Q: The first images from a rover dispatched centuries earlier to the sandy beach of a watery exoplanet have arrived back home. For these first images, the rover stopped and scanned the terrain and sea for an hour, then slewed its camera and photographed its wheel tracks in the sand. While the science team is naturally most curious about the apparent sneaker prints, the other notable thing is that the tracks are raised rather than sunken into the sand. Do physics work differently on this exoplanet, or is there an explanation?
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From the September issue: ROTATING MOON
We asked you to fill in the blanks of a sentence explaining how the moon could suddenly start turning in the night sky. Your responses were reviewed by Caltech planetary scientist Simon Lock, who is currently conducting research at the University of Bristol. Multiple respondents answered correctly. We were looking for “asteroid” to fill the second blank, but reader Chris Limbach accurately noted that maybe a large “starship” flew by. In the end, however, one response stood out for its creativity:
WINNER: “The tidal lock between the moon and Earth was broken when a large asteroid flew by.” The moon is tidally locked to Earth. This means that the moon takes the same amount of time to rotate once about its axis as it takes to revolve once around the Earth. This is why we see the same face of the moon every night we look up. To understand this, we can do a thought experiment. Imagine listening to a deaf person singing karaoke, being cued in at the right time and knowing the song perfectly. The voice and the music are in perfect sync, or one could say “tidally locked.” Now, midway through, due to some disturbance in the music system, the speed of the music starts increasing slowly. Since the singer does not adapt to this increasing speed, there is a lag between the singer and the music. As the song progresses, this lag keeps increasing. The “tidal lock” between the singer and the music is lost. This is similar to what happens when a large body like an asteroid moves past the moon. During this passage, there is some energy exchange between the asteroid and the moon: Either the asteroid gives some of its energy to the moon and the moon speeds up, or the asteroid gains some energy, which slows down the moon just like the disturbance in the music system. The tidal lock is lost, and there is a lag between the rotation speed and the revolution speed of the moon, causing it to rotate in the sky.
Roshan Eapen, AIAA Young Professional member
University Park, Pennsylvania
Roshan is an assistant professor of aerospace engineering at Pennsylvania State University.