OK, thinking time. Let's see if we can do the calculation that +Brooks Moses
' comment implies we're going to need: find the rate at which a lump of degenerate neutron matter deposits energy into ordinary atomic matter, a.k.a. dirt.
As a baseline, when normal matter hits normal matter, the main reaction is just the repulsion of electrons in the valence shells of the respective atoms. These electrons are basically smeared out over the entire impact surface, so the effective cross-sectional area is equal to the geometric area of the impact. That forms nice vibrational waves which cart the energy off in the form of sound waves moving through the earth.
For degenerate neutron matter, though, the reactions are instead going to be
nN -> nN (elastic collisions of neutrons from Mjölnir against nucleons from the dirt)
ne -> pbar nu (electron capture by the neutrons from Mjölnir against the electrons from the dirt)
But the second reaction is weak-mediated, so it's going to be tiny compared to the electromagnetic/strong nucleon scattering. (NB that this scattering gets a big EM component, even though neutrons are electrically neutral, because their component quarks aren't) So we can treat this as a straight-up nuclear scattering problem.
The Earth's crust is about 47% Oxygen, 28% Silicon, and the rest miscellaneous other stuff. The O is almost entirely 16O and the Si 28Si. Their neutron scattering cross-sections are 4.29 and 2.12 barns, respectively, so we can estimate that the average cross-section of a terrestrial nucleon is going to be about 3.1 barns.
These are thermal nucleon cross-sections, which is good because the neutrons are coming in at very nonrelativistic speeds, but realistically degenerate neutron matter behaves pretty much nothing like free neutrons. (We're assuming here, like Tyson, that Mjölnir is made out of the free neutron drip part of a neutron star, despite the obvious questions of what would hold that together against a rather spectacularly rapid weak decay. I'm going to assume that it was bound that way by some particularly crafty Svártalfar technology, or maybe it's been wrapped in the hide of the world-serpent from a previous incarnation of Midgard by Odin. That sounds like the sort of thing he would do.) I am not a neutron star astrophysicist so I don't have a good guess off the top of my head about how this would interact. But since it is a degenerate gas, I would expect that the effective radius of each neutron would grow to be roughly the size of the hammer as a whole, basically a continuum of nuclear matter rather than individual nucleons. Fortunately, the density of the stuff roughly accounts for that, so I'm going to wave my hands in the air and assume this doesn't matter.
OK, so we have two intersecting columns of nucleons, in (as previously supposed) a circle of diameter 0.5m, one at a density of roughly 1.2*10^3 kg/m^3 (the density of dirt, which if we guess that the average nucleon has a mass of about 30u translates to 2.5*10^28 nucleons/m^3) and one at a density of about 2*10^14 kg/m^3 (or 1.2*10^41 neutrons/m^3). Each matter nucleon has an effective cross-sectional area of 3.1 barns.
The number of scattering events per unit length of dirt is therefore sigma*rho(dirt)*rho(neutrons)*volume(Mjölnir), where the rho's are number densities of nucleons, or 6.1*10^40 / m. (Using standard formulae for the definition of cross-section; I yoinked this out of Peskin & Schroeder, eqn 4.59, because I couldn't remember it. Too many years doing CS instead of physics.)
Since this is a nonrelativistic elastic collision, we can easily estimate the momentum transfer per collision as
delta p = 2mMv/(m+M),
where m and M are the masses of the neutron and the nucleus, and v is the impact velocity. Over a period of time (dt), the total number of collisions will be X*v*dt, where X is the number of scattering events per unit length computed above, and so the total momentum transfer will be
dp = 2mMv^2Xdt / (M+m)
and so the force is simply dp/dt = 2mMXv^2/(m+M). Plugging in those numbers, that translates to
F = (1.96*10^14 kg/m) * v^2
At the moment of impact, that translates to 3.9*10^16N, which in turn should translate to a deceleration of the bolide at 187 m/s^2, enough to bring it to a halt within 75msec.
This is comparable to the stopping time of ordinary matter against ordinary matter, so I conclude that the degenerate nature of the neutron matter does not, in fact, affect the original calculation.