# Gravity on Mars

1. Nov 16, 2009

### Coda

Scientist says that gravity on Mars is around 38% compare to Earth. Is they proof this merely by caculation?

I not sure whether scientist really land any space mechanic/machine on Mars, if yes, can someone provide me the information?

Your help is much appreaciate, thx :)

2. Nov 16, 2009

### Sorry!

Yes we can caluclate the approximate force of gravity a planet in our solar system has (or even objects outside our solar system given enough observation time).

Yes we also have landed crafts on Mars. Many times actually:

http://en.wikipedia.org/wiki/Mars#Exploration

3. Nov 16, 2009

### ideasrule

Figuring out the mass of Mars is easy: it has moons. By observing the orbit of its two moons, scientists calculated the mass of the planet long before anybody dreamed of building spacecraft to explore it. As Sorry mentioned, there have been a lot of spacecraft sent to Mars, and their orbits give even more precise information about the planet's gravity. See this wonderful gravity map, for instance: http://img253.imageshack.us/i/gravityfieldle7.jpg/

It shows differences in the acceleration of gravity as small as 0.001 m/s^2

4. Nov 16, 2009

### Stratosphere

You don't even need to test it by landing on mars, you can find it out mathematically.

$$g=-\frac{GM}{r^{2}}$$

5. Nov 17, 2009

### qraal

Theorists! They never appreciate a good experimental proof!

"Viking 1" & "Viking 2" proved the Martian surface gravity was about 3.71 m.s-2 via landing there. Other probes have followed, but no others have used retro-rockets all the way to touch-down.

6. Nov 17, 2009

### Coda

Thx for the information you provide, I have been learning alot.

Anyway graal, does the rockets have to be touch-down to the planet to confirm the exactly gravity amount?

7. Nov 17, 2009

Staff Emeritus
I don't like this argument, because there is no measurement of M. What's actually measured is g, and from that M is inferred.

What's missing in this discussion is that we know g from direct observation, and have since 1877. There are two objects where we can see them fall and measure their acceleration - Deimos and Phobos, the two moons of Mars.

8. Nov 17, 2009

### qraal

Well you did ask if it had been directly measured. That's as direct a measurement as you can get. But if you can monitor the orbits of anything nearby a planet, then you can work out the surface gravity fairly simply via Newton's equations. Not direct, but always trustworthy within the limits of measurement error.

One thing in science is the presence of limitations of measurement. The Gravitational Constant, G, is a notoriously difficult constant to measure and is best avoided whenever possible for high precision tasks. Fortunately GM, the product of G and the mass of any particular planet, is usually known to very high precision, so it is used when computing orbits.