<P> The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System . It is mostly made of molecular hydrogen and helium in roughly solar proportions; other chemical compounds are present only in small amounts and include methane, ammonia, hydrogen sulfide and water . Although water is thought to reside deep in the atmosphere, its directly measured concentration is very low . The nitrogen, sulfur, and noble gas abundances in Jupiter's atmosphere exceed solar values by a factor of about three . </P> <P> The atmosphere of Jupiter lacks a clear lower boundary and gradually transitions into the liquid interior of the planet . From lowest to highest, the atmospheric layers are the troposphere, stratosphere, thermosphere and exosphere . Each layer has characteristic temperature gradients . The lowest layer, the troposphere, has a complicated system of clouds and hazes, comprising layers of ammonia, ammonium hydrosulfide and water . The upper ammonia clouds visible at Jupiter's surface are organized in a dozen zonal bands parallel to the equator and are bounded by powerful zonal atmospheric flows (winds) known as jets . The bands alternate in color: the dark bands are called belts, while light ones are called zones . Zones, which are colder than belts, correspond to upwellings, while belts mark descending gas . The zones' lighter color is believed to result from ammonia ice; what gives the belts their darker colors is uncertain . The origins of the banded structure and jets are not well understood, though a "shallow model" and a "deep model" exist . </P> <P> The Jovian atmosphere shows a wide range of active phenomena, including band instabilities, vortices (cyclones and anticyclones), storms and lightning . The vortices reveal themselves as large red, white or brown spots (ovals). The largest two spots are the Great Red Spot (GRS) and Oval BA, which is also red . These two and most of the other large spots are anticyclonic . Smaller anticyclones tend to be white . Vortices are thought to be relatively shallow structures with depths not exceeding several hundred kilometers . Located in the southern hemisphere, the GRS is the largest known vortex in the Solar System . It could engulf two or three Earths and has existed for at least three hundred years . Oval BA, south of GRS, is a red spot a third the size of GRS that formed in 2000 from the merging of three white ovals . </P> <P> Jupiter has powerful storms, often accompanied by lightning strikes . The storms are a result of moist convection in the atmosphere connected to the evaporation and condensation of water . They are sites of strong upward motion of the air, which leads to the formation of bright and dense clouds . The storms form mainly in belt regions . The lightning strikes on Jupiter are hundreds of times more powerful than those seen on Earth, and are assumed to be associated with the water clouds . This storm near the red spot is called Red Spot Junior . </P>

What is the most prominent feature in jupiter's atmosphere called