Scientific Background Information
  Mars is an exciting planet to study because the Martian surface features are easily visible from Earth-based telescopes.  As early as 1666, Italian Giovanni Cassini noted after watching Martian features that the rotational period of Mars is 24 hours and 37 minutes, almost the same as Earth's 24 hour day.  A century later, Englishman William Herschel (discoverer of Uranus) determined that Mars' rotational axis is tilted at 25.2 degrees, which is almost the same as Earth's 23.5 degree tilt.  Because the Martian year of 686 Earth days is almost two Earth years, Martian seasons are almost twice as long as Earth's seasons. 
 
Animated GIF of Martian Northern Polar Ice Cap courtesy of NASA
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Two major features on Mars are the polar caps.  Careful observation of the surface has revealed changes in the brightness and color of surface markings that correlate with the annual growth and shrinkage of the Martian polar cap.  The south polar cap shrinks more than the north polar cap because Mars is closer to the sun at the time of the southern hemisphere summer.  The caps recede very rapidly in late spring but retreat only gradually in the summer.  The rapid retreat of the polar cap indicates that the seasonal cap is covered with a very thin layer of ice, probably CO2 ice.  The permanent polar cap is a thick layer of mainly water ice (H20) that remains year round.  These regular changes in the polar caps and in the brightness and color of other visible features on Mars clearly indicate that Mars has seasons. 

Today, astronomers observing with the NASA Hubble Space Telescope study Mars in great detail.  NASA scientists have created a Quick Time rotation movie showing the rotation of Mars.  NASA scientists are also able to create a high-quality Quick Time time-lapse movie showing how the polar ice caps change over time. 

In October of 1996 the maximum extent of the northern ice cap extended down to almost 60 degrees latitude.  In March, 1997, the northern ice cap shrunk to a little more than 1000 km in diameter. 

Mars polar cap sequenceStudents might be very curious as to how a space telescope in near Earth-orbit can take pictures that seem to be taken from above the Martian north pole.  These images (Photo No.: STScI-PRC97-15b) were actually created by assembling combinations of three sets of images taken by HST in October 1996, and in January and March 1997, and projecting them to appear as they would if seen from above the pole.  

The far left picture, October 1996, was assembled from images obtained between Oct. 8 and 15.  The cap extends down to 60 degrees N latitude, nearly its maximum winter extent.  (The notches are areas where HST data were not available).  

The middle picture shows mid-spring in January 1997.  Increased warming as spring progresses in the northern hemisphere has sublimated the carbon dioxide ice and frost below 70 degrees north latitude.  Particularly obvious is the marked hexagonal shape of the polar cap at this season, noted previously by HST in 1995 and Mariner 9 in 1972; this may be due to the underlying topography, which isn't well understood. 

Finally, on the far right, is early summer, March, 1997.  The cap has fully retreated to its year-round cap of permanent water-ice.  This residual cap is actually almost split into two by a large, horn-shaped canyon called Chasma Borealis which is cut deeply into the polar terrain.  The darker circle marks the location of circumpolar dark sand dunes.  The bright circular features at 3, 6, and 9 o'clock are ice-filled craters. 

Extra Resources 

  • Hubble Space Telescope Science Institute (NASA STSci)
Cross Curricular Connections 
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