Does the axial tilt of Mercury and Venus vary? The axial tilt of Mars varies greatly over time, with a tilt varying between 15 and 25° over the last million years, and varying even more over the last 5 million years. The Earth's tilt on the other hand is stabilized by the presence of a large Moon, so its tilt doesn't vary so much.
Venus and Mercury don't have any moons at all, so does their axial tilt also vary a lot over time? If not, why not? Do we have enough data to know how much their axial tilt has varied over time?
There is almost no data available on the internet as such.
Though, the theory that they (Mercury and Venus) are stabilised by the Sun's Tidal Force seems very likely as they are much closer to the sun than any other planet. The Sun very easily dwarfs these planets in both size and gravitational and tidal attraction.
I found this Wikipedia page that I think might be useful : https://en.wikipedia.org/wiki/Axial_tilt
It says that it 'might' be that they are stabilised due to the Sun.
But this article I found changed my mind : http://hosting.astro.cornell.edu/~jlm/publications/2005I09374.pdf
It clearly says that Mercury's obliquity is not consistent with time. It also contains other info that makes it must-read if you are interested in this topic.
And although I couldn't find anything convincing on the subject of the obliquity of Venus,  I found this one article that seemed quite interesting :
http://articles.adsabs.harvard.edu/cgi-bin/t2png?bg=%23FFFFFF&amp;/seri/AJ.../0075/600/0000273.000&amp;db_key=AST&amp;bits=4&amp;res=100&amp;filetype=.gif
Venus' obliquity is 177.4 degrees, to be exact, according to this : http://solarviews.com/cap/misc/obliquity.htm
It also displays the approximated obliquity of other planets in the Solar System.