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10.5 Water and life on mars  (Page 3/8)

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Images taken from orbit also show a distinctive type of terrain surrounding the permanent polar caps, as shown in [link] . At latitudes above 80° in both hemispheres, the surface consists of recent layered deposits that cover the older cratered ground below. Individual layers are typically ten to a few tens of meters thick, marked by alternating light and dark bands of sediment. Probably the material in the polar deposits includes dust carried by wind from the equatorial regions of Mars.

What do these terraced layers tell us about Mars? Some cyclic process is depositing dust and ice over periods of time. The time scales represented by the polar layers are tens of thousands of years. Apparently the martian climate experiences periodic changes at intervals similar to those between ice ages on Earth. Calculations indicate that the causes are probably also similar: the gravitational pull of the other planets produces variations in Mars’ orbit and tilt as the great clockwork of the solar system goes through its paces.

The Phoenix spacecraft landed near the north polar cap in summer ( [link] ). Controllers knew that it would not be able to survive a polar winter, but directly measuring the characteristics of the polar region was deemed important enough to send a dedicated mission. The most exciting discovery came when the spacecraft tried to dig a shallow trench under the spacecraft. When the overlying dust was stripped off, they saw bright white material, apparently some kind of ice. From the way this ice sublimated over the next few days, it was clear that it was frozen water.

Evaporating ice on mars.

Evaporating ice on Mars. This image has two panels, and each of them show the same little “trench” excavated by the Phoenix lander. On the left, in the lower left portion of the rectangular trench, three gray spots can be discerned. The scale at lower left reads 1.7 c m. In the right hand image, taken four days later, the three spots have vanished.
We see a trench dug by the Phoenix lander in the north polar region four martian days apart in June 2008. If you look at the shadowed region in the bottom left of the trench, you can see three spots of ice in the left image which have sublimated away in the right image. (credit: modification of work by NASA/JPL-Caltech/University of Arizona/Texas A&M University)

Comparing the amount of water on mars and earth

It is interesting to estimate the amount of water (in the form of ice) on Mars and to compare this with the amount of water on Earth. In each case, we can find the total volume of a layer on a sphere by multiplying the area of the sphere (4π R 2 ) by the thickness of the layer. For Earth, the ocean water is equivalent to a layer 3 km thick spread over the entire planet, and the radius of Earth is 6.378 × 10 6 m (see Appendix F ). For Mars, most of the water we are sure of is in the form of ice near the poles. We can calculate the amount of ice in one of the residual polar caps if it is (for example) 2 km thick and has a radius of 400 km (the area of a circle is π R 2 ).

Solution

The volume of Earth’s water is therefore the area 4π R 2

4 π ( 6.378 × 10 6 m ) 2 = 5.1 × 10 14 m 2

multiplied by the thickness of 3000 m:

5.1 × 10 14 m 2 × 3000 m = 1.5 × 10 18 m 3

This gives 1.5 × 10 18 m 3 of water. Since water has a density of 1 ton per cubic meter (1000 kg/m 3 ), we can calculate the mass:

1.5 × 10 18 m 3 × 1 ton/m 3 = 1.5 × 10 18 tons

For Mars, the ice doesn’t cover the whole planet, only the caps; the polar cap area is

π R 2 = π ( 4 × 10 5 m ) 2 = 5 × 10 11 m 2

(Note that we converted kilometers to meters.)

The volume = area × height, so we have:

( 2 × 10 3 m ) ( 5 × 10 11 m 2 ) = 1 × 10 15 m 3 = 10 15 m 3

Therefore, the mass is:

10 15 m 3 × 1 ton/m 3 = 10 15 tons

This is about 0.1% that of Earth’s oceans.

Check your learning

A better comparison might be to compare the amount of ice in the Mars polar ice caps to the amount of ice in the Greenland ice sheet on Earth, which has been estimated as 2.85 × 10 15 m 3 . How does this compare with the ice on Mars?

Answer:

The Greenland ice sheet has about 2.85 times as much ice as in the polar ice caps on Mars. They are about the same to the nearest power of 10.

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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