Based on telescopic observations, it is easy to see that the maria (dark regions) on the Moon have far fewer craters than the highlands (bright regions). This led scientists to believe that _____.

Saturn’s moon Titan is unique because it has _____ composed mostly of _____.

Because they are the closest planets to the Sun, Mercury and Venus are generally easiest to observe _____.

The summer in the southern hemisphere on Mars is hotter than in the northern hemisphere because during summer in the southern hemisphere _____.

The huge “Valles Marineris” (Mariner Valley) on Mars was formed by _____. The Grand Canyon on Earth was formed by _____.

Active water outflows on Mars are observed _____.

Water ice is able to survive near the _____ on the Moon and Mercury because it is _____ and there is no _____.

The thick muscular layer of the heart wall is called the and is responsible for .

Most oxygen is transported in the blood bound to to form .

The prevents backflow of blood into the right atrium during .