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Water crystallizes into ice at 32 degrees F, but not always (courtesy of flickr user s.alt)

The title of this post would seem an appropriate question for an elementary-school science exam, but the answer is far more complicated than it first appears. We’ve all been taught that water freezes at 32 degrees Fahrenheit, 0 degrees Celsius, 273.15 Kelvin. That’s not always the case, though. Scientists have found liquid water as cold as -40 degrees F in clouds and even cooled water down to -42 degrees F in the lab. How low could they go?

That turns out to be a tricky problem to answer. When liquid water is cooled below -42 degrees F, it crystallizes into ice too quickly for scientists to measure the temperature of the liquid. So Emily Moore and Valeria Molinero of the University of Utah developed a sophisticated computer simulation of 32,768 water molecules (fewer molecules than can be found in a raindrop) that let them see what happened to the water’s heat capacity, density and compressibility as it supercooled and determine what happened as 4,000 of those molecules froze. Their results appear in the journal Nature.

As the temperature of the water approaches -55 degrees F, the water molecules form tetrahedrons, with each molecule loosely bonding to four other molecules. The density of the water decreases, its heat capacity increases and its compressibility increases. “The change in structure of water controls the rate at which ice forms,” Molinero says. “We show both the thermodynamics of water and the crystallization rate are controlled by the change in structure of liquid water that approaches the structure of ice.” Below -55 degrees F, tiny bits of liquid water may still exist, but it would do so only for an incredibly short time, Molinero says.

This supercooling of water is possible because water needs a small nucleus or seed of ice for the molecules to form crystals and in very pure water “the only way you can form a nucleus is by spontaneously changing the structure of the liquid,” Molinero says. Those nuclei won’t form or grow large enough until the structure of the liquid water molecules approaches that of solid ice, which doesn’t happen until the water gets so incredibly cold.

(HT: io9)

A pigeon's turn is very different from that of an airplane (courtesy of S. A. Combes)

If you were to compare a pigeon to a flying machine, you’d probably think airplane–they’ve both got wings, a tail and landing gear. But when it comes to turning in the air, pigeons have more in common with helicopters than planes, say scientists who report their findings this week in PNAS.

There are two ways for a flying object, whether it be living or mechanical, to change its direction: it can (A) alter the direction of its body or (B) alter the direction of the force of propulsion. Helicopters and most insects use method A, while airplanes use method B.

Researchers from Harvard University and Harvey Mudd College filmed pigeons on high-speed video as they flew at low speed down a corridor with a sharp, 90-degree turn. They found that a bird, as it turns, changes the orientation of its whole body and redirects the aerodynamic forces so that they stay in line with its body. “We didn’t expect the forces to change direction relative to the body this little–as little as is observed in helicopters,” lead author Ivo Ross of Harvard University told New Scientist.

Technicans work on the Mars Science Laboratory, aka Curiosity. Image by Jim Grossman/NASA via In Focus

As we’re catching up on our e-mails and to-do lists after the long Thanksgiving break, we thought we’d give you some recommended reading on Saturday’s launch of Curiosity, the newest Mars rover, which is scheduled to touch down on the red planet in August 2012.

In our sister publication, Air and Space, editor Tony Reichhardt outlines the physics behind the science laboratory’s descent to the planet’s surface in a fascinating photo essay:

The landing should be precise and gentle, and because the descent stage flies away after dropping off the rover, there’s no rocket exhaust to contaminate the arrival site, as happens with conventional landers. The journey to Mars takes nine months, but the final, stomach-churning landing sequence, scheduled for August 5, 2012, takes less than a minute.

In 2005, science writer Carl Zimmer wrote a piece for Smithsonian about what it would mean to find life on Mars:

If all goes as planned, a new generation of rovers will arrive on Mars within the next decade. These missions will incorporate cutting-edge biotechnology designed to detect individual molecules made by Martian organisms, either living or long dead.

The search for life on Mars has become more urgent thanks in part to probes by the two rovers now roaming Mars’ surface and another spaceship that is orbiting the planet. In recent months, they’ve made a series of astonishing discoveries that, once again, tempt scientists to believe that Mars harbors life—or did so in the past. At a February conference in the Netherlands, an audience of Mars experts was surveyed about Martian life. Some 75 percent of the scientists said they thought life once existed there, and of them, 25 percent think that Mars harbors life today.

Elsewhere on the web, Bad Astronomy gives you a rundown of what to look for in the liftoff video and has extended thoughts on Curiosity’s landing procedure; Alan Taylor of the Atlantic’s In Focus blog has an up close and personal look at the rover; and the Planetary Society highlights a great infographic by Jason Davis on the history of Mars exploration.

A male eastern wild turkey (via wikimedia commons)

Tomorrow may officially be called Thanksgiving, but you all know what it really is–Turkey Day! But how well do you really know Meleagris gallopavo, the wild turkey from which the domesticated version, the one likely to be on your plate, was derived?

1 ) Turkeys are more than just big chickens–more than 45 million years of evolution separates the two species.

2 ) The wild turkey was hunted nearly to extinction by the early 1900s, when the population reached a low of around 30,000 birds. But restoration programs across North America have brought the numbers up to seven million today.

3 ) There are six subspecies of wild turkey, all native to North America. The pilgrims hunted and ate the eastern wild turkey, M. gallopavo silvestris, which today has a range that covers the eastern half of the United States and extends into Canada. These birds, sometimes called the forest turkey, are the most numerous of all the turkey subspecies, numbering more than five million.

4 ) The Aztecs domesticated another subspecies, M. gallapavo gallopavo, the south Mexican wild turkey, and the Spanish brought those turkeys to Europe. The pilgrims then brought several of these domestic turkeys back to North America.

5 ) Male turkeys are called “gobblers,” after the “gobble” call they make to announce themselves to females (which are called “hens”) and compete with other males. Other turkey sounds include “purrs,” “yelps” and “kee-kees.”

6 ) An adult gobbler weighs 16 to 22 pounds on average, has a beard of modified feathers on his breast that reaches seven inches or more long, and has sharp spurs on his legs for fighting. A hen is smaller, weighing around 8 to 12 pounds, and has no beard or spurs. Both genders have a snood (a dangly appendage on the face), wattle (the red dangly bit under the chin) and only a few feathers on the head.

7 ) Studies have shown that snood length is associated with male turkey health. In addition, a 1997 study in the Journal of Avian Biology found that female turkeys prefer males with long snoods and that snood length can also be used to predict the winner of a competition between two males.

8 ) A turkey’s gender can be determined from its droppings–males produce spiral-shaped poop and females’ poop is shaped like the letter J.

9 ) Turkeys can run at speeds of up to 25 miles per hour and fly as fast as 55 miles per hour.

10 ) A group of related male turkeys will band together to court females, though only one member of the group gets to mate.

11 ) When a hen is ready to make little turkeys, she’ll lay about 10 to 12 eggs, one egg per day, over a period of about two weeks. The eggs will incubate for about 28 days before hatching.

12) Baby turkeys, called poults, eat berries, seeds and insects, while adults have a more varied diet that can include acorns and even small reptiles.

13 ) There is one other species of turkey, the ocellated turkey (Meleagris ocellata), which can be found on the Yucatan Peninsula in Mexico.

14 ) Benjamin Franklin never proposed the turkey as a symbol for America, but he did once praise it as being “a much more respectable bird” than the bald eagle.

To welcome the newest member of Smithsonian.com’s blog family, Just One More Story: What’s new and novel in children’s books, I’ve rounded up some of my favorite kids’ science books that have been published in the past year:

Pond Walk, by Nancy Elizabeth Wallace

Pond Walk by Nancy Elizabeth Wallace (ages 4 to 7)
This must be how biologists go to the park with their kids–pointing out all the interesting plants and animals, teaching about how these organisms interact, encouraging their children to document it all in drawings and telling silly jokes along the way.

11 Experiments That Failed by Jenny Offill and Nancy Carpenter (ages 4 to 8 )
Each bizarre experiment starts with a question–Can a kid survive the winter on ketchup-covered snowballs?–and presents a hypothesis, list of materials, set of methods and an outcome; they’re mini scientific papers and great for teaching the basics of the scientific method in a hilarious way.

In the Bag! Margaret Knight Wraps It Up by Monica Kulling, illustrated by David Parkins (ages 5 to 8 )
This is the wonderful story of of an early female inventor. Margaret Knight began inventing at the age of 12, when she was working in a cotton mill and created a device that made looms safer. But her most famous invention is one we’re all familiar with–she created a machine that made flat-bottomed paper bags.

Coral Reefs by Jason Chin (ages 5 to 9)
What makes this book special is the premise of the illustrations–a little girl goes to the New York Public Library and opens a book on coral reefs only to have the library, and then the entire city, turn into a vibrant reef for her to explore.

How the Dinosaur Got to the Museum by Jessie Hartland (ages 6 to 9)
By following a Diplodocus from its discovery to its eventual display in the Smithsonian Institution, Hartland has created a lovely tribute to all of the people who help to make a museum’s dinosaur exhibit possible.

Nature’s Adventures by Mick Manning and Brita Granstrom (ages 6 to 9)
Anyone can have an adventure in nature, no matter where they live. Manning and Granstrom giving budding naturalists a head start with some simple advice on what to bring and what to look for, whether you’re at the beach, in a forest or at home in the city.

North: The Amazing Story of Arctic Migration by Nick Dowson, illustrated by Patrick Benson (ages 7 to 10)
Dowson follows birds, whales, caribou and other animals as they migrate from as far away as New Zealand to the Arctic in the spring, and sees them through fall, when the weather turns for the worse.

The Secret World of Whales by Charles Siebert, illustrated by Molly Baker (ages 8 to 12)
A comprehensive look at the whale world, including whales in literature, the history of whaling and highlights of current whale science. For example, one page mentions the work of Hal Whitehead, who was featured in the recent Smithsonian story about sperm whales.

Far from Shore: Chronicles of an Open Ocean Voyage by Sophie Webb (ages 9 to 12)
Webb, a naturalist and artist, documents a four-month research voyage in the Pacific on the NOAA ship McArthur II with fascinating diary entries (marked by latitude and longitude so readers can map out her journey for themselves) and beautiful illustrations of the creatures she saw along the way.