Which planet has the lowest gravity at its surface?

Answer 1

Mars, bizarrely...The mean surface gravity of a planet is determined primarily by its mass and its radius.

Prior to the reclassification of the major bodies in our solar system, the smallest planet by volume and mass was Pluto. Pluto has a mean surface gravity of 0.062 Earth equivalents.

However, Pluto has been reclassified as a dwarf planet.

This demotes the planet Mercury into last place in terms of volume and mass. You might expect, therefore, Mercury to have the lowest surface gravity of the eight major planets. However, the planet Mars, despite having about twice the mass of Mercury, has a radius about 1000 km larger. So, actually Mars has a fractionally lower mean surface gravity.

Mercury = 0.3772 Earth equivalents

Mars = 0.3769 Earth equivalents

(measured at the equator).

For "surface gravity", the mass of a planet is effectively concentrated at the center (the center of mass). So, the gravitational force is less when the surface is further from the center of mass.

So, a planet's surface gravity increases with its mass, but reduces with its radius.

One of the small contributory factors to this reversal in expected ranking may be due to the comparatively slow rotation of Mercury (58.6 Earth days per rotation with an average speed of 10.9 km/h. Compare Mars: 1.03 Earth days and 868 km/h).

The small rotational speed on Mercury means the outward acting centrifugal effect, which opposes the inward pull of gravity, is small.

(Strictly speaking this only matters when we are talking about what scientists call "apparent surface gravity".)

Answer 2

Mars has the lowest surface gravity (0.376g), just a little lower than that of Mercury (0.38g). Although Mars is larger and more massive than Mercury, Mercury has a higher density, which effects the surface gravity comparison.