At http://phys.org/print327069680.html … comet Siding Spring, the other comet of interest this autumn, will pass by Mars in October, just 132,000km away – which is the equivalent of a comet passing the Earth at one third the distance away from the Moon. A spokesman is quoted, 'we hope to witness two atmospheres colliding'. Planets have atmospheres (Mars is on the thin side) but so do comets, he claims – only it is called a coma and is capable of stretching to the width of Jupiter. In other words, they do not have what is normally recognised as an atmosphere, but a coma composed of dust and gases that tends to get stretched after contact with the solar wind. What they are hoping for, then, is for the coma of Siding Spring to sweep across the puny atmosphere of Mars, just to see what happens – and don't we all.
Meanwhile, to stir the broth, NASA has a spacecraft, the MAVEN mission, due to arrive in orbit around Mars, in September, a month before the close encounter with the comet. This was totally unplanned as the comet was a complete unknown when the mission was being brought together. MAVEN is designed to study the atmosphere of Mars – why it lost as much of it in the past, or even if it had a larger atmosphere in the first instance. It was also designed to study external stimuli affecting the atmosphere – the solar wind. Now, they have the added pleasure of a comet encounter.
NASA are fiendishly clever. Not only will they have three spacecraft orbiting Mars in October they will manoevre them into a position, placing the body of Mars between the spacecraft and the comet (when it is thought to be offloading lots of dust and debris which might damage them).
See also http://tallbloke.wordpress.com/2014/08/13/what-happened-to-the-atmospher… … which reports on the PhysOrg posting. The comments are interesting and the first one invokes the Expanding Earth theory to account for the lack of atmosphere on Mars. In another one a commenter claims the Earth is shrinking and a third person debunks mainstream ideas about the small atmosphere – taking the Michael out of the Wiki entry.