I have read this news article and I wonder how Mars water is lost to space. Mars has mass and hence it has also got gravity to pull objects towards it like the Earth does. So how can water be lost to space? Does not Mars pull the water body towards it?
UV tends to crack H2O into hydrogen and oxygen and the hydrogen is easily lost to space. The oxygen is more massive and leaks out to space more slowly. The Earth is also losing gas to space, albeit so slowly that our atmosphere will not change much due to the effect before the Sun heats up enough to give Earth a runaway greenhouse effect.
Mars has mass and hence it has also got gravity to pull objects towards it like the Earth does. So how can water be lost to space?
At any specific temperature for a group of gas molecules, the state energy of any one gas molecule follows a Boltzmann distribution. Some are traveling very slow, most are at a median velocity, and some are traveling very fast. If the ones traveling very fast are traveling at greater than the escape velocity of the parent body, and don't run into something else that slows them down, they will likely be lost to space.
By the way, a grain of dust has a gravitational field, but you don't see any grains of dust with an atmosphere.
Wikipedia in its http://en.wikipedia.org/wiki/Mars_ocean_hypothesis says (I added links to the full text of papers):
Fate of the ocean
As the Martian climate cooled, the surface of the ocean would have frozen.
- One hypothesis states that part of the ocean remains in a frozen state buried beneath a thin layer of rock, debris, and dust on the flat northern plain Vastitas Borealis.[33 Janhunen, P., 2002: Are the northern plains of Mars a frozen ocean?, Journal of Geophysical Research, 107, 5103.]
- The water could have also been absorbed into the subsurface cryosphere[3 Clifford, S. M. and T. J. Parker, 2001: The Evolution of the Martian Hydrosphere: Implications for the Fate of a Primordial Ocean and the Current State of the Northern Plains, Icarus 154, 40-79, pii S0019103501966710.] or
- been lost to the atmosphere (by sublimation) and eventually to space through atmospheric sputtering.[26 Kass, D. M. and Y. L. Yung, 1995: Loss of atmosphere from Mars due to solar wind-induced sputtering, Science, 268, 697-699.]
So, water possibly migrated under the surface, or was "sputtered" - particles in upper atmosphere hit by solar wind and get accelerated enough to start into space (reach escape velocity):
According to The Planetary Air Leak (SciAm 2009) http://libserver.wlsh.tyc.edu.tw/sa/pdf.file/en/e088/e088p070.pdf (table on last page) Mars loses hydrogen by thermal methods, and Carbon, Oxygen, Nitrogen, Argon by nonthermal methods: photo-chemical and sputtering.
First of all, look at this picture: the phase diagram for water. Any water on the surface of Mars would be frozen. Under the influence of solar radiation, ice can undergo a process named sublimation: a direct transition from ice to water vapor (you can see this on cold winter days with the sun shining on a snow cover, which then very gradually thins). Of course Mars would attract the ice, and it also attracts the water vapour molecules resulting from sublimation. However, as Andrew Thompson points out here, some of these molecules, their speeds following the Boltzmann distribution, would escape into space. As solar radiation influx, in W/m², is much lower on Mars than on Earth, the sublimation process would also be slower. But in the end it would still happen: any surface water, frozen into ice, would slowly sublimate away.