I propose two Principle of Least Action explanations, BROADSIDE TRAVEL
and SPIN RE-ADJUSTMENT, of why huge, super huge impact centres are so
often polar in paper 4.2 of my ebook at www.nodrift.com :

"BROADSIDE TRAVEL
The 1st mechanism is implied by C Johnston's explanation of a
longstanding, regular ~26 myr impact cycle, at
http://mb-soft.com/public/extinct.html  , which I favour in w.2, 3.1.

        The orientation of the plane of the Solar System would tend to be
"broadside" to the direction of its Johnstonian oscillation, across
the width of the revolving Galactic System's spiral arm, the most
likely direction of impactors . . ..
        Planetary spin axes tend to point in this same direction,
perpendicular to the plane of the Solar System, so the spin poles are
most likely places to be impacted, impact antipoded.

SPIN RE-ADJUSTMENT
Spin re-adjustment follows the path of Least Action to minimize wobble,
caused by impact tectonogenetic uplift imbalance between impacts and
their less energised antipodes . . ."

In paper w.2, I refer to "Cassini spacecraft photos of Saturn's
"walnut-shaped" moon Iapetus . . . at:
http://www.nasa.gov/mission_pages/cassini/multimedia/pia06169.html
http://ciclops.lpl.arizona.edu/view.php?id=706

I propose that the "equatorial" ridge of the "walnut" is a
classic example of a global bisectional faultline manifestation
(GLOBIF), produced by a polar super huge impact upon this dry rocky
moon of Saturn.
This explanation is corroborated by the fact that the ridge is roughly
equatorial. Huge, super huge planetary impacts are often polar . . .

HEMISPHERIC DICHOTOMIES
I further propose that planetary hemispheric dichotomies, such as on
Mars, are the effect of the same process on an oceanic rocky planet,
when the impact hemisphere has low relief and is mostly icy or oceanic.
The Martian hemispheric dichotomy is thus seen as due to a Northern
Hemispheric super huge impact (w.1a, Slides 23-30) upon an earlier, wet
or icy, low relief Mars.

MARS, EARTH, MOON
Mars' Northern hemisphere has been depressed by a Freeze/Foraze
Effect, y.3, much as Earth's Impact Hemisphere (IH) has been
similarly depressed at Pacific, Arctic, North Atlantic Ocean impacts,
the Moon's poles at and antipodal to impact, w.1a, Slide 31 . . .

CUMULATIVE DICHOTOMIES
Earth's octo-, hexadeci-chotomous rhythms of my protothesis, this
ebook's Vols 3, 4, is consistent with  cumulative hemispheric
dichotomy, ( ½ )2.( ½ )2.( ½ )2.( ½ )2 . . . genesis.

OTHERWISE INEXPLICABLE
The Martian hemispheric dichotomy, Iapetus', Earth's GLOBIFS,
Earth, Moon and Mars' polar uplifts/depressions are otherwise
inexplicable."