单选题 NASA is releasing several images taken by the Hubble Space Telescope, which is the closest yet look at the red planet. Altogether there are four images, which show the entire planet. Each view shows the planet as it completes one quarter of its daily rotation. In these views the north polar cap is turned toward the Earth and is clearly visible at the top of each picture. The images were taken in the middle of the Martian northern summer, when the polar cap was at its smallest size. During this season the sun shines continuously on the polar cap. Previous spacecraft observations have shown that this summertime polar cap is composed of water ice, just like Earth's polar caps.
The Hubble Telescope pictures reveal that great changes have occurred on the surface of Mars in the past 20 years. The Martian surface is ever changing. Some regions that were dark 20 years ago are now bright red; some areas that were bright red are now dark. Winds move sand and dust from region to region, often in huge dust storms. Over long timescales many of the larger bright and dark markings remain stable, but smaller details come and go as they are covered and then uncovered by sand and dust.

单选题 How is the image of the north polar cap of Mars?

单选题 What is the summertime polar cap composed of?

单选题 When were the images of Mars taken?

单选题 Which of the following is TRUE about the north polar cap of Mars in summertime?

单选题 What is the characteristic of the Martian surface according to observations over the past 20 years?