is comparable. Thebe gossamer ring The Thebe gossamer ring is the faintest Jovian ring. It appears as a very faint structure with a rectangular cross section, stretching from the Thebean orbit at () to about (;). Its inner boundary is not clearly defined because of the presence of the much brighter main ring and halo. The thickness of the ring is approximately 8400 km near the orbit of Thebe and slightly decreases in the direction of the planet. The Thebe gossamer ring is brightest near its top and bottom edges and gradually becomes brighter towards Jupiter—much like the Amalthea ring. The outer boundary of the ring is not especially steep, stretching over . There is a barely visible continuation of the ring beyond the orbit of Thebe, extending up to () and called the Thebe Extension. In forward-scattered light the ring appears to be about 3 times fainter than the Amalthea gossamer ring. In back-scattered light it has been detected only by the Keck telescope. Back-scattering images show a peak of brightness just inside the orbit of Thebe. In 2002–2003 the dust counter of the Galileo spacecraft detected dust particles in the size range 0.2–5 μm—similar to those in the Amalthea ring—and confirmed the results obtained from imaging. The optical depth of the Thebe gossamer ring is about 3, which is three times lower than the Amalthea gossamer ring, but the total mass of the dust is the same—about 107–109 kg. However the particle size distribution of the dust is somewhat shallower than in the Amalthea ring. It follows a power law with q < 2. In the Thebe extension the parameter q may be even smaller. Origin of the gossamer rings The dust in the gossamer rings originates in essentially the same way as that in the main ring and halo. Its sources are the inner Jovian moons Amalthea and Thebe respectively. High velocity impacts by projectiles coming from outside the Jovian system eject dust particles from their surfaces. These particles initially retain the same orbits as their moons but then gradually spiral inward by Poynting–Robertson drag. The thickness of the gossamer rings is determined by vertical excursions of the moons due to their nonzero orbital inclinations. This hypothesis naturally explains almost all observable properties of the rings: rectangular cross-section, decrease of thickness in the direction of Jupiter and brightening of the top and bottom edges of the rings.