Here's how the gravity data works: as a spacecraft orbits Mars, its orbit gets tugged from time to time by dense concentrations of mass below it. (A prominent example would be that Martian volcanic region, Tharsis, but smaller mass concentrations also exist.) As the spacecraft's orbit moves, its transmission time to Earth is ever-slightly affected. Scientists can then estimate mass concentrations by mapping the Doppler shift, or a change in the frequency of the radio waves sending the spacecraft's information to Earth.
Ojha's team did consider other ideas for the deposit. Some suggested that perhaps one volcanic eruption did all the work, but Ojha said it would have to a "diabolical volcano" because the eruption force would be at a magnitude of 11; the highest ever recorded on Earth is only an 8 or a 9.
Others thought the volcanic ash, which hardened into rock over the eons, was actually a dust deposit, but density and volume calculations show there would have to be enough dust deposited there to cover the entire surface of Mars in a layer of dust 10 meters (32 feet) deep. "There also would need to be a cementing agent so the dust particles are stuck to each other; you'd need water [deposit] after water after water to hold all this dust together," Ojha said.
Ojha's team has another paper coming out about the volcanic deposit that is currently under embargo. He also points out that studies of Medusae Fossae provide more information about an important transition time in Martian history, when the planet was losing its water and was undergoing massive volcanic eruptions. So expect to see more work here in the near future.