View Full Version : How Strong is Gravity on Other Planets?

2015-Dec-30, 05:40 PM
Gravity is a fundamental force of physics, one which we Earthlings tend to take for granted. You can't really blame us. Having evolved over the course of billions of years in Earth's environment, we are used to living with the pull of a steady 1 g (or 9.8 m/s). However, for those who have gone into space or set foot on the Moon, gravity is a very tenuous and precious thing.
Basically, gravity is dependent on mass, where all things - from stars, planets, and galaxies to light and sub-atomic particles - are attracted to one another. Depending on the size, mass and density of the object, the gravitational force it exerts varies. And when it comes to the planets of our Solar System (http://www.universetoday.com/15451/the-solar-system/), which vary in size and mass, the strength of gravity on their surfaces varies considerably.
For example, Earth (http://www.universetoday.com/14367/earth/)'s gravity, which as already noted is equivalent to 9.80665 m/s (or 32.174 ft/s). This means that an object, if held above the ground and let go, will accelerate towards the surface at a speed of about 9.8 meters for every second of free fall. This is the standard for measuring gravity on other planets, which is also expressed as a single g.
In accordance with Isaac Newton's law of universal gravitation (http://hyperphysics.phy-astr.gsu.edu/hbase/grav.html), the gravitational attraction between two bodies can be expressed mathematically as F = G (mm/r) - where*F is the force, m1 and m2 are the masses of the objects interacting, r is the distance between the centers of the masses and G is the gravitational constant (http://www.universetoday.com/34838/gravitational-constant/) ( 6.67410?11*N?m2/kg2 ).
Based on their sizes and masses, the gravity on another planet is often expressed in terms of g units as well as in terms of the rate of free-fall acceleration. So how exactly do the planets of our Solar System stack up in terms of their gravity? Like this:
Gravity on Mercury:With a mean radius of about 2,440 km and a mass of 3.30 1023*kg, Mercury (http://www.universetoday.com/13943/mercury/) is approximately 0.383 times the size of Earth and only 0.055 as massive. This makes Mercury the smallest and least massive planet in the Solar System. However, thanks to its high density - a robust 5.427*g/cm3 which is slightly higher than Earth's density of 5.514*g/cm3 - Mercury has a surface gravity of 3.7*m/s, which is the equivalent of* 0.38 g.
Gravity on Venus:Venus (http://www.universetoday.com/14069/venus/) is similar to Earth in many ways, which is why it is often referred to as "Earths twin". With a mean radius of 4.6023108*km2, a mass of 4.86751024*kg, and a density of 5.243*g/cm3, Venus is equivalent in size to 0.9499*Earths, 0.815 times as massive, and roughly 0.95 times as dense. Hence, it is no surprise why the gravity on Venus is very close to that of Earth's - 8.87*m/s2, or 0.904 g.
Gravity on the Moon:This is one astronomical body where human beings have been able to test out the affects of diminish gravity in person. Calculations based on its mean radius (1737 km), mass (7.3477 x 10 kg), and density (3.3464 g/cm), and the missions conducted by the Apollo crews, the surface gravity on the Moon (http://www.universetoday.com/19424/the-moon/) has been measured to be 1.62*m/s2 , or 0.1654*g.
Gravity on Mars:Mars (http://www.universetoday.com/14701/mars/) is also similar to Earth in many key respects. However, when it comes to size, mass and density, Mars is comparatively small. In fact, its mean radius is 3.389 km is the equivalent of roughly 0.53 Earths, while its mass (6.41711023*kg) is just 0.107 Earths. Its density, meanwhile, is only about o.71 of Earths, coming in at a modest 3.93 g/cm. Because of this, Mars has 0.38 times the gravity of Earth, which works out to 3.711 m/s.
Gravity on Jupiter:Jupiter (http://www.universetoday.com/14469/jupiter/) is the largest and most massive planet in the Solar System. Its mean radius, at 69,911 6*km, is 10.97 the times of Earth, while its mass (1.89861027*kg) is the equivalent of 317.8*Earths. But being a gas giant, Jupiter is naturally less dense than Earth and other terrestrial planets, with a mean density of 1.326*g/cm3. As a result, Jupiter's surface gravity (which is defined as the force of gravity at its cloud tops), is 24.79*m/s, or 2.528*g.
Gravity on Saturn:Like Jupiter, Saturn (http://www.universetoday.com/15298/saturn/) is a huge gas giant that is significantly larger and more massive than Earth, but is far less dense. In short, its mean radius is 582326*km (9.13 Earths), its mass is 5.68461026*kg (95.15 times as massive), and has a density of 0.687*g/cm3. As a result, its surface gravity (again, measured from the top of its clouds) is just slightly more than Earth's, which is 10.44 m/s or 1.065 g.

Gravity on Uranus:With a mean radius of 25,360 km and a mass of 8.68 1025*kg, Uranus (http://www.universetoday.com/18855/uranus/) is approximately 4 times the size of Earth and*14.536 times as massive. However, as a gas giant, its density (1.27*g/cm3) is significantly lower than Earth's. Hence, why its surface gravity is slightly weaker than Earth's - 8.69*m/s2, or 0.886*g.
Gravity on Neptune:With a mean radius of 24,622 19*km and a mass of 1.02431026*kg, Neptune (http://www.universetoday.com/21581/neptune/) is the fourth largest planet in the Solar System, is 3.86 times the size of Earth and 17 times as massive. But, being a gas giant, it has a low density of 1.638*g/cm3. All of this works out to a surface gravity of 11.15*m/s2 (or 1.14 g), which again is measured at Neptune's cloud tops.
All in all, gravity runs the gambit here in the Solar System, ranging from 0.38 g on Mercury and Mars to a powerful 2.528*g atop Jupiter's clouds. And on the Moon, were astronauts have ventured, it is a very mild 0.1654*g, which allowed from some fun experiments in near-weightlessness.
Understanding the effect of zero-gravity on the human body has been essential to space travel, especially where long-duration missions to orbit and the ISS have been concerned. In the coming decades, knowing how to simulate it will come in handy when we send astronauts on deep space missions. And of course, knowing just how strong it is on other planets will be essential to manned missions (and perhaps even settlement) there.
We have written many interesting articles about gravity here at Universe Today. Here's How Fast Is Gravity? (http://www.universetoday.com/121284/how-fast-is-gravity/), Where Does Gravity Come From? (http://www.universetoday.com/75705/where-does-gravity-come-from/) and How We Know Gravity Is Not (Just) A Force (http://www.universetoday.com/108740/how-we-know-gravity-is-not-just-a-force/).
And here's Could We Make Artificial Gravity? (http://www.universetoday.com/121621/could-we-make-artificial-gravity/) and Does "Spooky Action" Define Gravity? (http://www.universetoday.com/106968/could-particle-spooky-action-define-the-nature-of-gravity/)
For more information, check out NASA's page titled "The Constant Pull of Gravity" (http://www.nasa.gov/audience/foreducators/topnav/materials/listbytype/The_Constant_Pull_of_Gravity.html) and Newton's law of gravity (http://physics.about.com/od/classicalmechanics/a/gravity.htm).
Astronomy Cast also has an episode, titled Episode 102: Gravity (http://www.astronomycast.com/2008/08/ep-102-gravity/).
The post How Strong is Gravity on Other Planets? (http://www.universetoday.com/35565/gravity-on-other-planets/) appeared first on Universe Today (http://www.universetoday.com).

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