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Comparative Data
Mass
Because both the Earth and Mars have moons whose periods are well known, it is easy to calculate masses (in kilograms) for both of these planets based on simple orbital mechanics.
The Earth is about 9.3 times more massive than Mars, partly because it is bigger, and partly because the Earth has a greater average density than Mars.
Volume
Both planets are approximately spherical in shape, therefore, if their diameters are known it is easy to calculate the volume of each planet.
The Earth has about 6.6 times the volume of Mars, having almost (but not quite) twice the diameter of Mars.
Length of the Day
Both the Earth and Mars rotate on an axis which points approximately to the same North Star, Polaris.
On Earth we have defined the length of the day (on average) to be exactly 24 hours. That is, 24 hours is the average length of time required for the earth to rotate exactly once on its axis with respect to the position of the Sun.
Mars spins slightly more slowly than the Earth and its takes 24.6597 (Earth) hours to complete one full day.
Surface Gravity
The force of gravity on Earth is 9.8 N (Newtons) per kg (kilogram). This force is commonly called weight. On Earth, for example, a 25 kilogram mass would be pulled down with a force of 245 Newtons (i.e.25 x 9.8). We would say its weight is 245 N.
On Mars gravity pulls down on objects with a force of about 3.7 Newtons per kilogram of mass. Objects on Mars weigh less (than on Earth), but their mass is unchanged.
An equally convenient way to express this is to say that the acceleration (on an object) at the Earth's surface is 9.8 m/s2 and the acceleration due to gravity on Mars' surface is 3.7 m/s2.
Solar Irradiance
The Earth receives about 1300 watts of power per square metre (above the atmosphere) from the Sun. At the Earth's surface, with the Sun directly overhead, the Earth receives about 1000 watts of power per square metre. The Earth's relatively dense atmosphere filters out about 300 W/m2 including most of the solar ultraviolet radiation.
Mars, because it is farther from the Sun than the Earth, receives about 590 watts of power per square metre, but almost all the solar ultraviolet radiation reaches Mars' surface. This ultraviolet light is very harmful (and would be lethal) to most plants and animals.
Relative Sizes
By a strange coincidence, Mars is almost exactly half the diameter of the Earth, and the Earth's Moon is almost exactly half the diameter of Mars.
This simple rule makes the construction of models fairly straightforward. Students often forget that Mars is really quite small, and the Earth's Moon is even smaller.
Distance from Earth (closest)
Because of the orbital motion of both the Earth and Mars, they are alternately approaching and receding from each other.
At closet approach (called opposition) Mars can be as close as 56 million kilometres from the Earth, about 135 times the average distance to the Earth's Moon.
Distance from Earth (farthest)
Because of the orbital motion of both the Earth and Mars, they are alternately approaching and receding from each other.
When Mars and the Earth are on opposite sides of their respective orbits (called conjunction), the distance separating the two planets is more than 1000 times the average distance to the Earth's Moon.
Diameter
Mars has a diameter which is slightly greater than the distance across Canada, at its widest East-West distance from St. John's, Newfoundland to Victoria, British Columbia.
As far as can be determined, Mars has no liquid water on its surface so all travel on its surface must be by overland methods.
Mars has some very large canyons, valleys, and mountains on its surface.
Surface Area (Canada)
Mercator Projections do not accurately represent the area of Mars' surface, but they are shown here for illustrative purposes.
Nevertheless, when the area of Mars is accurately calculated it has about fourteen-and-a-half times more surface than Canada.
Surface Area (World [total])
![Surface Area (World [total])](/images/mars_comp_surf_world_t.jpg)
On a global scale the surface area of the Earth is more than three-and-a-half times larger than the surface area of Mars. This includes the area of all the Earths' oceans and land masses.
The surface areas are calculated using the mean diameter of each planet and treating each as if it were a perfect sphere.
Surface Area (World [land])
![Surface Area (World [land])](/images/mars_comp_surf_land_t.jpg)
Humans usually prefer to build cities and towns on land. This limits the actual habitable part of the Earth to the surface of its land masses.
When the oceans are excluded from the total surface area of the Earth, we find that Mars has slightly more, by about ten percent, habitable surface area than the Earth. This assumes that all land on Mars, and all land on Earth, are potentially habitable by humans.
Seasons
Both the Earth and Mars have seasons due to the fact that their rotational axes are tilted with respect to the plane of their orbital motion.
The Earth's year is about 365.25 days long, each season being roughly one quarter of an Earth year long.
Mars has about 686.98 Earth days in its year, each Martian season being roughly one quarter of a Martian year-long.
Since the Martian year is about twice as long as an Earth year, each Martian season is therefore twice as long as each Earth season.
for the Canadian Space Agency.
