The Earth Story

Graded with Filmconvert Nitrate Music: Jūra by Ardie Son / Tokyo Street by Airtone

It's not just auroras that fill our sky with eerie light. The weaker phenomenon in the photo is called airglow (aka nightglow), and unlike the aurora, which is focussed towards the poles by Earth's magnetic field, it can be seen from anywhere on Earth with luck and a dark sky or long exposure photo. Both phenomena arise from excited atmospheric atoms, but with different solar sources for the excitation energy. Like aurorae, airglow can be patchy and shift on a scale of minutes across the night sky. It is also present during the day but hidden in the glare. Aurorae are due to high speed collisions with the high energy particles in the solar wind, usually during a coronal mass ejection that is pointed towards our planet and funnelled down our world's field lines. Airglow arises from high energy components of ordinary sunlight, in this case short wave ultraviolet and X-Rays. Several mechanisms combine in the lower reaches of space to produce this glow. Between 80 and 100 Km up from the surface, oxygen atoms get chemically excited and ionised (the electrons are stripped off from the nuclei by the energy). They then react with hydroxyl molecules (OH) to form water, or recombine into O2 and start to glow green from both chemically stimulated energy and the decay of those atoms excited by cosmic rays (just once in my life I think I've had the blue flash of Cherenkov radiation in one eye reported by astronauts). They only occur at high altitude because lower down the nitrogen in the atmosphere quenches the reaction. Other types of atom also recombine to contribute to the effect, such as nitrogen and oxygen forming nitric oxide (NO), emitting a photon of light as they do so. It was first identified by the Swede Angstrom in 1868, and subsequent laboratory studies have shown the chemical pathways that create the light as an energetic by product of the photochemical reactions. It limits the sensitivity of ground based telescopes at visible wavelengths, and is one of the reasons space telescopes are so useful to astronomers, as they can see faint objects normally masked by airglow. It usually appears bluish green, and seems brightest about ten degrees above the horizon. Only part of the layer of air that forms our bubble glows, too high up and the atoms are too tenuous to combine, too low and their density means that the energy is dissipated by collisions rather than photochemical reactions. While it is normally quite weak, and the photographer tells me that this picture has an unusual intensity for the phenomenon, he states that a friend obtained similar results when shooting from the same place in the same direction. Sometime in the next couple of days, we'll put up a photo of airglow from space, so you can see the shimmering band that encircles our wonderful Blue Marble. Loz Image credit: Yuri Beletsky (Carnegie Las Campanas Observatory) via APOD. http://bit.ly/1JGwDta http://bit.ly/1VByYuy http://bit.ly/1JSJrcK

The specimen comes from the Mogok stone tract of Burma, source of the traditional pigeon's blood ruby. The original limestone was transformed from sea bottom to land and limestone to marble by the collision of India with Eurasia that eliminated the eastern half of the near gone palaeoocean Tethys (the other half is but a sad remnant called the Mediterranean as Africa removes its last vesiges). The mineral is named after its type location from which it was first scientifically described. Unlike the 4.9 x 2.9 x 1.7 cm Burmese example in the photo which was born in what is known as a regional metamorphic event (when a large area of land is transformed in belts that reflect variations in temperature and pressure conditions during a mountain building event), the original material from Norberg in Sweden came from a different kind of metamorphism called contact metamorphism, which occurs when existing rocks surrounding an intruding mass of molten rock rising from the bowels of the Earth are baked and stewed in the igneous juices. The granites (in this case) pushed into carbonate rocks and the last refined water rich remnants of the magma gave up its fluorine as a pegmatite. The mineral fluoresces in UV light, as electrons are excited by the higher energy and wavelength rays and give up that energy after some time, emitting lower energy visible light wavelengths as it does so. Other sources include the Franklin mines in New Jersey, Canada, Finland and Italy. As well as the colour of the specimen, crystals can also be white, orange or red. The Mohs hardness weighs in at 6.5, between feldspar and quartz. Loz Image credit: Rob Lavinsky/iRocks.com http://www.mindat.org/min-2925.html http://bit.ly/1NItqgt http://bit.ly/1KWJncT http://www.galleries.com/Norbergite