Jupiter is perpetually covered with clouds composed of ammonia crystals and possibly ammonium hydrosulfide. The clouds are located in the tropopause and are arranged into bands of different latitudes, known as tropical regions. These are sub-divided into lighter-hued zones and darker belts. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 100 m/s (360 km/h) are common in zonal jets.[43] The zones have been observed to vary in width, color and intensity from year to year, but they have remained sufficiently stable for astronomers to give them identifying designations.[23]
The cloud layer is only about 50 km deep, and consists of at least two decks of clouds: a thick lower deck and a thin clearer region. There may also be a thin layer of water clouds underlying the ammonia layer, as evidenced by flashes of lightning detected in the atmosphere of Jupiter. This is caused by water’s polarity, which makes it capable of creating the charge separation needed to produce lightning.[31] These electrical discharges can be up to a thousand times as powerful as lightning on the Earth.[44] The water clouds can form thunderstorms driven by the heat rising from the interior.[45]
The orange and brown coloration in the clouds of Jupiter are caused by upwelling compounds that change color when they are exposed to ultraviolet light from the Sun. The exact makeup remains uncertain, but the substances are believed to be phosphorus, sulfur or possibly hydrocarbons.[31][46] These colorful compounds, known as chromophores, mix with the warmer, lower deck of clouds. The zones are formed when rising convection cells form crystallizing ammonia that masks out these lower clouds from view.[47]
Jupiter’s low axial tilt means that the poles constantly receive less solar radiation than at the planet’s equatorial region. Convection within the interior of the planet transports more energy to the poles, balancing out the temperatures at the cloud layer.[23]
When the North American Plate on its slow journey westwards encountered the Pacific Plate approximately 250 million years ago during the Paleozoic, the latter began to subduct under the North American continent. Intense pressure underground caused some of the Pacific Plate to melt, and the resulting upwelling magma pushed up and hardened into the granite batholith that makes up much of the Sierra Nevada.[19] Extensive layers of marine sedimentary rock that originally made up the ancient Pacific seabed were also pushed up by the rising granite, and the ancestral Merced River formed on this layer of rock. Over millions of years, the Merced cut a deep canyon through the softer sedimentary rock, eventually hitting the hard granite beneath. The encounter with this resilient rock layer caused the Merced River to mostly stop its downcutting, although tributary streams continued to widen the ancient canyon.[20]
Over about 80 million years, erosion caused the transportation of massive amounts of alluvial sediment to the floor of the Central Valley, where it was trapped between the California Coast Range on the west and the Sierra Nevada on the east, forming an incredibly flat and fertile land surface. The present-day form of the upper Merced River watershed, however, was formed by glaciers, and the lower watershed was indirectly but significantly affected.[21][22]
When the last glacial period or Ice Age arrived, a series of four tremendous valley glaciers filled the upper basin of the Merced River. These glaciers rose in branches upstream of Yosemite Valley, descending from the Merced River headwaters, Tenaya Canyon and Illilouette Creek. Tenaya Canyon was actually eroded even deeper by an arm of the Tuolumne Glacier, which formed the Grand Canyon of the Tuolumne and Hetch Hetchy Valley on the Tuolumne River in the north. Little Yosemite Valley formed as a result of the underlying rock being harder than that below the Giant Staircase, the cliff wall containing Vernal Fall and Nevada Fall. These three branches of each glacier combined to form one large glacier about 7,000 feet (2,100 m) thick at maximum, stretching 25 miles (40 km) downstream past the mouth of Yosemite Valley, well into Merced Canyon. These glaciers formed the granite cliffs that now constitute landmarks such as Half Dome, El Capitán, and Cloud’s Rest.[21][22][23]
The first and largest glacier was the Sherwin or Pre-Tahoe glacier, which eroded the upper Merced watershed to an extent close to its present form. Three stages followed during the Wisconsinian glaciation; these were the Tahoe, Tenaya and Tioga stages, of which the Tioga was the smallest. The Tioga glacier left at the mouth of Yosemite Valley a rocky moraine. This moraine was actually one of several moraines deposited by the four glaciations, which include Medial Moraine and Bridalveil Moraine.[24] After the Tioga Glacier retreated this moraine formed a lake that flooded nearly the entire valley. Gradual sedimentation filled Lake Yosemite, creating a broad and flat valley floor. Sediments of glacial origin continued to travel down the Merced River following then, helping to form the flat floor of the Central Valley.[21][22][23]
This is a view of the clouds over the Merced River Valley, Yosemite, just as the Sun decided to set. We were exhausted from two days of hikes and a day on the slopes. The kids were in the truck, stripping off their clothes, only to curl up and sleep during the ride home. More images pending time in the darkroom.
This image is dedicated to Michelle Budziak who helped me debug my enlarger and get rid of that dreaded halo effect. Thank you, Michelle.
