Acetone

Acetone[1]
Names
Preferred IUPAC name
Propan-2-one
Other names
  • Acetone
  • Dimethyl ketone[2]
  • Dimethyl carbonyl
  • β-Ketopropane[2]
  • Propanone[3]
  • 2-Propanone[2]
  • Dimethyl formaldehyde[4]
  • Pyroacetic spirit (archaic)[5]
  • Ketone propane[6]
Identifiers
3D model (JSmol)
3DMet B00058
Abbreviations DMK
635680
ChEBI
ChemSpider
ECHA InfoCard 100.000.602
EC Number 200-662-2
1466
KEGG
MeSH Acetone
RTECS number AL3150000
UNII
UN number 1090
Properties
C3H6O
Molar mass 58.08 g·mol−1
Appearance colorless liquid
Odor pungent, irritating, floral
Density 0.7845 g cm−3 (25 °C)
Melting point −94.7 °C (−138.5 °F; 178.5 K)[7]
Boiling point 56.05 °C (132.89 °F; 329.20 K)[7]
miscible
Solubility miscible in benzene, diethyl ether, methanol, chloroform, ethanol[8]
log P −0.042
Vapor pressure 9.39 kPa (0 °C)
30.6 kPa (25 °C)
374 kPa (100 °C)
2.8 MPa (200 °C)[9]
Acidity (pKa) 19.2
Basicity (pKb) −5.2 (for conjugate base)
-33.78·10−6 cm3/mol
1.3588 (VD=54.46)
Viscosity 0.295 mPa·s (25 °C)[8]
Structure
Trigonal planar at C2
Dihedral at C2
2.91 D
Thermochemistry
125.45 J/mol·K
200.4 J/mol·K
−250.03-(−248.77) kJ/mol
−1.772 MJ/mol
Hazards
Safety data sheet See: data page
GHS pictograms
GHS signal word DANGER
H225, H319, H336
P210, P261, P305+351+338
NFPA 704
Flammability code 3: Liquids and solids that can be ignited under almost all ambient temperature conditions. Flash point between 23 and 38 °C (73 and 100 °F). E.g., gasoline) Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
3
1
0
Flash point −20 °C (−4 °F; 253 K)
465 °C (869 °F; 738 K)
Explosive limits 2.6–12.8%[10]
1185 mg/m3 (TWA), 2375 mg/m3 (STEL)
Lethal dose or concentration (LD, LC):
5800 mg/kg (rat, oral)
3000 mg/kg (mouse, oral)
5340 mg/kg (rabbit, oral)[11]
20,702 ppm (rat, 8 hr)[11]
45,455 ppm (mouse, 1 hr)[11]
US health exposure limits (NIOSH):
PEL (Permissible)
 1000 ppm (2400 mg/m3)[6]
REL (Recommended)
 TWA 250 ppm (590 mg/m3)[6]
IDLH (Immediate danger)
 2500 ppm[6]
Related compounds
Related compounds
 Butanone
Isopropanol
Urea
Carbonic acid
Carbonyl fluoride
Supplementary data page
Refractive index (n),
Dielectric constant (εr), etc.
Thermodynamic
data
 Phase behaviour
solid–liquid–gas
UV, IR, NMR, MS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
YesY verify (what is YesYN ?)
Infobox references

Acetone (systematically named propanone) is the organic compound with the formula (CH3)2CO.[12] It is a colorless, volatile, flammable liquid, and is the simplest ketone.

Acetone is miscible with water and serves as an important solvent in its own right, typically for cleaning purposes in the laboratory. About 6.7 million tonnes were produced worldwide in 2010, mainly for use as a solvent and production of methyl methacrylate and bisphenol A.[13][14] It is a common building block in organic chemistry. Familiar household uses of acetone are as the active ingredient in nail polish remover, and as paint thinner.

Acetone is produced and disposed of in the human body through normal metabolic processes. It is normally present in blood and urine. People with diabetes produce it in larger amounts. Reproductive toxicity tests show that it has low potential to cause reproductive problems. Pregnant women, nursing mothers and children have higher levels of acetone.[15] Ketogenic diets that increase acetone in the body are used to counter epileptic attacks in infants and children who suffer from recalcitrant refractory epilepsy.

History

Acetone was first produced by alchemists during the late Middle Ages via the dry distillation of metal acetates (e.g., lead acetate, which produced "spirit of Saturn" (since the alchemical symbol for lead was also the astrological symbol for the planet Saturn)).[16]

In 1832, French chemist Jean-Baptiste Dumas and German chemist Justus von Liebig determined the empirical formula for acetone.[17][18] In 1833, the French chemist Antoine Bussy named acetone by adding the suffix -one to the stem of the corresponding acid (viz, acetic acid).[19] By 1852, English chemist Alexander William Williamson realized that acetone was methyl acetyl;[20] the following year, the French chemist Charles Frédéric Gerhardt concurred.[21] In 1865, the German chemist August Kekulé published the modern structural formula for acetone.[22][23][24] During WWI, Chaim Weizmann developed the process for industrial production of acetone (Weizmann Process).[25]

Metabolism

Biosynthesis

Small amounts of acetone are produced in the body by the decarboxylation of ketone bodies. Certain dietary patterns, including prolonged fasting and high-fat low-carbohydrate dieting, can produce ketosis, in which acetone is formed in body tissue. Certain health conditions, such as alcoholism and diabetes, can produce ketoacidosis, uncontrollable ketosis that leads to a sharp, and potentially fatal, increase in the acidity of the blood. Since it is a byproduct of fermentation, acetone is a byproduct of the distillery industry.

Metabolic use

Although some biochemistry textbooks and current research publications[26] indicate that acetone cannot be metabolized, there is evidence to the contrary, some dating back thirty years. Acetone can be produced from the oxidation of ingested isopropanol, or from the spontaneous/enzymatic breakdown of acetoacetate (a ketone body) in ketotic individuals. It can then be metabolized either by CYP2E1 via methylglyoxal to D-lactate and pyruvate, and ultimately glucose/energy, or by a different pathway via propylene glycol to pyruvate, lactate, acetate (usable for energy) and propionaldehyde.[27][28][29]

Production

In 2010, the worldwide production capacity for acetone was estimated at 6.7 million tonnes per year.[30] With 1.56 million tonnes per year, the United States had the highest production capacity,[31] followed by Taiwan and mainland China. The largest producer of acetone is INEOS Phenol, owning 17% of the world's capacity, with also significant capacity (7–8%) by Mitsui, Sunoco and Shell in 2010.[30] INEOS Phenol also owns the world's largest production site (420,000 tonnes/annum) in Beveren (Belgium). Spot price of acetone in summer 2011 was 1100–1250 USD/tonne in the United States.[32]

Current method

Acetone is produced directly or indirectly from propylene. Approximately 83% of acetone is produced via the cumene process;[14] as a result, acetone production is tied to phenol production. In the cumene process, benzene is alkylated with propylene to produce cumene, which is oxidized by air to produce phenol and acetone:

Other processes involve the direct oxidation of propylene (Wacker-Hoechst process), or the hydration of propylene to give 2-propanol, which is oxidized to acetone.[14]

Older methods

Previously, acetone was produced by the dry distillation of acetates, for example calcium acetate in ketonic decarboxylation.

Ca(CH3COO)2 → CaO(s) + CO2(g) + (CH3)2CO (v)

After that time, during World War I, acetone was produced using acetone-butanol-ethanol fermentation with Clostridium acetobutylicum bacteria, which was developed by Chaim Weizmann (later the first president of Israel) in order to help the British war effort,[14] in the preparation of Cordite.[33] This acetone-butanol-ethanol fermentation was eventually abandoned when newer methods with better yields were found.[14]

Uses

About a third of the world's acetone is used as a solvent, and a quarter is consumed as acetone cyanohydrin, a precursor to methyl methacrylate.[13]

Solvent

Acetone is a good solvent for many plastics and some synthetic fibers. It is used for thinning polyester resin, cleaning tools used with it, and dissolving two-part epoxies and superglue before they harden. It is used as one of the volatile components of some paints and varnishes. As a heavy-duty degreaser, it is useful in the preparation of metal prior to painting. It is also useful for high reliability soldering applications to remove rosin flux after soldering is complete; this helps to prevent the rusty bolt effect.

Acetone is used as a solvent by the pharmaceutical industry and as a denaturant in denatured alcohol.[34] Acetone is also present as an excipient in some pharmaceutical drugs.[35]

Although itself flammable, acetone is used extensively as a solvent for the safe transportation and storage of acetylene, which cannot be safely pressurized as a pure compound. Vessels containing a porous material are first filled with acetone followed by acetylene, which dissolves into the acetone. One liter of acetone can dissolve around 250 liters of acetylene at a pressure of 10 bar.[36][37]

Chemical intermediate

Acetone is used to synthesize methyl methacrylate. It begins with the initial conversion of acetone to acetone cyanohydrin:

(CH3)2CO + HCN → (CH3)2C(OH)CN

In a subsequent step, the nitrile is hydrolyzed to the unsaturated amide, which is esterified:

(CH3)2C(OH)CN + CH3OH → CH2=(CH3)CCO2CH3 + NH3

The third major use of acetone (about 20%)[13] is synthesizing bisphenol A. Bisphenol A is a component of many polymers such as polycarbonates, polyurethanes, and epoxy resins. The synthesis involves the condensation of acetone with phenol:

(CH3)2CO + 2 C6H5OH → (CH3)2C(C6H4OH)2 + H2O

Many millions of kilograms of acetone are consumed in the production of the solvents methyl isobutyl alcohol and methyl isobutyl ketone. These products arise via an initial aldol condensation to give diacetone alcohol.[14]

2 (CH3)2CO → (CH3)2C(OH)CH2C(O)CH3

Laboratory

In the laboratory, acetone is used as a polar, aprotic solvent in a variety of organic reactions, such as SN2 reactions. The use of acetone solvent is critical for the Jones oxidation. It does not form an azeotrope with water (see azeotrope (data)).[38] It is a common solvent for rinsing laboratory glassware because of its low cost and volatility. Despite its common use as a supposed drying agent, it is not effective except by bulk displacement and dilution. Acetone can be cooled with dry ice to −78 °C without freezing; acetone/dry ice baths are commonly used to conduct reactions at low temperatures. Acetone is fluorescent under ultraviolet light, and its vapor can be used as a fluorescent tracer in fluid flow experiments.[39]

Medical and cosmetic uses

Acetone is used in a variety of general medical and cosmetic applications and is also listed as a component in food additives and food packaging and also in nail polish remover. Dermatologists use acetone with alcohol for acne treatments to peel dry skin.

Acetone is commonly used in chemical peeling. Common agents used today for chemical peels are salicylic acid, glycolic acid, 30% salicylic acid in ethanol, and trichloroacetic acid (TCA). Prior to chemexfoliation, the skin is cleaned and excess fat removed in a process called defatting. Acetone, Septisol, or a combination of these agents is commonly used in this process.

Domestic and other niche uses

Acetone is often the primary component in cleaning agents such as nail polish remover. Acetone is a component of superglue remover and easily removes residues from glass and porcelain. Make-up artists use acetone to remove skin adhesive from the netting of wigs and moustaches by immersing the item in an acetone bath, then removing the softened glue residue with a stiff brush.

Acetone can also be used in combination with automatic transmission fluid to create an effective penetrating oil. Brake fluid is sometimes used in place of ATF. These mixtures (usually 1:1) can be useful in loosening rusted or stuck bolts.

Acetone is often used for vapor polishing of printing artifacts on 3D-printed models printed with ABS plastic. The technique, called acetone vapor bath smoothing, involves placing the printed part in a sealed chamber containing a small amount of acetone, and heating to around 80 degrees Celsius for 10 minutes. This creates a vapor of acetone in the container. The acetone condenses evenly all over the part, causing the surface to soften and liquefy. Surface tension then smooths the semi-liquid plastic. When the part is removed from the chamber, the acetone component evaporates leaving a glassy-smooth part free of striation, patterning, and visible layer edges, common features in untreated 3D printed parts.[40]

Low-grade acetone is also commonly used in academic laboratory settings as a glassware rinsing agent for removing residue and solids before a final wash.[41]

Safety

Flammability

The most hazardous property of acetone is its extreme flammability. At temperatures greater than acetone's flash point of −20 °C (−4 °F), air mixtures of between 2.5% and 12.8% acetone, by volume, may explode or cause a flash fire. Vapors can flow along surfaces to distant ignition sources and flash back. Static discharge may also ignite acetone vapors, though acetone has a very high ignition initiation energy point and therefore accidental ignition is rare. Even pouring or spraying acetone over red-glowing coal will not ignite it, due to the high concentration of vapour and the cooling effect of evaporation of the liquid.[42] It auto-ignites at 465 °C (869 °F). Auto-ignition temperature is also dependent upon the exposure time, thus at some tests it is quoted as 525 °C. Also, industrial acetone is likely to contain a small amount of water which also inhibits ignition.

Acetone peroxide

When oxidized, acetone forms acetone peroxide as a byproduct, which is a highly unstable, primary high explosive compound. It may be formed accidentally, e.g. when waste hydrogen peroxide is poured into waste solvent containing acetone. Due to its instability, it is rarely used, despite its simple chemical synthesis.

Health information

Acetone has been studied extensively and is generally recognized to have low acute and chronic toxicity if ingested and/or inhaled.[43] Acetone is not currently regarded as a carcinogen, a mutagenic chemical or a concern for chronic neurotoxicity effects.[42]

Acetone can be found as an ingredient in a variety of consumer products ranging from cosmetics to processed and unprocessed foods. Acetone has been rated as a generally recognized as safe (GRAS) substance when present in beverages, baked foods, desserts, and preserves at concentrations ranging from 5 to 8 mg/L.[43]

Toxicology

Acetone is believed to exhibit only slight toxicity in normal use, and there is no strong evidence of chronic health effects if basic precautions are followed.[44]

At very high vapor concentrations, acetone is irritating and, like many other solvents, may depress the central nervous system. It is also a severe irritant on contact with eyes, and a potential pulmonary aspiration risk. In one documented case, ingestion of a substantial amount of acetone led to systemic toxicity, although the patient eventually fully recovered.[45] Some sources estimate LD50 for human ingestion at 0.621 g/kg; LD50 inhalation by mice is given as 23 g/m3, over 4 hours.[46]

Acetone has been shown to have anticonvulsant effects in animal models of epilepsy, in the absence of toxicity, when administered in millimolar concentrations.[47] It has been hypothesized that the high-fat low-carbohydrate ketogenic diet used clinically to control drug-resistant epilepsy in children works by elevating acetone in the brain.[47]

Environmental effects

Although acetone occurs naturally in the environment in plants, trees, volcanic gases, forest fires, and as a product of the breakdown of body fat,[48] the majority of the acetone released into the environment is of industrial origin. Acetone evaporates rapidly, even from water and soil. Once in the atmosphere, it has a 22-day half-life and is degraded by UV light via photolysis (primarily into methane and ethane.[49]) Consumption by microorganisms contributes to the dissipation of acetone in soil, animals, or waterways.[48] The LD50 of acetone for fish is 8.3 g/L of water (or about 1%) over 96 hours, and its environmental half-life in water is about 1 to 10 days. Acetone may pose a significant risk of oxygen depletion in aquatic systems due to the microbial consumption.[50]

Extraterrestrial occurrence

On 30 July 2015, scientists reported that upon the first touchdown of the Philae lander on comet 67P's surface, measurements by the COSAC and Ptolemy instruments revealed sixteen organic compounds, four of which were seen for the first time on a comet, including acetamide, acetone, methyl isocyanate, and propionaldehyde.[51][52][53]

References

  1. ↑ The Merck Index, 15th Ed. (2013), p. 13, Acetone Monograph 65, O'Neil: The Royal Society of Chemistry.(subscription required)
  2. 1 2 3 "Acetone". NIST Chemistry WebBook. USA: National Institute of Standards and Technology.
  3. ↑ Klamt, Andreas (2005). COSMO-RS: From Quantum Chemistry to Fluid Phase Thermodynamics and Drug Design. Elsevier. pp. 92–94. ISBN 978-0-444-51994-8.
  4. ↑ Ash, Michael; Ash, Irene (2004). Handbook of preservatives. Synapse Information Resources, Inc. p. 369. ISBN 1-890595-66-7.
  5. ↑ Myers, Richard L. (2007). The 100 Most Important Chemical Compounds: A Reference Guide. Greenwood. pp. 4–6. ISBN 978-0-313-08057-9.
  6. 1 2 3 4 "NIOSH Pocket Guide to Chemical Hazards #0260". National Institute for Occupational Safety and Health (NIOSH).
  7. 1 2 David R. Lide (ed), CRC Handbook of Chemistry and Physics, 84th Edition. CRC Press. Boca Raton, Florida, 2003; Section 3, Physical Constants of Organic Compounds
  8. 1 2 Properties of substance: acetone. chemister.ru
  9. ↑ Acetone in Linstrom, P. J.; Mallard, W. G. (eds.) NIST Chemistry WebBook, NIST Standard Reference Database Number 69. National Institute of Standards and Technology, Gaithersburg MD. http://webbook.nist.gov (retrieved 2014-05-11)
  10. ↑ Working with modern hydrocarbon and oxygenated solvents: a guide to flammability Archived 2009-06-01 at the Wayback Machine. American Chemistry Council Solvents Industry Group, pg. 7, January 2008
  11. 1 2 3 "Acetone". Immediately Dangerous to Life and Health. National Institute for Occupational Safety and Health (NIOSH).
  12. ↑ Allen, P. W.; Bowen, H. J. M.; Sutton, L. E.; Bastiansen, O. (1952). "The molecular structure of acetone". Transactions of the Faraday Society. 48: 991. doi:10.1039/TF9524800991.
  13. 1 2 3 Acetone, World Petrochemicals report, January 2010
  14. 1 2 3 4 5 6 Stylianos Sifniades, Alan B. Levy, "Acetone" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.
  15. ↑ American Chemistry Council, Acetone VCCEP Submission, September 10, 2003, page 9
  16. ↑ Gorman, Mel & Doering, Charles (1959). "History of the structure of acetone". Chymia. 5: 202–208. JSTOR 27757186.
  17. ↑ Dumas, J. (1832) "Sur l'esprit pyro-acétique" (On pyro-acetic spirit), Annales de Chimie et de Physique, 2nd series, 49 : 208–210.
  18. ↑ Liebig, Justus (1832) "Sur les combinaisons produites par l'action du gas oléfiant et l'esprit acétique" (On compounds produced by the action of ethylene and acetic spirit), Annales de Chimie et de Physique, 2nd series, 49 : 146–204 (especially 193–204).
  19. ↑ Bussy, Antoine (1833) "De quelques Produits nouveaux obtenus par l’action des Alcalis sur les Corps gras à une haute température" (On some new products obtained by the action of alkalies on fatty substances at a high temperature), Annales de Chimie et de Physique, 2nd series, 53 : 398–412 ; see footnote on p. 409, continued from p. 408.
  20. ↑ Williamson, A. W. (1852) "On Etherification," Journal of the Chemical Society, 4 : 229–239 ; see especially pp. 237–239.
  21. ↑ Gerhardt, Charles (1853) "Researches sur les acids organiques anhydres" (Research on anhydrous organic acids), Annales de Chimie et de Physique, 3rd series, 37 : 285–342 ; see p. 339.
  22. ↑ Kekulé, Auguste (1865) "Sur la constitution des substances aromatiques," Bulletin de la Société chimique de Paris, 1 : 98–110 ; see especially p. 110.
  23. ↑ Kekulé, Auguste (1866) "Untersuchungen über aromatischen Verbindungen" (Investigations into aromatic compounds), Annalen der Chemie und Pharmacie, 137 : 129–196 ; see especially pp. 143–144.
  24. ↑ Johann Josef Loschmidt had presented the structure of acetone in 1861, but his privately published booklet received little attention. See: J. Loschmidt, Chemische Studien (Vienna, Austria-Hungary: Carl Gerold's Sohn, 1861).
  25. ↑ http://www.chemistryexplained.com/Va-Z/Weizmann-Chaim.html
  26. ↑ Vujasinovic, M; Kocar, M; Kramer, K; Bunc, M; Brvar, M (2007). "Poisoning with 1-propanol and 2-propanol". Human & Experimental Toxicology. 26 (12): 975. doi:10.1177/0960327107087794.
  27. ↑ Glew, Robert H (2010). "You Can Get There From Here: Acetone, Anionic Ketones and Even-Carbon Fatty Acids can Provide Substrates for Gluconeogenesis". Nig. J. Physiol. Sci. 25: 2–4.
  28. ↑ Miller DN, Bazzano G; Bazzano (1965). "Propanediol metabolism and its relation to lactic acid metabolism". Ann NY Acad Sci. 119 (3): 957–973. Bibcode:1965NYASA.119..957M. PMID 4285478. doi:10.1111/j.1749-6632.1965.tb47455.x.
  29. ↑ Ruddick JA (1972). "Toxicology, metabolism, and biochemistry of 1,2-propanediol". Toxicol App Pharmacol. 21: 102–111. doi:10.1016/0041-008X(72)90032-4.
  30. 1 2 Greiner, Camara; Funada, C (June 2010). "CEH Marketing Research Report: ACETONE". Chemical Economics Handbook. SRI consulting. Retrieved 2 September 2016.(subscription required)
  31. ↑ "Acetone Uses and Market Data". ICIS.com. October 2010. Retrieved 2011-03-21.
  32. ↑ Acetone (US Gulf) Price Report – Chemical pricing information. ICIS Pricing. Retrieved on 2012-11-26.
  33. ↑ Wittcoff, M.M. Green ; H.A. (2003). Organic chemistry principles and industrial practice (1. ed., 1. reprint. ed.). Weinheim: Wiley-VCH. p. 4. ISBN 3-527-30289-1.
  34. ↑ Weiner, Myra L.; Lois A. Kotkoskie (1999). Excipient Toxicity and Safety. p. 32. ISBN 978-0-8247-8210-8.
  35. ↑ Inactive Ingredient Search for Approved Drug Products, FDA/Center for Drug Evaluation and Research
  36. ↑ Mine Safety and Health Administration (MSHA) – Safety Hazard Information – Special Hazards of Acetylene. Msha.gov. Retrieved on 2012-11-26.
  37. ↑ History – Acetylene dissolved in acetone. Aga.com. Retrieved on 2012-11-26.
  38. ↑ What is an Azeotrope?. Solvent—recycling.com. Retrieved on 2012-11-26.
  39. ↑ Lozano, A.; Yip, B.; Hanson, R.K. (1992). "Acetone: a tracer for concentration measurements in gaseous flows by planar laser-induced fluorescence". Exp. Fluids. 13 (6): 369–376. Bibcode:1992ExFl...13..369L. doi:10.1007/BF00223244.
  40. ↑ "Quality Finish 3D Prints with Acetone". instructables.com
  41. ↑ "Cleaning Glassware" (PDF). Wesleyan University. September 2009. Retrieved July 7, 2016.
  42. 1 2 Acetone MSDS. Hazard.com (1998-04-21). Retrieved on 2012-11-26.
  43. 1 2 "SIDS Initial Assessment Report: Acetone" (PDF). Environmental Protection Agency.
  44. ↑ Basic Information on Acetone. Ccohs.ca (1999-02-19). Retrieved on 2012-11-26.
  45. ↑ Canadian Centre for Occupational Health and Safety. "Health Effects of Acetone". Archived from the original on 17 October 2008. Retrieved 2008-10-21.
  46. ↑ Safety (MSDS) data for propanone. Msds.chem.ox.ac.uk. Retrieved on 2012-11-26
  47. 1 2 Likhodii SS, Serbanescu I, Cortez MA, Murphy P, Snead OC, Burnham WM (2003). "Anticonvulsant properties of acetone, a brain ketone elevated by the ketogenic diet". Ann Neurol. 54 (2): 219–226. PMID 12891674. doi:10.1002/ana.10634.
  48. 1 2 Acetone, Agency for Toxic Substances and Disease Registry ToxFAQs, 1995
  49. ↑ Darwent, B. deB.; Allard, M. J.; Hartman, M. F.; Lange, L. J. (1960). "The Photolysis of Acetone". Journal of Physical Chemistry. 64 (12): 1847–1850. doi:10.1021/j100841a010.
  50. ↑ Safety Data Sheet Acetone Archived 2009-03-20 at the Wayback Machine.. jmloveridge.com. Retrieved on 2012-11-26.
  51. ↑ Jordans, Frank (30 July 2015). "Philae probe finds evidence that comets can be cosmic labs". The Washington Post. Associated Press. Retrieved 30 July 2015.
  52. ↑ "Science on the Surface of a Comet". European Space Agency. 30 July 2015. Retrieved 30 July 2015.
  53. ↑ Bibring, J.-P.; Taylor, M.G.G.T.; Alexander, C.; Auster, U.; Biele, J.; Finzi, A. Ercoli; Goesmann, F.; Klingehoefer, G.; Kofman, W.; Mottola, S.; Seidenstiker, K.J.; Spohn, T.; Wright, I. (31 July 2015). "Philae's First Days on the Comet – Introduction to Special Issue". Science. 349 (6247): 493. Bibcode:2015Sci...349..493B. PMID 26228139. doi:10.1126/science.aac5116. Retrieved 30 July 2015.
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