Above Image: The relative sizes of Earth and Mars © Public Domain

Earth and Mars: Comparison of Planetary Neighbours

Earth is the third planet from the Sun, and Mars, the fourth planet from the Sun, is our closest planetary neighbour. Scientists have studied Mars by peering through telescopes, sending robotic spacecraft to orbit the planet and placing landers and robotic rovers on the Martian surface. Thanks to this exploration, we have learned quite a lot about what Mars and Earth have in common, and what makes each of these planets unique (See Figure 1).

Planetary Basics

Mars is smaller and lighter than Earth. The equatorial diameter and circumference of Mars are half that of Earth (See Figure 1). The volume of Mars is 15% of the volume of Earth. If Earth could be cracked open and emptied like an eggshell, about 6.5 Mars planets would fit inside. Mars is not just smaller than Earth; it is also less dense. So even though Mars is 15% of Earth’s volume, it is only 11% of Earth’s mass.

The pull of gravity on the surface of Mars is 38% as strong as the pull of gravity on Earth’s surface. Weight is the measurement of the force of gravity. For a person standing on the surface of Mars, his or her mass would be exactly the same as on Earth, because that person would contain the same amount of matter. However, the person’s weight would be 38% of his or her weight on Earth, due to the lower force of gravity on Mars.

Two Rocky Planets

Earth and Mars are both rocky planets, with similar types of iron-rich rocks at the surface. Mars is sometimes called the Red Planet, because of the red iron oxides (rust) on its barren surface (See Figure 2). Mars has mountains and canyons like Earth, but they can be much bigger. The tallest Martian mountain, called Olympus Mons, is three times taller than Mount Everest and is the tallest known mountain in our whole solar system. The deepest Martian canyon, called Valles Marineris, is four times deeper than the Grand Canyon and is the deepest known canyon in the solar system. Mars had active volcanoes in the past, but they are all inactive now.

As the Worlds Turn – Days and Years

The time it takes for Earth to complete one rotation around its axis (like a spinning top) is called a day and it lasts 24 hours. Compared to Earth, Mars spins slightly slower around its axis, so each day is slightly longer and lasts 24 hours and 37 minutes. A Martian day is called a sol. 

The time it takes for Earth to complete one revolution in its orbit around the Sun (like a spinning top tracing a path around a point on the floor) is called a year and it lasts about 365 days. Mars has a larger orbit than Earth, and it moves through the solar system more slowly, so a Martian year lasts 687 Earth days or 669 sols.

Martian Environment

Deadly Martian Atmosphere

The mixture of gases that surround a planet (or a moon) is called its atmosphere. Earth’s atmosphere, commonly called air, contains 78% nitrogen and 20% oxygen. There are also small amounts of other gases, including carbon dioxide (0.04%). For humans, the atmosphere on Mars would be suffocating. It is 96% carbon dioxide and only 0.145% oxygen. The Martian atmosphere is also “thin”, because it is 100 times less dense than Earth’s atmosphere.

Harsh Martian Climate

Both Earth and Mars experience four seasons, but each season is longer on Mars because the Martian year is longer. While the two planets have changing seasons in common, the climate on Mars is colder, drier and harsher than any place on Earth.

Earth’s average temperature is 14^°C, but the average temperature on Mars is a chilly -63^°C. Mars is colder than Earth because it is farther from the Sun, and the atmosphere is too thin to retain heat at the surface.

A vast ocean system of liquid water covers 71% of the surface of Earth. There is water on Mars too, but the cold temperatures and thin atmosphere mean that liquid water cannot exist for long on the surface. There is evidence that briny (salty) liquid water flows in some areas on the Martian surface when the temperature is above -23°C. Mars has solid water ice in its polar ice caps (like Earth) and just below the planet’s surface. There is also a tiny amount of water vapour in the atmosphere, and water ice in clouds.

Weather on Earth often involves some kind of precipitation such as rain or snow. Sometimes it “snows” on the Martian surface too, but in a surprising way. The only kind of snow that makes it all the way through the atmosphere to land on Mars is made of frozen carbon dioxide that comes from carbon dioxide ice clouds. Frozen water snow can fall from water ice clouds, but it does not reach the planet’s surface because it turns to water vapour in the thin atmosphere.

Dust storms are common on Mars, sometimes covering the whole planet. The biggest storms usually start during summer in Mars’ southern hemisphere when strong winds, caused by uneven heating of the atmosphere, pick up dust from the surface. Wind is a major weather feature on Mars and shapes the planet’s surface. Smaller dust devils are caused by more localized swirling winds, just like on Earth (See Figure 3).

Glossary

Air

The mixture of gases that surrounds Earth; the common name of Earth’s atmosphere.

Atmosphere

The gases that surround some planets and moons, held in place by gravity. 

Climate

The normal weather pattern over a long period of time in a large geographical area. Climate can change over hundreds or thousands of years, not day-to-day.

Day

The length of time for a planet to complete one full rotation around its axis.

Density

A measure of how much mass (matter) is in a given volume. Objects with higher density have more matter than objects with lower density, for the same volume (metal ball versus cotton ball of equal volume).

Equatorial circumference

The distance around a planet, as measured on the surface following the line of the equator.

Equatorial diameter

The distance across (through) a planet, as measured from opposite points on the equator.

Gravity

The attractive (pulling) force between any objects that have mass. For example, Earth’s gravity keeps people and objects on the surface of the planet and the Sun’s gravity keeps all the planets in orbit.

Mass

A measure of the amount of matter in an object.

Sol

The name for one day on Mars.

Weather

The local mix of events in the atmosphere, such as clouds, precipitation or wind. Weather can change day-to-day and over short distances (e.g., between nearby cities in the same province).

Weight

The measurement of the force of gravity on an object. An object’s mass (amount of matter it has) is a fixed property, but its weight depends on the force of gravity at its location. 

Year

The length of time for a planet to complete one full revolution around the Sun, following its orbital path.

References

• Planet Mars in Numbers - Canadian Space Agency (Retrieved April 19, 2016). This infographic is a highly visual comparison of Earth and Mars.
• Detailed Facts About Mars - NASA (Retrieved April 19, 2016). This webpage is a comparison of the properties of Earth and Mars (click to expand the categories).
• Mars: In Depth - NASA (Retrieved April 19, 2016). This article discusses the importance of water on Mars, and highlights the milestones in Mars exploration.
• NASA Confirms Evidence That Liquid Water Flows on Today’s Mars - NASA (Retrieved May 4, 2016). This press release describes how NASA’s Mars Reconnaissance Orbiter identified salts on the surface of Mars that point to recurring liquid water flows.
• NASA Orbiter Observations Point to 'Dry Ice' Snowfall on Mars - NASA (Retrieved April 19, 2016). This article discusses the frozen carbon dioxide snow that can fall on Mars.
• The Fact and Fiction of Martian Dust Storms - NASA (Retrieved April 19, 2016). This article discusses the cause and characteristics of dust storms on Mars, and how these storms affect robotic equipment.

Earth and Mars by the Numbers

1. Rounded to the nearest whole number
2. Measured as gravitational acceleration at the planet's surface
3. Maximum temperature on Mars is during the summer at the equator when the Sun is shining on the surface.
4. Earth's orbit is nearly circular so perihelion and aphelion are similar; Mars' orbit is more elliptical.
5. Tilted axis of rotation causes seasons on Earth and Mars.
6. No magnetic field on Mars (plus thin atmosphere) means no protection from solar radiation.

Table adapted from All About Mars