Touma and Wisdom 1993, Science, 259, 1294
A final thought about the Earth's habitability:
It is known that Mars' obliquity -- angle
of rotation from the orbital plane -- varies
chaotically:
Touma and Wisdom 1993, Science, 259, 1294
The Earth's obliquity does NOT vary because the
Moon acts as an anchor, holding the obliquity
more or less constant.
Imagine what the effect on climate -- and therefore
on life -- might be if the Earth's obliquity varied
as much as Mars' does!