Why are the inner planets also called the rocky planets?

Answer 1

Because they are chieflly rock and metal.

MERCURY - Astronomers have estimated that the Mercury composition is made up of approximately 70% metals and 30% silicate material. In fact, it's only slightly less dense than the Earth, with 5.43 g/cm3. Since Mercury is smaller than Earth, its gravity doesn't compress it down as much, so it actually has much more heavier elements inside it. Geologists estimate that its core is very large, and mostly iron. The core probably accounts to 42% of Mercury's volume, while Earth is just 17%.

VENUS - The crust of Venus is thought to be about 50 km thick, and composed of silicious rocks. Beneath that is the mantle, which is thought to be about 3,000 km thick. The composition of the mantle is unknown. And then at the center of Venus is a solid or liquid core of iron or nickel. Since Venus doesn't have a global magnetic field, scientists think that the planet doesn't have convection in its core.

EARTH - The mass of the Earth is approximately 5.98×1024 kg (6.6x1021 tons). It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium(13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is estimated to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.

MARS - Mars composition is similar to Earth in its basic composition. It has a core that is mainly iron and sulfur, wrapped in a mantle that is silicates, all of which is cocooned by a crust that is made ofbasalt and just enough iron oxide to give the planet it characteristic reddish hue. Knowing the basic components of Mars is just brushing the surface.

Where as the gas giants - Jupiter and Saturn consist mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass.[5] Their structures are thought to consist of an outer layer of molecular hydrogen, surrounding a layer of liquid metallic hydrogen, with a probable molten core with a rocky composition. The outermost portion of the hydrogen atmosphere is characterized by many layers of visible clouds that are mostly composed of water and ammonia. The metallic hydrogen layer makes up the bulk of each planet, and is described as "metallic" because the great pressure turns hydrogen into an electrical conductor. The core is thought to consist of heavier elements at such high temperatures (20,000 K) and pressures that their properties are poorly understood.

Uranus and Neptune have distinctly different interior compositions from Jupiter and Saturn. Models of their interior begin with a hydrogen-rich atmosphere that extends from the cloud-tops down to about 85% of Neptune's radius and 80% of Uranus'. Below the point is predominantly "icy", composed of water, methane and ammonia. There is also some rock and gas but various proportions of ice/rock/gas could mimic pure ice so the exact proportions are unknown.

Very hazy atmosphere layers with a small amount of methane gives them aquamarine colors such as baby blue and ultramarine colors respectively. Both have magnetic fields that are sharply inclined to their axes of rotation.

So to sum up - The term gas giant was coined in 1952 by the Science Fiction writer James Blish. Arguably it is something of a misnomer, since throughout most of the volume of these planets all the components (other than solid materials in the core) are above the critical point and therefore there is no distinction between liquids and gases. Fluid planet would be a more accurate term. Jupiter is an exceptional case, having metallic hydrogen near the center, but much of its volume is hydrogen, helium and traces of other gases above their critical points. The observable atmospheres of any of these planets (at less than unit optical depth) are quite thin compared to the planetary radii, only extending perhaps one percent of the way to the center. Thus the observable portions are gaseous (in contrast to Mars and Earth, which have gaseous atmospheres through which the crust may be seen).

Answer 2

The inner planets, Mercury, Venus, Earth, and Mars, are called rocky planets because they have solid rocky surfaces. These planets are predominantly made up of rocks and metals, with solid surfaces that are distinct from the gas giants in the outer solar system.