Science

Richat Structure, Mauritania, 2001

The 31-mile-wide bull's-eye in the western Sahara is a landmark for astronauts. The structure formed when a volcanic dome hardened and gradually eroded, exposing the onion-like layers of rock. Desert sands appear white and pale yellow at the corners; less sandy, rocky areas are green; and volcanic rocks are blue.

Kalahari Desert, Southern Africa, 2000

The large stretch of semiarid, sandy savanna covers part of Botswana, Namibia and South Africa. The desert has vast areas covered by red sand without any permanent surface water. The red dot near the Nossob River in the center of this image represents a farm made possible by a center-pivot irrigation system.

Bombetoka Bay, Madagascar, 2000

Islands and sandbars have formed where the Betsiboka River flows into the Mozambique Channel. The past few decades have seen a dramatic increase in the amount of sediment moved by the river and deposited in the estuary. Dense vegetation is deep green, and water is sapphire, tinged with pink where sediment is particularly thick.

Great Salt Desert, Iran, 2003

A mix of salt marshes, mud flats, wadis, steppes and desert plateaus color the landscape of Iran'’s Great Salt Desert, Dasht-e Kavir. The region covers an area of more than 29,000 square miles. Dramatic daily temperature swings and violent storms are the norm, and extreme heat leaves the marshes and mud grounds with crusts of salt.

Carnegie Lake, Australia, 1999

Carnegie Lake in Western Australia fills with water only during periods of significant rainfall. In dry years, it is reduced to a muddy marsh. Flooded areas appear dark blue or black, vegetation appears in shades of dark and light green, and sands, soils and minerals appear in a variety of colors.

Ice Waves, Greenland, 2001

The undulating swirls shown here along the eastern coast of Greenland are slurries of sea ice, newly calved icebergs, and older weathered bergs. During the summer melting season, the southward-flowing East Greenland Current twirls these mixtures into stunning shapes. The exposed rock of mountain peaks are tinted red.

Lena River Delta, Russia, 2000

The Delta extends 62 miles into the Laptev Sea and Arctic Ocean, and includes a protected wilderness area and wildlife refuge. The delta is frozen tundra for about seven months of the year, and spring transforms it into a lush wetland. Vegetation appears as shades of green, sandy areas as shades of red, and water as purples and blues.

Nazca Lines, Peru, 2000

The ancient geoglyphs, located in southern Peru, are estimated to be created by the Nazca culture between 400 and 650 A.D. The Nazca Lines were made by removing reddish iron-oxide pebbles that cover the surface of the desert. When the gravel is removed, the lines contrast with the light color underneath.

Meandering Mississippi, U.S., 2003

Graceful swirls and whorls of the Mississippi River encircle fields and pastures on the border between Arkansas and Mississippi. The Mississippi is the largest river system in North America and forms the second largest watershed in the world.

Garden City, Kan., U.S., 2000

Garden City, Kan., has a semi-arid steppe climate with hot, dry summers and cold, dry winters. Center-pivot irrigation systems created the circular patterns. The red circles indicate irrigated crops of healthy vegetation, and the light-colored circles denote harvested crops.

Mayn River, Russia, 2000

The Mayn River is a tributary of the larger Anadyr River, which flows through the far northeastern corner of Siberia. While these rivers are frozen for about eight to nine months in a year, they are home to chum and sockeye salmon during the summer months.

Von Kármán Vortices, Southern Pacific Ocean, 1999

Swirling clouds line up in a formation known as a von Kármán street. They appear when wind-driven clouds encounter an obstacle, in this instance Alexander Selkirk Island in the southern Pacific Ocean.

Terkezi Oasis, Chad, 2000

A series of rocky outcroppings emerge from the sand in the Sahara Desert near the Terkezi Oasis. Stretching across the immense desert are vast plains of sand and gravel; seas of sand dunes; and barren, rocky mountains. Only 10,000 years ago, grasses covered the region, and mammals such as lions and elephants roamed the land.

Himalayas, Central Asia, 2001

The soaring, snow-capped peaks and ridges of the eastern Himalaya Mountains create an irregular patchwork between major rivers in Tibet and southwestern China. Covered by snow and glaciers, the mountains here rise to altitudes of more than 16,000 feet. Vegetation at lower elevations is colored red.