A Christmas surprise
Q: It’s the future. Mr. Grinch has a cousin in the United Kingdom whose heart is still several sizes too small. This Mr. Grinch prefers flying, and he has decided to upgrade to a jet. Being a Grinch, he sets out from his lair to find the loudest, dirtiest engine he can. Sneaking into a warehouse, he finds a huge engine that says “Ultra . . .” on it, but he can’t quite make out the rest in the darkness. He shrugs, assuming “ultra” must mean big, and big must mean loud and dirty. What engine has he found, and is this another stroke of evil genius?
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From the October issue: ROVER MYSTERY
We asked you to explain why a rover’s tracks are raised rather than sunken in the sands of a watery exoplanet.
WINNER: The sand is probably moist with sea water and behaves like a non-newtonian dilatant fluid and thus “Unlike most other solid materials, the tendency of a compacted dense granular material is to dilate (expand in volume) as it is sheared,” as explained on Wikipedia [https://en.wikipedia.org/wiki/Dilatancy_(granular_material)]. As the rover moves, the sand expands, and thus the tracks are elevated.
George Kyriakou, AIAA Young Professional member
New York, New York
Kyriakou is chief operating officer at BotFactory, a New York startup that additively manufactures circuits.
More about sand: Here is a expanded explanation I crafted after corresponding with civil engineer Stein Sture, principal investigator for the Mechanics of Granular Material experiments flown on two space shuttle missions in the 1990s and the ill-fated Columbia mission in 2003. In between the grains of sand are water molecules. When compressed, the sand in the structure of the imprint comes into tighter contact with those molecules. The result is a capillary effect in which the water molecules adhere to the sandy walls around them and align with each other to create surface tension. The combined effect supports and dilates the structure of the print. Meanwhile, the surrounding moist sand is not compressed, so those water molecules evaporate and drain away, and the sand sinks. Now you have a raised track instead of a sunken one. — Editor-in-chief Ben Iannotta