A plan to drop a quarter-tonne copper ball through Mars鈥檚 atmosphere and study the ejecta it blasts away from the planet鈥檚 surface on impact is to be proposed to NASA.
The mission, called THOR, would test models suggesting the planet鈥檚 tilt 鈥� and therefore its climate 鈥� swings through extreme changes every 50,000 years.
Robotic landers and rovers have previously visited the Red Planet鈥檚 equatorial regions, and an upcoming mission called Phoenix is due to touch down near the north pole in 2008. But no probe has visited the planet鈥檚 mid-latitudes, where gullies and glacier-like features suggest there may be large amounts of pure water ice beneath a layer of dusty soil.
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Now, researchers led by Phil Christensen at Arizona State University in Tempe, US, are proposing a mission to search for that ice directly. The idea behind THOR (Tracing Habitability, Organics, and Resources) is to fly an observer spacecraft to Mars and, hours before it reaches the planet, release an 鈥渋mpactor鈥� ball. It could be up to 230 kilograms in mass and would be aimed at a region about 40掳 north or south of the equator.
The impactor, likely to be a giant copper sphere, would crash to the surface at more than 4 kilometres per second, blasting a crater about 10 metres deep. Copper is not found on Mars in large quantities 鈥� unlike iron 鈥� so when observers see it in the ejecta, they will know its from the ball and not the soil. And it is also 鈥渉eavy, dense, relatively inexpensive, and easy to work with鈥�, says Christensen. The observer spacecraft would record the impact from orbit, studying the composition of the ejected soil with spectrometers.
Pure snow
鈥淚t鈥檚 neat because it鈥檚 a brute force way to gain access to the subsurface of Mars,鈥� says David Spencer, a team member at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, US. 鈥淭he impactor will be very simple and we鈥檒l get our first look at material from that depth.鈥�
Christensen says that will provide a crucial test for models of Mars鈥檚 past climate. 鈥淭he climate models predict that as the orbit of Mars evolves, the tilt of its spin axis changes,鈥� he told New 杏吧原创. Over a 50,000-year timescale, the planet can tilt from 10掳 to 40掳 鈥� or anywhere in between, he says.
When the planet is tilted most drastically on its side, the planet鈥檚 poles receive a lot of sunshine. Any water locked in ice there is thought to vaporise and move towards the equator, where it . 鈥淭he climate can change dramatically and deposit as much as 10 or 20 metres of pretty much pure snow in the mid-latitudes,鈥� Christensen says.
Reconnaissance mission
THOR, named after the hammer-wielding Norse god of thunder, would prove this snow theory right if it measures mostly water in the plume of ejected material, he says. But if it reveals mostly dirt, that would lead researchers back to the drawing board, he says: 鈥淢aybe the idea of climate change, and tilt, and the deposition of snow [is wrong], or maybe not as much ice moves around.鈥�
The mission would also be able to detect organic compounds, such as methane, in the ejecta and the atmosphere. The compounds are intriguing because they might signal the presence of life, but THOR would not carry the instruments to prove it unequivocally.
鈥淚 view this as a reconnaissance mission to see if these regions are rich in ice,鈥� says Christensen. 鈥淎nd if they are, to use that as a rationale to go back to these regions with future rovers and explore them in more detail.鈥�
Christensen鈥檚 team will submit a preliminary proposal for the $450 million mission to NASA in July. If it is selected to fly, it could launch as soon as 2011.
