Chp 6. 5) Impact craters are a useful tool for estimating the relative ages of planetary surfaces. Surfaces that show few craters must be relatively young, since various geological processes erased the craters. The four terrestrial planets can be ranked in the order of surface age by relative numbers of carters on their surfaces. Thus, from youngest to oldest: Earth, Venus, Mars, Mercury.

Chp 7. 2) The Earth is too hot (too close to the Sun) and its mass is too low to retain the original hydrogen-helium atmosphere typical of the presolar nebula gas. The combination of high temperature (very high velocities for H and He) and low mass (relatively low escape velocity) makes it impossible to hold onto much of the original hydrogen and helium.

5) Europa would have lower surface gravity. Since ice is less dense than rock, the mass of the Europa must be substantially less than the mass of the Moon, within the same volume and radius and g = GM/ R2

Cht 8 2) Keplerís laws apply to comets just as they do to planets. Applying Keplerís third law to a typical comet in the Oort cloud gives

P2 = a3 = ( 105)3 = 1015

P = 3.16 * 107 years

6) a) The warning time that we would have is determined simply by the time it would take the asteroid to cover the 15 million kilometers to the Earth at a rate of 15 kilometers per sec:

Velocity = distance / time

Warning Time = distance covered / velocity = 15 * 106/15 = 11- 12 days

b) A kilometer- scale asteroid impact is the equivalent of many extremely large nuclear explosion, and the shock wave would cause a great deal of devastation. The impact would also raise a large dust cloud, which would block sunlight for weeks or months.

c) At the distance of the Moon (about 400,000 km), repeating the same as in part (a) gives a warning time of about 7 hours.

d) To cross an atmosphere of 100 km, repeat the same as in part (a)

100 km / 15 km/s = 6.67 sec