  We present ALMA multiwavelength observations of the protoplanetary disk
around the nearby (d$\sim$100 pc) young solar analog MP Mus (PDS 66). These
observations at 0.89 mm, 1.3 mm, and 2.2 mm have angular resolutions of $\sim$
1", 0.05", and 0.25", respectively, and probe the dust and gas in the system
with unprecedented detail and sensitivity. The disk appears smooth down to the
4 au resolution of the 1.3 mm observations, in contrast with most disks
observed at comparable spatial scales. The dust disk has a radius of 60$\pm$5
au, a dust mass of $0.14_{-0.06}^{+0.11} M_{\rm Jup}$, and a mm spectral index
$<2$ in the inner 30 au, suggesting optically thick emission from grains with
high albedo in this region. Several molecular gas lines are also detected
extending up to 130$\pm$15 au, similar to small grains traced by scattered
light observations. Comparing the fluxes of different CO isotopologues with
previous models yields a gas mass of $0.1-1 M_{\rm Jup}$, implying a gas to
dust ratio of 1-10. We also measure a dynamical stellar mass of $M_{\rm
dyn}$=1.30$\pm$0.08 $M_\odot$ and derive an age of 7-10 Myr for the system. The
survival of large grains in an evolved disk without gaps/rings is surprising,
and it is possible that existing substructures remain undetected due to
optically thick emission at 1.3 mm. Alternatively, small structures may still
remain unresolved with the current observations. Based on simple scaling
relations for gap-opening planets and gap widths, this lack of substructures
places upper limits to the masses of planets in the disk as low as 2
$M_\oplus$-0.06 $M_{\rm Jup}$ at $r > 40$ au. The lack of mm emission at radii
$r > 60$ au also suggests that the gap in scattered light between 30-80 au is
likely not a gap in the disk density, but a shadow cast by a puffed-up inner
disk.
