In the inner solar system, Mercury, Venus, and Mars are similar in size to the Earth, so the four are collectively called terrestrial planets. Their small masses and sizes are in marked contrast to those of the giant planets—Jupiter, Saturn, Uranus, and Neptune—at far greater distances from the Sun. The four terrestrial planets and Earth's Moon offer a rich variety in physical makeup and environment. Traditionally, Mercury and the Moon were described as airless bodies, classic cases of objects too small in mass to hold on to gases to form a permanent atmosphere. With advances in modern technology for ground-based remote sensing and space-flight opportunities for direct samplings, both Mercury and the Moon are now known to have transient atmospheres, continuously being formed and lost.