"Is frozen carbon dioxide a key to features in some Martian gullies? To find out, scientists grabbed a bag of dry ice and took a road trip." Martian Gullies are gullies found on the planet of Mars. First discovered on images from Mars Global Surveyor... Gullies occur on steep slopes, especially on the walls of craters. These are believed to be relatively young because they have few, if any craters. Moreover, they lie on top of sand dunes which themselves are considered to be quite young... Most occur 30 degrees poleward in each hemisphere with greater numbers in the southern hemisphere. In the northern hemisphere, they have been found in Arcadia Planitia, Tempe Terra, Acidalia Planitia, and Utopia Planitia. In the south, high concentrations are found on the northern edge of Argyre basin, in northern Noachis Terra, and along the walls of the Hellas outflow channels... Dry ice, sometimes referred to as "Cardice" or as "card ice" (chiefly British English), is the solid form of carbon dioxide. It is used primarily as a cooling agent. Its advantages include lower temperature than that of water ice and not leaving any residue (other than incidental frost from moisture in the atmosphere). It is useful for preserving frozen foods, ice cream, etc., where mechanical cooling is unavailable. Dry ice sublimates at −78.5 °C (−109.3 °F) at Earth atmospheric pressures. This extreme cold makes the solid dangerous to handle without protection due to burns caused by freezing (frostbite). While generally not very toxic, the outgassing from it can cause hypercapnia due to buildup in confined locations... Properties Dry ice is the solid form of carbon dioxide (chemical formula CO2), comprising two oxygen atoms bonded to a single carbon atom. It is colorless, with a sour zesty odor, non-flammable, and slightly acidic. At temperatures below −56.4 °C (−69.5 °F) and pressures below 5.13 atm (the triple point), CO2 changes from a solid to a gas with no intervening liquid form, through a process called sublimation. The opposite process is called deposition, where CO2 changes from the gas to solid phase (dry ice). At atmospheric pressure, sublimation/deposition occurs at −78.5 °C (−109.3 °F). The density of dry ice varies, but usually ranges between about 1.4 and 1.6 g/cm3 (87--100 lb/ft3). The low temperature and direct sublimation to a gas makes dry ice an effective coolant, since it is colder than water ice and leaves no residue as it changes state. Its enthalpy of sublimation is 571 kJ/kg (25.2 kJ/mol). Dry ice is non-polar, with a dipole moment of zero, so attractive intermolecular van der Waals forces operate. The composition results in low thermal and electrical conductivity. History It is generally accepted that dry ice was first observed in 1834 by French chemist Charles Thilorier, who published the first account of the substance. In his experiments, he noted that when opening the lid of a large cylinder containing liquid carbon dioxide, most of the liquid CO2 quickly evaporated. This left only solid dry ice in the container. In 1924, Thomas B. Slate applied for a US patent to sell dry ice commercially. Subsequently, he became the first to make dry ice successful as an industry. In 1925, this solid form of CO2 was trademarked by the DryIce Corporation of America as "Dry ice", thus leading to its common name.[9] That same year the DryIce Co. sold the substance commercially for the first time; marketing it for refrigerating purposes. The alternative name "Cardice" is a registered trademark of Air Liquide UK Ltd. It is sometimes written as "card ice". Manufacture Dry ice is easily manufactured. First, gases with a high concentration of carbon dioxide are produced. Such gases can be a byproduct of another process, such as producing ammonia from nitrogen and natural gas, or large-scale fermentation. Second, the carbon dioxide-rich gas is pressurized and refrigerated until it liquifies. Next, the pressure is reduced. When this occurs some liquid carbon dioxide vaporizes, causing a rapid lowering of temperature of the remaining liquid. As a result, the extreme cold causes the liquid to solidify into a snow-like consistency. Finally, the snow-like solid carbon dioxide is compressed into either small pellets or larger blocks of dry ice. Dry ice is typically produced in two standard forms: blocks and cylindrical pellets. A standard block weighing approximately 30 kg[citation needed] is most common. These are commonly used in shipping, because they sublimate relatively slowly due to a low ratio of surface area to volume. Pellets are around 1 cm (0.4 in) in diameter and can be bagged easily. This form is suited to small scale use, for example at grocery stores and laboratories...
Posted on: Fri, 14 Jun 2013 01:11:15 +0000