Q:

Is SSTO viable on Mars?

I know that Single-Stage-to-Orbit is quite challenging from the surface of Earth, but it's relatively straightforward from the moon (as demonstrated by the Apollo LM). What about Mars? Let's assume in-situ fueling of methane-LOX. 

A:

It's definitely viable. By the rocket equation:
$∆v = v_e \ln \frac {m_0} {m_f}$
Methane-LOX gives an exhaust velocity around 3500m/s. Mars surface to orbit requires the expenditure of about 3800 m/s of ∆v. 
So the ratio of initial (fully fueled) mass to final (fuel spent) mass need only be around:
$e^{\frac {3800} {3500}}$
or
$e^{1.09}$
or about 3:1 -- 2 tons of propellant per 1 ton of empty ship/payload. 
This is as compared to an Earth-launched SSTO, which needs about 9500 m/s, about a 15:1 mass ratio for methalox!

