The Solar System and Its Place in the Galaxy 15
FIGURE 6 Four cometary nuclei photographed by flyby spacecraft: Halley’s comet in 1986
(Giotto, top left), Borrelly in 2001 (Deep Space 1, top right), Wild 2 in 2004 (Stardust, bottom
left), and Tempel 1 in 2005 (Deep Impact, bottom right). The nuclei show considerable diversity
both in shape and in surface topography. The Halley nucleus is about 15×8 km in diameter, the
Borrelly nucleus is 8×3.2 km, the Wild 2 nucleus is 5.2×4.0 km, and the Tempel 2 nucleus is
7.6×4.9 km. The Halley image shows bright dust jets emanating from active areas on the
nucleus surface. The other three nuclei were also active during their respective flybys but the
activity was too faint to show in these images.
Like almost all other satellites in the solar system, the
Moon has no substantial atmosphere. There is a transient at-
mosphere due to helium atoms in the solar wind striking the
lunar surface and being captured. Argon has been detected
escaping from surface rocks and being temporarily cold-
trapped during the lunar night. Also, sodium and potassium
have been detected, likely the result of sputtering of surface
materials due to solar wind particles, as on Mercury.
Unlike the Earth’s Moon, the two natural satellites of
Mars are both small, irregular bodies and in orbits relatively