Could water molecules from Mars come to Earth?
The High Resolution Stereo Camera (HRSC) on board ESA's Mars Express orbiter has been delivering high-resolution images from the surface of Mars since January 2004 – in colour and 3-D. A monthly selection of these images is published on the dedicated page that covers the Mars Express mission. Quite often, we receive questions about these images and the geological peculiarities they reveal. Other questions are simply about Mars in general. In this blog post, Ulrich Köhler from the DLR Institute of Planetary Research provides an answer to a particularly interesting question.##markend##
Question: Is it possible that if the atmosphere enveloping Mars were to collapse, gaseous water would reach Earth due to gravitational pull? (Posted on the DLR Facebook page.)
In principle, yes. However, there are a couple of limitations. Firstly, it is not entirely clear what is meant by the 'collapse of the atmosphere enveloping Mars'. Although the current Martian atmosphere is substantially thinner than Earth's (1/100 to 1/150 of the gas pressure found in Earth's atmosphere, depending on altitude), it remains relatively stable. A process that could trigger something similar to a 'collapse' in the Martian atmosphere is a collision with an extremely large asteroid that hurtles towards the planet, passes through its atmosphere and impacts the surface. As it travels through the atmosphere, the asteroid would tear a hole, which would then be enlarged by the processes unfolding after impact itself. One could say that an impact like this, which would occur at a speed of several tens of thousands of kilometres per hour, would 'erode' the atmosphere at its point of entry. Complete collapse of the Martian atmosphere would require a collision with an asteroid measuring perhaps 500 to 1000 kilometres across. However, a definitive answer would require more precise modelling.
Although it would be thinner, the atmosphere would not disappear entirely, even after a massive collision with this kind of asteroid. At the moment, the volume of gases that are produced beneath the surface of Mars and fed into the atmosphere via volcanoes would be insufficient to replace the quantity of gas lost as a consequence of this kind of incident. This would make the atmosphere even thinner than it was before impact. First of all, though, we are unaware of any asteroid of this magnitude currently on a collision course with Mars. Secondly, there are no processes underway that could lead to a collapse in the Martian atmosphere.
Nevertheless, if an incident of this kind were to eject water molecules into space, once they have acquired 'escape velocity', they would no longer be influenced by the gravitational pull of Mars. Therefore, theoretically, these molecules could drift further into the Solar System and at some stage might be captured by Earth's gravitational field. The majority of this 'Martian water' would originate from the surface of Mars (ice deposits at the north and south poles), or from layers of ice beneath the planet's surface. Today’s Martian atmosphere contains just 210 parts per million water molecules, which is negligible. So initially, it would be molecules, atoms or ions of carbon dioxide (96 percent), nitrogen (1.9 percent, argon (1.9 percent), oxygen (0.15 percent) and carbon monoxide (0.06 percent) – in descending order of their relative contribution to the Martian atmosphere – that would arrive before water molecules.
If this transfer of water molecules from Mars to Earth did indeed occur, it would be possible, in theory at least, to identify them as 'Martian', based on their slightly different isotope composition (the relative proportions of different 'types' of hydrogen and oxygen atoms in the water molecules). However, given that it is likely to be very, very, very few atoms, any kind of identification using current analysis techniques is probably impossible.
The situation with tiny gaseous inclusions found in meteorites is slightly different. Some meteorites have been found that must have originated from Mars, as they contain small pockets of gas in which the noble gas argon is found in similar concentrations to those that orbiters have measured in the Martian atmosphere. This is considered definite proof of their origin.