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AstroBob: What does a comet smell like?

The human nose is practically useless for astronomy. Photo illustration by Bob King / 1 / 5
Jets of carbon dioxide blast from beneath the surface of mile-wide Comet Hartley 2 in this photo taken during by NASA’s EPOXI/Deep Impact spacecraft during its flyby of the comet in 2010. The jets carry water ice in the form of large snowballs (white dots) and dust particles. Solar heating is the cause of all the activity. Credit: NASA 2 / 5
Dark lines and bands stripe this spectrum of the sun. Patterns of lines represent light absorbed by different elements and compounds in the sun’s atmosphere from light that streams from its surface. Credit: N.A. Sharp, NOAO, NSO, Kitt Peak, FTS, AURA, NSF3 / 5
A hunk of the aromatic Murchison meteorite that fell in 1969. Murchison, a carbonaceous chondrite, contains lots of hydrocarbons including amino acids, the building blocks life uses to make proteins. Credit: Basilicofresco 4 / 5
A new study shows that Comet 45P/Honda-Mrkos-Pajdusakova, which put in a nice appearance earlier this year, is rich in methane and methanol. Credit: Gerald Rhemann 5 / 5

We rarely use our sense of smell in astronomy except for the occasional run-in with a skunk during late night sky sweeping. We can't smell the irony-dusty air on Mars, the pungent ammonia clouds that swaddle Jupiter or the reeking sulfur plumes of Io's many active volcanoes. Our noses are helpless.

I love hunting for and looking at comets and sometimes try to imagine what it might be like to smell one of those. First, we have to know what they're made of. Astronomers use an instrument called a spectrograph to puzzle out the composition of comets. It uses a piece of finely-scored glass called a diffraction grating to split and spread the light of an astronomical object into a full rainbow of colors called a spectrum. A DVD works much like a grating. Its millions of microscopic, laser-etched pits scatter light into a rainbow of color when held against a light.

Within an object's spectrum are seen patterns of thin, black lines where narrow slivers of this or that color is missing. The missing colors are those absorbed by gases in the atmosphere of a comet or by dust on the surface of an asteroid. Particular gases or minerals absorb light in their own unique way, leaving a "fingerprint" or pattern of dark lines in a spectrum. Those versed in reading spectra can see at a glance what's in a comet, a star or nebula like reading the ingredients on a package of cookies.

How do we know one group of lines represents one element and another set a different element? Scientists perform flame tests back in the lab on all manner of materials — set them on fire, basically — and record each unique signature with a spectroscope.

Comets are made of ices mixed with dust. Some of that ice vaporizes when a comet nears the sun to form a thin atmosphere called a "coma." A coma makes a comet look like a fuzzy blob through our telescopes. From their spectra, we can drill down into the particulars. Ingredients that end up in a comet's atmosphere include vaporized ices. First and foremost is water, then carbon monoxide and carbon dioxide (dry ice) — all odorless — followed by ammonia, methane (also odorless), methyl alcohol or methanol, hydrogen cyanide, formaldehyde, acetone and dozens of other compounds in ever smaller quantities. The dust released from the boiled-away ice is rich in carbon compounds, rocky materials like olivine and pyroxene, graphite, carborundum and even smidges of diamond dust.

The most fragrant materials in a typical comet would include:

• Ammonia: sharp, pungent-smelling like cat urine or cleaning fluid

• Methanol: alcohol smell similar to drinkable alcohol

• Hydrogen cyanide: not everyone can smell this, but those that can describe it as "bitter almond"

• Acetone: intense sweet or fruity chemical smell of nail polish

• Formaldehyde: like a high school biology dissecting class. Smells a little like pickles.

So if we could break off a chunk of a comet's fragile, icy surface, place it in a sealed container and return it to Earth, what might you smell when you twist open the lid? Most definitely a bouquet of fragrances intense enough to make you yank your head back, then approach the jar with great care for another whiff. I can almost smell it! (Although cyanide is poisonous, the amount in a fistful of comet ice is probably too little for concern.)

Some meteorites contain hydrocarbons that have distinct odors. One of the smelliest meteorites, Murchison, came crashing down in pieces over southern Australia in September 1969. Many described it as reeking of methanol especially when kept in a jar for a while and then unsealed. Other pieces still retain a sulfurous smell similar to tar and asphalt.

We know we can't directly smell a comet, but don't despair. Let's make one right here on Earth. Using ammonia, alcohol and other familiar ingredients, we can come close to not only how a comet appears and behaves but also get a sniff of its heady aroma. Just follow the instructions in the video. When you're done, hold the icy monster (remember to use thick gloves!) next to a warm light and watch jets of carbon dioxide gas shoot out just like the real thing.

Watch: How to make your own comet at " target="_blank">