by Murray Anderson 
Gosses Bluff, Northern Territory, Australia
23°50'S, 132°19'E; rim diameter: 24 kilometers; age: 142.5 +- 0.5 million years 142 million years ago, an asteroid or comet slammed into what is now the Missionary Plains in Australia's Northern Territory, forming a crater 24 kilometers in diameter and 5 kilometers deep. Today, like a bull's eye, the circular ring of hills that defines Gosses Bluff stands as a stark reminder of the event. The crater is located just south of MacDonnel Ranges (top of the picture). It is highly eroded. The circular ring of hills (5 kilometers or 3 miles diameter) is actually the results from differential erosion of the central uplift within this large complex crater. The crater rim is eroded to the point that it is no longer visible though it is probably located along the grayish colored drainage system outside the inner ring. (Courtesy USGS)
Bosumtwi, Ghana 06°32'N, 01°25'W; rim diameter: 10.5 kilometers (6.5 miles); age: 1.3 +- 0.2 million years This crater is situated in crystalline bedrocks of the West African Shield and is filled almost entirely by Lake Bosumtwi. Chemical, isotopic, and age studies demonstrate that the crater is the most probable source for the Ivory Coast tektites, which are found on land in the Ivory Coast region of central Africa and as microtektites in nearby ocean sediments. In this photo the crater lake is partly obscured by clouds. (Courtesy NASA/LPI)
Deep Bay, Saskatchewan, Canada 56°24'N, 102°59'W; rim diameter: 13 kilometers (8 miles); age: 100 +- 50 million years This crater consists of a near-circular bay, about 5 kilometers (3 miles) wide and 220 meters (720 feet) deep, in the otherwise shallow Reindeer Lake. Such deep circular lakes are unusual in this region, which is dominated by the shallow gouging of glacial erosion. The circular shoreline, at a diameter of 11 kilometers (6.8 miles), is partially surrounded by a ridge with heights to 100 meters (328 feet) above the lake surface. The diameter of this ridge, ~13 kilometers (8 miles), is likely the outer rim of the impact structure. The structure was formed in Precambrian metamorphic crystalline rocks with a conspicuous northwest trending fabric. Although not obvious from the surface, Deep Bay is a complex impact structure with a low, totally submerged central uplift. Samples obtained in the 1960's from drilling into the central structure revealed shocked and fractured metamorphic rocks flanked by deposits of allocthonous, mixed breccias. (Courtesy NASA/LPI)
Clearwater Lakes, Quebec, Canada Clearwater Lake West: 56°13'N, 74°30'W; rim diameter: 32 kilometers (20 miles) Clearwater Lake East: 56°05'N, 74°07'W; rim diameter: 22 kilometers (13.7 miles) age: 290 +- 20 million years These twin circular lakes (large dark features) were formed simultaneously by the impact of an asteroidal pair which slammed into the planet approximately 290 million years ago. The lakes are located near the eastern shore of Hudson Bay within the Canadian Shield in a region of generally low relief in northern Quebec province. Notice that the larger western structure contains a ring of islands with a diameter of about 10 kilometers that surrounds the center of the impact zone. They constitute a central uplifted area and are covered with impact melts. The central peak of the smaller Clearwater Lake East is submerged. The lakes are named after their exceedingly clear water. Also notice that the surrounding terrain shows widespread scarring from glaciation. The multitude of linear and irregular shaped lakes (dark features) are the result of gouging or scouring action caused by the continental ice sheets that once moved across this area. (Courtesy NASA)
Manicouagan, Quebec, Canada 51°23'N, 68°42'W; rim diameter: ~100 kilometers (62 miles); age: 212 +- 1 million years The Manicouagan impact structure is one of the largest impact craters still preserved on the surface of the Earth. This shuttle view shows the prominent 70 kilometers (43 miles) diameter, ice-covered annular lake that fills a ring where impact-brecciated rock has been eroded by glaciation. The lake surrounds the more erosion-resistant melt sheet created by impact into metamorphic and igneous rock types. Shock metamorphic effects are abundant in the target rocks of the crater floor. Although the original rim has been removed, the distribution of shock metamorphic effects and morphological comparisons with other impact structures indicates an original rim diameter of approximately 100 kilometers (62 miles). (Courtesy NASA/LPI) 
Mistastin Lake, Newfoundland and Labrador, Canada 55°53'N, 63°18'W; rim diameter: 28 kilometers (17.4 miles); age: 38 +- 4 million years This shuttle image shows a winter view of the Mistastin Crater, a heavily eroded complex structure. Eastward moving glaciers have drastically reduced the surface expression of this structure, removing most of the impact melt sheet and breccias and exposing the crater floor. Glacial erosion has also imparted an eastward elongation to the crater that is particularly evident in the shape of the lake that occupies the central 10 kilometers (6 miles) of the structure. Horseshoe Island, in the center of the lake, is part of the central uplift and contains shocked Precambrian crystalline target rocks. Just beyond the margins of the lake are vestiges of the impact melt sheet that contains evidence of meteoritic features in quartz, feldspar and diaplectic glasses. (Courtesy NASA/LPI)
Roter Kamm, South West Africa/Namibia 27°46'S, 16°18'E; rim diameter: 2.5 kilometers (1.55 miles); age: 5 +- 0.3 million years Located in the Namibia Desert, the raised crater rim is clearly visible against darker background vegetation. Target rocks include primarily Precambrian crystalline rocks and modest amounts of younger sedimentary rocks. Outcrops of impact melt breccias are found exclusively on the crater rim. The crater floor is covered by broad, shifting sand dunes. This image shows an oblique view of the crater, from about 150 meters (492 feet) above ground looking southeast. (Courtesy of W. U. Reimold and LPI)
Roter Kamm, SAR-C/X-SAR Image
Wolfe Creek, Australia 19°10' S, 127° 48' E; rim diameter: 0.875 kilometers (.544 miles); age: 300,000 years Wolfe Creek is a relatively well-preserved crater that is partly buried under wind blown sand. The crater is situated in the flat desert plains of north-central Australia. Its crater rim rises ~25 meters (82 feet) above the surrounding plains and the crater floor is ~50 meters (164 feet) below the rim. Oxidized remnants of iron meteoritic material as well as some impact glass have been found a Wolf Creek. This photograph is a south-looking, oblique aerial view of the crater. (Courtesy of V. L. Sharpton, LPI)
Chicxulub, Yucatan Peninsula, Mexico 21°20'N, 89°30'W; diameter: 170 km; age: 64.98 million years This three-dimensional map of local gravity and magnetic field variations shows a multiringed structure called Chicxulub named after a village located near its center. The impact basin is buried by several hundred meters of sediment, hiding it from view. This image shows the basin viewed obliquely from approximately 60° above the surface looking north, with artificial lighting from the south. The image covers 88 to 90.5° west longitude and 19.5 to 22.5° north latitude. NASA scientists believe that an asteroid 10 to 20 kilometers (6 to 12 miles) in diameter produced this impact basin. The asteroid hit a geologically unique, sulfur-rich region of the Yucatan Peninsula and kicked up billions of tons of sulfur and other materials into the atmosphere. Darkness prevailed for about half a year after the collision. This caused global temperatures to plunge near freezing. Half of the species on Earth became extinct including the dinosaurs. (Image courtesy of V. L. Sharpton, LPI) 
The Tyndall effect is responsible for some other blue coloration's in nature: such as blue eyes, the opalescence of some gem stones, and the colour in the blue jay's wing. The colours can vary according to the size of the scattering particles. When a fluid is near its critical temperature and pressure, tiny density fluctuations are responsible for a blue coloration known as critical opalescence. People have also copied these natural effects by making ornamental glasses impregnated with particles, to give the glass a blue sheen. But not all blue colouring in nature is caused by scattering. Light under the sea is blue because water absorbs longer wavelength of light through distances over about 20 metres. When viewed from the beach, the sea is also blue because it reflects the sky, of course. Some birds and butterflies get their blue colorations by diffraction effects.
A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. When we look towards the sun at sunset, we see red and orange colours because the blue light has been scattered out and away from the line of sight.
The white light from the sun is a mixture of all colours of the rainbow. This was demonstrated by Isaac Newton, who used a prism to separate the different colours and so form a spectrum. The colours of light are distinguished by their different wavelengths. The visible part of the spectrum ranges from red light with a wavelength of about 720 nm, to violet with a wavelength of about 380 nm, with orange, yellow, green, blue and indigo between. The three different types of colour receptors in the retina of the human eye respond most strongly to red, green and blue wavelengths, giving us our colour vision.
The first steps towards correctly explaining the colour of the sky were taken by John Tyndall in 1859. He discovered that when light passes through a clear fluid holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than the red. This can be demonstrated by shining a beam of white light through a tank of water with a little milk or soap mixed in. From the side, the beam can be seen by the blue light it scatters; but the light seen directly from the end is reddened after it has passed through the tank. The scattered light can also be shown to be polarised using a filter of polarised light, just as the sky appears a deeper blue through polaroid sun glasses.
This is most correctly called the Tyndall effect, but it is more commonly known to physicists as Rayleigh scattering--after Lord Rayleigh, who studied it in more detail a few years later. He showed that the amount of light scattered is inversely proportional to the fourth power of wavelength for sufficiently small particles. It follows that blue light is scattered more than red light by a factor of (700/400)4 ~= 10.
Tyndall and Rayleigh thought that the blue colour of the sky must be due to small particles of dust and droplets of water vapour in the atmosphere. Even today, people sometimes incorrectly say that this is the case. Later scientists realised that if this were true, there would be more variation of sky colour with humidity or haze conditions than was actually observed, so they supposed correctly that the molecules of oxygen and nitrogen in the air are sufficient to account for the scattering. The case was finally settled by Einstein in 1911, who calculated the detailed formula for the scattering of light from molecules; and this was found to be in agreement with experiment. He was even able to use the calculation as a further verification of Avogadro's number when compared with observation. The molecules are able to scatter light because the electromagnetic field of the light waves induces electric dipole moments in the molecules.
