An analysis of data returned by the Curiosity rover, which landed on the planet a year ago, suggests there was a major upheaval which could have been caused by volcanic eruptions or a massive collision which stripped away the atmosphere.

The rover has returned its first measurements of the makeup of gases, including argon, oxygen, nitrogen, carbon monoxide and carbon dioxide, in the Martian atmosphere.

The results, published in two parallel studies in the journal Science, allow scientists to better understand how the Martian climate changed, and understand whether it ever had the right conditions for life.

Dr Chris Webster at Nasa's Jet Propulsion Laboratory in Pasadena, lead author on one of the studies, said the data enabled direct comparisons with the Earth's climate.

"As Mars became a planet and its magma solidified, catastrophic outgassing occurred while volatiles were delivered by impact of comets and other small bodies", Dr Webster said.

"Our Curiosity measurements are - for the first time - accurate enough to make direct comparisons with measurements done on Earth on meteorites using sophisticated large instrumentation that gives high accuracy results."

The team believe a major event destroying the atmosphere must have happened around four billion years ago.

The different ratio of two forms of the gas argon on Mars and Earth suggests some huge event changed their relative amounts, the scientists said.

Monica Grady, professor of planetary sciences at The Open University, who did not write the studies, told The Guardian: "It's really great that two separate studies using different instruments and techniques have given the same composition.

"These findings reverse the results from the Phoenix mission and clear up some confusion over the composition of the Martian atmosphere."

According to a study of rock samples published last month, Mars had an oxygen-rich atmosphere more than a billion years before Earth.

Rocks collected from the surface of the Gusev crater by Nasa's Spirit rover were found to contain five times as much nickel as Martian meteorites found on Earth.

This suggests that the surface rocks, which are at least 3.7 billion years old, formed in an oxygen-rich environment while the meteorites, aged between 180 million and 1.4 billion years, did not.

Dr Paul Mahaffy from Nasa's Goddard Space Flight Centre, who was the lead author on the other paper, said further analysis needed to be done before humans could be put on Mars.

"From a practical standpoint we need to know the composition [of the atmosphere] today, and how it is changing, so that we can prepare for the eventual arrival of human explorers."

i wonder if it is just not big enough to hold an atmosphere for a long time?

Graham Hancock wrote a fascinating book on Mars back in 1998, entitled The Mars Mystery, in which he examined the NASA Mars Global Surveyor mission and many of the anomalies that NASA scientists have discovered about Mars including the lack of atmosphere and its geography including the laborious efforts to identify what to many are Megalithic structures on the shores of a long dried up sea shore at Cydonia including the Sphinx-like structure and the huge efforts by scientists to clear those images up, along with evidence of a once lush environment that may only have been lost several thousand years ago and not millions as alleged by NASA. It is a very compelling read.

from wiki:
The atmospheric pressure on the Martian surface averages 600 pascals (0.087 psi), about 0.6% of Earth's mean sea level pressure of 101.3 kilopascals (14.69 psi) and only .0065% that of Venus's 9.2 megapascals (1,330 psi). It ranges from a low of 30 pascals (0.0044 psi) on Olympus Mons's peak to over 1,155 pascals (0.1675 psi) in the depths of Hellas Planitia. Mars's atmospheric mass of 25 teratonnes compares to Earth's 5148 teratonnes with a scale height of about 11 kilometres (6.8 mi) versus Earth's 7 kilometres (4.3 mi). The Martian atmosphere is about 95% carbon dioxide, 3% nitrogen, 1.6% argon, and traces of free oxygen, carbon monoxide, water and methane, among other gases, for a mean molar mass of 43.34 g/mol.[1][5] The atmosphere is quite dusty, giving the Martian sky a light brown or orange color when seen from the surface; data from the Mars Exploration Rovers indicate that suspended dust particles within the atmosphere are roughly 1.5 micrometres across.[6]

so mars could support some kind of baterial/plant life with the carbon dioxde levels, but mars is about a third the size of earth...
so i'm not sure it could ever have enough air to support animal life as we know it...

I know a man here in town who is so huge and fat, he has his own atmosphere!

i think he has a couple of brothers and sisters here in Houston...