Quote

The air on Mars — what there is of it —
> is leaking away, about half a pound a second
> sputtering into space, scientists announced on
> Thursday.
>
> The planet’s early atmosphere is thought to have
> been as thick as or thicker than Earth’s today,
> and even over the 4.5-billion-year history of the
> solar system, that slow leak would not explain how
> it atrophied to its current wisps.
>
> But new readings from NASA’s Mars Atmosphere and
> Volatile Evolution mission — Maven, for short
> — show that when Mars is hit by a solar storm,
> the ferocious bombardment of particles from the
> sun strips away the upper atmosphere much more
> quickly.
>
> That could help explain the disappearance of the
> atmosphere. The sun during its youth was more
> unsettled, with many more solar storm eruptions,
> and it shone brighter in the ultraviolet
> wavelengths that also help knock atoms out of
> Mars’ atmosphere.

Quote

Such solar storms were stronger and more
> frequent about 4 billion years ago, and the sun's
> emissions in extreme ultraviolet (UV) light were
> more powerful back then as well, he added. The
> solar wind — the flow of charged particles from
> the sun, which is a major driver of Martian
> atmosphere loss today — was also more potent
> during the sun's youth.
>
> "All of these [factors] point to the loss of the
> Martian atmosphere in the earliest stages,"
> Jakosky said.
>
> This all happened right after Mars lost its
> global magnetic field, which had protected the
> planet's air against solar-driven stripping, he
> added. (Earth, which is much larger than Mars,
> retains a global field to this day.)
>
> That stripping would then have proceeded very
> quickly, "within a few hundred million years after
> the shutoff of the magnetic field," Jakosky
> said.