What Makes Mercury and Venus Unique Planets?


By Robert Hazen, Ph.D.George Mason University

A careful examination of the objects in the solar system provides convincing evidence of the unique characteristics of planets; that the Earth and other planets are unique worlds. The Earth and the other planets are distinct, each with its own personality and idiosyncrasies. Each planet holds surprises. Each planet is begging for more research, begging for more discoveries.

Illustration of the Earth in space with the Sun in the background
The planets of the solar system have unique characteristics. (Image: buradaki/Shutterstock)

The Solar System

The solar system was formed from a nebula of dust and gas. It began as an irregular cloud of dust and gas, just floating around; but perhaps a shockwave from a supernova created dense local areas, and those dense areas became the focus of gravitational collapse. 

As the dust collapsed, it began to rotate faster and faster, forming a flat disc with a bulging center, and other dust spread out. The central bulge became the Sun, and the flat disc became the planets. The Sun ignites; it sends out heat and solar wind.

That strips the inner planets of hydrogen and helium, so there are four inner terrestrial planets. Then there are four larger, Jovian planets, or gas giant planets farther out, made primarily of that hydrogen and helium that’s been pushed out in the outer reaches of the solar system.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

Mercury: A Blasted and Dead World

Mercury, the innermost planet, is desolate; it’s a forbidding world. When you hear the name of Mercury mentioned, I want you to remember two words: “blasted” and “dead.” Its small size and proximity to the Sun have made Mercury extremely difficult to observe, at least with Earth-based telescopes. 

An illustration of Mercury.
Mercury’s poles reside in eternal twilight. (Image: Vadim Sadovski/Shutterstock)

Mercury’s elliptical orbit takes it as close as 43 million kilometers to the Sun and as far away as 70 million kilometers. Even so, it’s much too close to observe clearly against the bright light of the Sun. The swift orbital period of Mercury is just 88 days, and surprisingly, a day on Mercury lasts 176 days which is twice that long; so on Mercury, the day is twice as long as the year. 

Photographs reveal a heavily cratered surface alternately baked to 500 degrees centigrade on the sunny side and frozen to minus 200 degrees celsius on the dark side. Mercury has no atmosphere whatsoever, and thus there’s no erosion to wear down the deeply cratered and pockmarked surface. 

The poles on Mercury are fascinating places. That’s because the planet’s tilt is absolutely zero—it’s locked in at zero by the Sun—thus, the poles are in eternal twilight, with the Sun grazing across them.

Learn more about the nebular hypothesis.

Venus, the Earth-like Planet

An illustration of the planet Venus.
The Venusian day is longer than the Venusian year. (Image: Artsiom P/Shutterstock)

The second planet is Venus. Venus is our nearest planetary neighbor, both in distance and in size. When you think of Venus, remember two phrases: “Earth-like” and “greenhouse effect.”

The orbit of Venus is nearly circular and has an average distance from the Sun of 108 million kilometers. It has an orbital period of 225 days—that’s about two-thirds of Earth’s year. The Venusian day is 243 Earth days. Once again, the Venusian day is slightly longer than the Venusian year, a strange situation.

Venus also has a slow retrograde rotation about its axis. It rotates the wrong way, if you will. That retrograde rotation is thought to be the result of a late-stage collision that occurred when two bodies were competing for the Venus orbit. The smaller one hit Venus. It didn’t destroy Venus; it just made it start rotating the wrong way.

Learn more about how astronomers collect, analyze, and interpret data to understand spatial distribution.

Greenhouse Effect on Venus

Venus is enshrouded in a very thick atmosphere, composed primarily of clouds of carbon dioxide and nitrogen. The daytime surface temperature reaches almost 500 degrees centigrade due to an exaggerated greenhouse effect.

Carbon dioxide on the Venusian atmosphere traps sunlight, and the planet gets hotter and hotter, in what’s called a runaway greenhouse effect. The surface pressures are tremendous: beneath the thick atmosphere, it’s supposed to be about 95 times the pressure of that on Earth’s atmosphere. That’s about 1,400 pounds-per-square-inch pressure at the surface of Venus. 

Though the surface is forever hidden from our view, at least with telescopes, the Venusian landscape has been mapped and detailed by satellites orbiting Venus that look at the surface with radar. The radar maps show a planet with mountain ranges. They have cratered plains and ancient volcanoes; evidence of a once-active planet, though not quite so rugged and not quite as dynamic as the Earth is today. 

Still, Venus is very Earth-like. The Soviet Union succeeded in a tremendous technological feat when they had four Venus landers: Venera 9, 10, 13, and 14. These all returned remarkable surface photographs of a rocky landscape. Venus might have been a lot like the Earth one day if it hadn’t been for that intense greenhouse effect.

Common Questions about the Unique Characteristics of Mercury and Venus

Q: What was the main difference between the inner and outer planets when the solar system was formed?

When the Sun sent out solar winds, it stripped the inner planets of their hydrogen and helium, creating four inner terrestrial planets. These gases went to the outer reaches of the solar system and eventually made up the Jovian planets.

Q: Why is Mercury hard to observe?

Observing Mercury with Earth-based telescopes is a difficult task because of the planet’s two unique characteristics: it is quite small and it is very close to the Sun.

Q: What is the runaway greenhouse effect observed on Venus?

Venus is enshrouded in a very thick atmosphere, composed primarily of clouds of carbon dioxide and nitrogen. Carbon dioxide on the Venusian atmosphere traps sunlight, and the planet gets hotter and hotter, in what’s called a runaway greenhouse effect.

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