years, so the dust must be continually renewed through collisions between larger bodies with sizes from 1 cm to 0.5 km and by the same high-speed bodies and particles from outside the Jovian system. These larger bodies are confined to the narrow (approximately 1000 km) bright outer part of the main ring, which also includes Metis and Adrastea. The size The maximum of these bodies must be less than 0.5 km in radius. This upper limit was obtained by the New Horizons probe. The previous upper limit, obtained by the Hubble telescope and the Cassini probe, was about 4 km.. The dust produced by the collisions retains approximately the same orbital elements of the larger bodies and slowly falls in a spiral in the direction of Jupiter forming the faint, backlit, innermost part of the main ring and the halo ring. Age of the main ring is currently unknown, but may be the last remnant of a past population of small satellites close to Jupiter.

Halo ring

Appearance and structure

The halo ring is the innermost and thickest of Jupiter's rings. Its outer edge coincides with the interior of the main ring at approximately a radius of 122,500 km from the center of the planet, 1.72 R_J. From this radius the ring rapidly becomes thicker and thicker in the direction of Jupiter. The actual extent in the vertical direction of the halo is unknown, but the presence of its material was detected as high as 10,000 km above the ring plane. The inner edge of the halo is relatively sharp and is located at a radius of 100,000 km. km, 1.4 R R J, but some material has been located even further inland, at approximately 92,000 km. Thus, the width of the ring halo is around 30,000 km. Its shape resembles a wide torus with no defined internal structure. Unlike the main ring, the appearance of the halo depends very little on the illumination geometry.

The halo is bright in frontal illumination, where it was extensively photographed by the Galileo probe. While its surface brightness is much lower than that of the main ring, in the vertical direction its photon flux is comparable due to its greater width. Although it extends in a vertical direction for more than 20,000 km, the brightness of the halo is concentrated towards the plane of the ring and follows a power law of the form: z^{-0, 6} to z^-1.5, where z is the altitude with respect to the ring plane. The appearance of the back-illuminated halo ring, as observed by the Keck Telescope and the Hubble Space Telescope, is basically the same. In any case the total photon flux is several times smaller than that of the main ring and is much more concentrated in the plane of the ring than in the front-illuminated images.

The spectral properties of the halo are different from those of the main ring. The flux distribution in the range 0.5 to 2.5 μm is flatter in the main ring. The halo is not red and may even be blue in color.

Origin of the halo ring

The optical properties of the halo ring can be explained by the hypothesis that it is composed only of dust with particle sizes less than 15 μm. The halo areas away from the ring plane may consist of submicrometric powder. This composition explains the highest brightness in front lighting, the most blue color and the absence of visible structure in the halo. Powder possibly originates in the main ring, a theory that rests on the fact that the optical depth ${displaystyle scriptstyle tau _{s},}$ ~10^-6 is comparable to the dust of the main ring. The large thickness of the ring can be attributed to the excitation of the orbital inclination and eccentricity of dust particles by the electromagnetic forces of the Jupiter magnetosphere. The outer edge of the halo coincides with the situation of a strong resonance of Lorentz 3:2.

Like the Poynting-Robertson drag It causes the particles to tend to fall in the direction of Jupiter, their orbital inclinations are excited as they pass through it. The thickening of the main ring may be the beginning of the halo ring. The inner edge of the ring is not far from the strong 2:1 Lorentz resonance. At this resonance the excitation is probably significant, forcing the particles to precipitate to the Jovian atmosphere and thus forming a very defined inner edge. Being originated by material from the main ring, the age of the halo ring is the same as that of the main ring.

Fuzzy rings

Amalthea's fuzzy ring

The diffuse ring of Amalthea is a very faint structure of rectangular section that extends from the orbit of Amalthea to 182 000 km from the center of Jupiter, 2.54 R_J to approximately 129,000 km, 1.80 R_J. Its inner edge does not It is clearly defined due to the presence of the relatively much brighter main ring and halo ring. The thickness of the ring is approximately 2300 km near the orbit of Amalthea and reduces slightly towards Jupiter. The diffuse ring of Amalthea is brightest near its upper and lower edges and gradually brighter toward Jupiter, with the upper edge being brighter than the lower edge. The outer edge of the ring is relatively well defined and there is a sharp drop in brightness just at the interior of Amalthea's orbit. In front-illuminated images the ring appears to be thirty times fainter than the main ring. In back-illuminated images it has only been detected by the Keck Telescope and the Hubble Space Telescope. These images show additional structure in the ring, a brightness peak just inside the orbit of Amalthea. In 2002 and 2003 the Galileo probe made two passes through the diffuse rings. The dust counter detected particles ranging in size from 0.2 to 5 μm and confirmed the results obtained by image analysis. Observations of the Amalthea diffuse ring from the Earth's surface and images from the probe Galileo's direct measurements of the dust have allowed us to determine the particle size distribution, which appears to follow the same power law as the main ring dust with q=2 ±0.5. The optical depth of the ring is approximately 10⁻⁷, which is an order of magnitude smaller than that of the main ring, but the total mass of the dust, between 10⁷ sup> and 10⁹ kg, is comparable.

Thebe's Fuzzy Ring

The Thebe diffuse ring is the weakest of the Jovian rings. It appears to be a rectangular-section structure extending from Thebe's orbit at 226,000 km from the center of Jupiter, 3.11 R_J to approximately 129,000 km, 1.80 R R J It is approximately 8400 km near Thebe's orbit and decreases slightly in the direction of the planet. The Thebe ring is, like Amalthea's, brighter at the upper and lower edges and its brightness increases in the direction of Jupiter. The outer edge of the ring is not well defined, extending for 15,000 km. There is a hardly observable continuation that extends to 280,000 km, 3.75 R J called Thebe Extension. In front-illuminated images the ring is three times fainter than the diffuse ring of Amalthea. In back-illuminated images obtained by the Keck telescope, the ring shows a peak of brightness just inside Thebe's orbit. In 2002 and 2003 the particle counter of the Galileo probe detected particles with a size between 0.2 and 5 μm (similar results to those of the of the Amalthea ring), confirming the results of the image analysis.

The optical depth of the Thebe diffuse ring is about 3 × 10^-8, which is three times smaller than that of the Amalthea diffuse ring, but the total mass of the dust is the same, approximately between 10⁷ and 10⁹ kg. The size distribution of dust particles is more dispersed than in the Amalthea ring, following a power law with q < 2. In the Tebe extension, this parameter can be even smaller.

Origin of diffuse rings

Dust in the diffuse rings originates in essentially the same way as that in the main rings and halo. Its source is the internal satellites Amalthea and Thebe respectively. The high speed of impact of objects from outside the Jovian system expels dust particles from their surfaces. These particles initially retain the same orbits as the satellites from which they come, but little by little those orbits decay, spiraling towards the planet due to the Poynting-Robertson drag effect. The thickness of the diffuse rings is determined by the orbital inclination of satellites. This would explain almost all the observable properties of the rings: rectangular section, drop in thickness in the direction of Jupiter and the greater brightness of the upper and lower edges of the rings. However, there are some properties that remain unexplained, such as the Thebe Extension, which may be due to unobserved bodies outside of Thebe's orbit, and structures observed in back-illuminated images.

A possible explanation for the Thebe Extension is the influence of the electromagnetic forces of Jupiter's magnetosphere. When dust enters the shadow behind the planet, it loses its electrical charge fairly quickly. As the small dust particles partially rotate with the planet, they will move outward during the passage through the shadow, creating an outer extension to the Thebe ring. The same forces can explain the transition in particle distribution and brightness that occurs between the orbits of Amalthea and Thebe.

Analysis of the diffuse ring images revealed a brightness peak just inside Amalthea's orbit due to dust particles trapped in the Lagrange points L₄ and L₅. The greater brightness observed at the upper edge of Amalthea's ring may also be caused by this same dust. There must also be dust particles trapped in the Lagrange points of Thebe's orbit. Its discovery would imply that there are two types of particle populations in the diffuse rings, one with orbits that slowly decay towards Jupiter while others remain trapped in 1:1 resonance with the satellite that produced them.