We're going to calculate how long it takes for dust to change the appearance of Mars' surface. Dust storms are quite common on Mars: can they change how the planet looks over a period of months? Decades?
From measurements of how red Mars' sky is - and how long it takes for the ground and sky to cool off at night - we can measure the size of a typical dust particle. This size is around 1 micron (1 10-6 m) -- much smaller than a sand grain.
a. What is the volume (in cm) of a dust particle on Mars?
b. Let's assume the average depth of dust on Mars is about 1 cm. Mars has a radius of 3400 km. What is its surface area? _________ cm2. What is the total volume of dust on Mars' surface? _________ cm3. Finally, how many dust grains are there on Mars? __________ grains.
c. A typical dust storm over the Sahara (on Earth) removes about 100 teragrams (ie, removes 1 1014 g) every year. Although Mars' atmosphere is much weaker than Earth's and can't lift up as much sand, the dust grains on Mars are smaller and the gravity weaker, so that about this same amount is lifted up every year on Mars.
How many years, at this rate, would it take for dust storms on Mars
to remove
d. Just like a very thin layer of chalk dust can change how a surface looks, so can a thin layer of dust. How long would it take to remove not 1 cm of dust from the surface, but 1 micron of dust? ______ yr.
What does this say about how quickly we can observe surface features
come and go on Mars? Should we expect large areas to disappear and
change color and appearance every year? (Hint: the answer should be
`yes' - and we do directly observe this change over periods of months
and years. Check out a
photo of one Martian dust storm.)
Last modified 3-Jul-2000