Building a Martian Phobos is tidally locked to Mars (as the Moon is to Earth), where the same side of the moon stays facing the planet. A space elevator could extend down from Phobos to Mars 6,000 km, about 28 kilometers from the surface, and just out of the atmosphere of Mars. A similar space elevator cable could extend out 6,000 km the opposite direction that would counterbalance Phobos. In total the space elevator would extend out over 12,000 km which would be below Areostationary orbit of Mars (17,032 km).
A rocket launch would still be needed to get the rocket and cargo to the beginning of the space elevator 28 km above the surface. The surface of Mars is rotating at 0.25 km/s at the equator and the bottom of the space elevator would be rotating around Mars at 0.77 km/s, so only 0.52 km/s of Delta-v would be needed to get to the space elevator. Phobos orbits at 2.15 km/s and the outer most part of the space elevator would rotate around Mars at 3.52 km/s.
In our speculative Mars Colonization Timeline we have estimated such a Phobos-based space elevator could be built as soon as in 2080s–2090s. In adition to the space elevator there could be built a shuttle port at the summit of Pavonis Mons – the Martian volcano practically on the equator – for shuttles heading to and coming from Phobos space elevator. The summit of Pavonis Mons is standing 14 km above Mars' mean surface level (way above the denser part of the atmosphere), halving the needed trip to the shuttle platform at space elavator's lower tip 28 km above the surface.
