杏吧原创s working with images from the Mars Phoenix mission are baffled by an unexpected difference between what they thought they would see and what Phoenix is now showing them.
The difference suggests that the northern plains of Mars may be a more complicated and active environment than previously imagined.
Phoenix landed on Monday at 0053 GMT (1643 PDT on Sunday). Since then it has been relaying images and data back to Earth via the Mars Odyssey orbiter, which periodically passes over the landing site.
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Among the most spectacular images thus far is a colour mosaic of the terrain looking out from the northward side of the lander. It clearly shows the so-called 鈥減olygons鈥� that are typical features of this region.
There鈥檚 just one problem: the polygons are too small.
Size surprise
During a press briefing yesterday, principal investigator of the University of Arizona, Tucson, US, said that preliminary estimates suggest the polygons in the foreground of the image are about 1.5 to 2.5 metres across.
This is much smaller than scientists estimated based on overhead views from the Mars Reconnaissance Orbiter and on models of the Martian climate.
鈥淚 think it means that there are polygons within polygons within polygons,鈥� said Smith. 鈥淎t different climate times there may have been big polygons and at other times there may be small polygons, and it just may go back and forth over time.鈥�
Polygons are produced by seasonal expansion and contraction of ground ice. When the ice is very cold it contracts and fractures in geometric patterns, much like mud cracks in the desert.
Ice depth
The fractures may only be a few millimetres across and are not visible on the surface. However, if they fill in with material, such as dry dust in the case of Mars, the ground ice will have no room to expand again when the temperature rises. As a result, the ground ice buckles, producing low mounds at the centres of polygons, and shallow troughs at the boundaries.
The potential for using polygon sizes to investigate the Martian climate is something that mission scientists have been preparing for since long before Phoenix was launched. 鈥淲e knew they would be there, but we didn鈥檛 know what they would look like鈥� says , a Phoenix mission team member at the University of Colorado.
According to Mellon, the size of these polygons should be affected by the thickness of the soil layer above the ground ice and also by the climate itself. In general, the nearer ground ice is to the surface, the more it is subjected to temperature extremes and the more fractured it becomes, which leads to smaller polygons.
Active troughs
Deeper ground ice makes bigger polygons. On Mars, if the ice is more than about 30 centimetres below ground, the polygons should disappear altogether. A colder climate also affects the size of the polygons by making ice fracture more readily at greater depths.
One feature that has generated excitement during the first day since touchdown is the small polygon seen nearest to the lander in the first colour 鈥減ostcard鈥� from Phoenix (see image, top right). The upper right-hand side of this diamond-shaped polygon is extremely sharp edged, creating a dark furrow in the soil.
鈥淭hat means it could be very recent,鈥� said Smith, because a geologically old feature would likely have been softened and filled in by wind-blown dust long before now. 鈥淲e think these are active troughs,鈥� says Mellon.
If the polygons are 鈥渁ctive鈥�, it means they are currently expanding and contracting with some regularity. In the process, they may be churning up soil and rocks through a process called cryoturbation. By looking at the spacing between clusters of rocks across the surface it is possible to get a sense of whether polygon sizes, and therefore climatic conditions, have been changing on Mars in the recent past.
For example, a warmer climate would have driven the ground ice to lower depths and produced larger polygons. Polygons sizes can also point out local variations in soil composition and insolating properties.
Local anomaly?
Meanwhile, Mellon and his colleagues are pondering what appears to be a geoclimatic mystery. Polygons on Earth tend to be big, measuring 15 to 20 metres across. On Mars, based on the best available knowledge of conditions in the northern plains, Mellon calculates that the polygons should be, on average, 5 metres across. They are not.
鈥淚t has certainly got our brains turning,鈥� says Mellon.
The simplest explanation is that the first images sent back by Phoenix show a collection of atypically small polygons. A view in another direction may well show the 5-metre polygons that Mellon and other scientists were expecting.
If that proves not to be the case, however, it means that scientists are missing a key piece of information about the northern plains 鈥� a piece they hope that Phoenix will eventually provide.
Mars Rovers 鈥� Mars is full of surprises, learn more in our continually updated .
