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Thursday, 9 March, 2000, 09:45 GMT
Martian poles like cheese

Nasa/JPL/Malin Space Science Systems
Carbon dioxide could have played a role in the creation of the "Swiss cheese"
What's the difference between the northern and southern poles on Mars? Well, according to Nasa scientists the answer can be found in the fridge.

New high-resolution images from the Mars Global Surveyor (MGS) spacecraft comparing the two ice caps show the difference between the two regions is in the "cheese".

The north polar cap has a relatively flat, pitted surface that resembles cottage cheese. The south polar cap, on the other hand, has larger pits, troughs and flat mesas that give it the appearance of Swiss cheese.

"Looking like pieces of sliced and broken Swiss cheese, the upper layer of the Martian south polar residual cap has been eroded, leaving flat-topped mesas into which are set circular depressions," said Dr Peter Thomas of New York's Cornell University.

"Nothing like this has ever been seen anywhere on Mars except within the south polar cap, leading to some speculation that these landforms may have something to do with the carbon dioxide thought to be frozen in the south polar region."

Unusual shapes

Research based on the new images is published in the journal Nature. In the paper, scientists examine what lies behind these unusual, cheese-like features.

Nasa/JPL/Malin Space Science Systems
North polar cap: The image covers an area 1.5 km wide by 3 km long
"The unusual shapes of the landforms on the north and south polar caps suggest that these regions have had different climates and histories for thousands or perhaps even millions of years," Dr Thomas said.

He described the research as one of a series of steps necessary to connect the present day climate of Mars with the geological features seen on the surface.

"We know the climate has been doing something - regularly changing - but we don't known how fast," said Dr Thomas.

By connecting the geology to the climate, scientists may be able to determine what is controlling climate change on Mars, if the planet ever had a climate that could have sustained life, and if it did not, why not.

"It doesn't answer all the questions all at once but it gives another piece to the puzzle of what the atmosphere does, how warm it is, how much water it has, what the winds are."

Water vapour

The circular features seen on the southern cap are depressions. The largest mesas are about four metres (13 feet) high and may be composed of frozen carbon dioxide.

The pits that have developed on the north polar cap surface are much more closely spaced than those in the south polar cap. The pits are estimated from the length of shadows cast in them to be less than two metres (5.5 feet) deep.

Based upon observations made by the Mariner Nine and Viking orbiters in the 1970s, the north polar cap is thought to contain mostly water ice. Its summer temperature is usually near the freezing point of water and water vapour was observed by the Viking craft to be steaming off the cap during summer.

The southern cap, however, is different. Its temperatures in summer remain cold enough to freeze carbon dioxide, and little or no water vapour has been observed to come off the cap in summer.

The southern cap is six kilometres (3.5 miles) higher than the north, and this could cause additional carbon-dioxide ice formation in the Martian winter.

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See also:

03 Nov 99 |  Sci/Tech
The shadow of a Martian moon
04 Oct 99 |  Sci/Tech
Hopes of Mars oceans dry up
28 May 99 |  Sci/Tech
Mars in sharper focus
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