L7. The Rocky Inner Planets

Unit 2 · The Solar System

R-report

L7. The Rocky Inner Planets

Venus is almost exactly Earth’s size and made of the same kind of rock. Yet its surface is hot enough to melt lead and its air would crush you in seconds. Two nearly identical planets, two completely different worlds — why?

Four worlds of rock and metal

The four planets closest to the Sun — Mercury, Venus, Earth, and Mars — are called the terrestrial or rocky planets. They all have a solid surface, a metal core, and are small compared with the giants further out. - Mercury is the smallest, barely larger than our Moon. It has almost no atmosphere, so it swings from about 430 °C in sunlight to -170 °C at night — the biggest temperature range of any planet. - Venus is nearly Earth’s twin in size and mass, but wrapped in a crushing carbon-dioxide atmosphere. - Earth is the only planet with liquid water on its surface and the only one known to have life. - Mars is about half Earth’s width, cold and dry, with a thin atmosphere and the largest volcano in the solar system, Olympus Mons. They formed close to the young Sun, where it was too hot for ice to survive. Only rock and metal could stay solid, which is why these worlds are small and dense while the outer planets grew huge on gas and ice.

The atmosphere makes the world

Compare Venus, Earth, and Mars and one factor stands out: it is the atmosphere, not the rock, that decides what a surface is like. - Venus has an atmosphere about 90 times heavier than ours, almost entirely carbon dioxide. Sunlight gets in, heat struggles to get out, and the surface sits at roughly 465 °C — hotter than Mercury, even though Venus is nearly twice as far from the Sun. This is a runaway greenhouse effect. - Earth has a moderate atmosphere with just enough greenhouse gas. Without any, our average temperature would be around -18 °C instead of about 15 °C. The balance is what makes Earth liveable. - Mars has an atmosphere less than 1% as thick as ours. It cannot hold heat, so the average temperature is about -60 °C, and liquid water on the surface would boil away or freeze. Same basic ingredients, wildly different outcomes, because of how much gas each world held on to.

Why Mars keeps our attention

Of all the planets, Mars is the one we keep going back to, and there are good reasons. - Its day is 24 hours 37 minutes, remarkably close to ours. - It has seasons, polar ice caps, and weather, including planet-wide dust storms. - Orbiters and rovers have found dried-up riverbeds, lake sediments, and minerals that only form in water. Mars was once warmer and wetter. - Water ice still exists at the poles and under the surface, which future missions could use. So far no evidence of life, past or present, has been found. That question is exactly what current rovers are drilling rock to answer. Mars also carries a warning. It probably lost most of its atmosphere to space after its magnetic field faded, and once the air was gone the water followed. A planet’s habitability is not permanent — it depends on conditions that can change.

Putting the investigation together

The four inner planets are all made of rock and metal, but they turned out completely differently. Mercury kept no atmosphere and swings between extremes, Venus kept far too much and cooked, Mars lost most of its air and froze, and Earth held on to just enough.

This is a case where comparison is the method. Studying Earth alone would never reveal how delicate our situation is. Placing three similar planets side by side shows that a modest change in atmosphere is the difference between an ocean world and a furnace — a lesson that applies directly to understanding our own climate.