Supposedly by some physics estimate of the Mars core still offers
1727°C, although there's no apparent surface measurement of such an
extent of geothermal energy leaving Mars at any great pace (perhaps
less than one mw/m2), so if its core is actually that hot it must be
extremely well insulated.

"The martian geothermal heat flux (q) has never been measured"
http://epubl.ltu.se/1402-1757/2006/4...IC-0648-SE.pdf
Most if not all of the geothermal heat flux figures for Mars are those
of SWAG estimates via remote science and applied physics math, as
providing perfectly valid conjectures based upon Mars being
essentially the same age as Earth and of X average density, of which
may or may not be the case. Of course the geothermal heat flux of our
moon hasn't been directly measured either, so what's the difference.

Who would be all that surprised if Mars were old enough and thus
becoming ice to the core?
- Brad Guth