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Comment Re:photo too blurry (Score 2) 78 78

Tidal dissipation occurs when the tidal forces vary with time, generally due to the orbit of the secondary being eccentric. That brings it alternately closer to and farther from the primary, stretching and squeezing the interior.

However, the orbit of Charon about Pluto is circular (Buie et al., 2012), so the tidal bulge is constant. There's no time-varying deformation and no dissipation.

Comment Re:Sniff the wind (Score 2) 33 33

The solar wind never gets anywhere near Jupiter's atmosphere. Jupiter has an absurdly strong magnetic field. The magnetopause is between 50 about 100 Jupiter radii from the planet, and the solar wind is deflected around it (Khurana et al., 2004). The jovian aurorae are powered by currents entirely within the magnetosphere (Jupiter's rotation and Io's plasma).

Comment Re:Counterclockwise? (Score 4, Interesting) 141 141

According to the IAU definition, the north pole for a major planet (or one of its satellites) is the pole on the same side of the ecliptic as the Earth's north pole, the North Celestial Hemisphere. By this definition, Venus and Uranus are retrograde rotators -- they rotate clockwise about their north poles.

For comets and minor planets (including Dwarf planets), the north pole is the pole about which the body rotates counterclockwise. So the north pole of a retrograde-rotating asteroid points into the South Celestial Hemisphere.

This brings us (as do all topics that mention the IAU) to Pluto. Pluto rotates retrograde. It was once considered a major planet, so it's north pole would have been on the same side of the ecliptic as ours. But as a dwarf planet, the opposite definition applies. Even before the 2006 decision, the convention was inconsistently applied. Papers have been published using each definition of the north pole, and they're not always good about stating which convention they used. With New Horizons on the doorstep, we're going to need consistency for mapping and navigation. So I believe the mission has decided to use the current IAU definition consistently to avoid any confusion. There was a huge fight over the coordinate system of Vesta on the Dawn mission, and we don't want that.

Comment Re:Antipodal eruptions (Score 2) 78 78

I've never found the antipodal argument convincing. Seismic waves converge at the antipode of an impact only if the target is spherically symmetric and isotropic. In the actual Earth, you have reflections off all kinds of laterally varying boundaries. Also, the sound speed differs substantially between continental and oceanic crust, so the path matters quite a bit.

The Chixulub impact is also not that big (as planetary-scale impacts go). The projectile was what, 10 km? Shock heating is only significant within a few times the projectile diameter.

Top Ten Things Overheard At The ANSI C Draft Committee Meetings: (10) Sorry, but that's too useful.