Answer to Question 1The most important of these internal processes was radiation pressure. When the Sun became a luminous object, light streaming from its photosphere pushed against the particles of the solar nebula. Large bits of matter like planetesimals and planets were not affected, but low-mass specks of dust and individual atoms and molecules were pushed outward and eventually driven from the system.
The second effect that helped clear the nebula was the solar wind, the flow of ionized hydrogen and other atoms away from the Sun's upper atmosphere (look back to Chapter
. This flow is a steady breeze that rushes past Earth at about 400 km/s (250 mi/s). Young stars have even stronger winds than stars of the Sun's age and also irregular fluctuations in luminosity, like those observed in young stars such as T Tauri stars, which can accelerate the wind. The strong surging wind from the young Sun may have helped push dust and gas out of the nebula.
The third effect that helped clear the nebula was the sweeping up of space debris by the planets. All of the old, solid surfaces in the Solar System are heavily cratered by meteorite impacts. Earth's Moon, Mercury, Venus, Mars, and most of the moons in the Solar System are covered with craters. A few of these craters have been formed recently by the steady rain of meteorites that falls on all the planets in the Solar System, but most of the craters appear to have been formed roughly 4 billion years ago in what is called the heavy bombardment, as the last of the debris in the solar nebula was swept up by the planets.
The fourth effect was the ejection of material from the Solar System by close encounters with planets. If a small object such as a planetesimal passes close to a planet, the small object's path will be affected by the planet's gravitational field. In some cases, the small object can gain energy from the planet's motion and be thrown out of the Solar System. Ejection is most probable in encounters with massive planets, so the Jovian planets were probably very efficient at ejecting the icy planetesimals that formed in their region of the nebula.
Attacked by the radiation and gravity of nearby stars and racked by internal processes, the solar nebula could not survive very long. Once the gas and dust were gone and most of the planetesimals were swept up, the planets could no longer gain significant mass, and the era of planet building ended.
Answer to Question 2c
(A) Normal spongy bone. (B) Spongy bone with osteoporosis, which is characterized by a loss of bone
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