How would you measure the density of the atmosphere of Mars?

This is important because the helicopter now heading for Mars can’t fly unless you know what the density of the atmosphere is. The question is complicated because this density varies not just with altitude but also with season (the atmosphere freezes at the poles) and the temperature.

I looked up the density of Mars’ atmosphere on wikipedia – it isn’t there. On Google – it isn’t there. On a property table for the atmosphere of Mars – it isn’t there.

So how would you measure it? I can think of four methods, none of which is satisfactory.

1. Evacuate a chamber and measure the change in weight. This is very inaccurate because the weight of the chamber is going to be very much more than the weight of the air. We’re losing three significant digits in accuracy this way, quite apart from the cost of the pump.

2. Drop a feather and time how long it takes to land. This gives the drag. You wouldn’t use an actual feather of course but a feather-like object that falls at a uniform speed. The drag is the velocity squared on 2 (known) times the air density (unknown) times the drag coefficient (also unknown). The inaccuracy comes from the drag coefficient being a function of the flow regime (Reynolds number) and other factors such as the viscosity of the atmosphere.

3. The third way is to make up a chamber on Earth and put in it a gas of the same composition, pressure and temperature and measure the density. But that’s a simulation, not a direct measurement.

4. Measure the speed of sound. But like dropping a feather this is also an indirect measure.

The density can be easily computed from the temperature, pressure and composition, all of which are well known now.

mollwollfumble said:

How would you measure the density of the atmosphere of Mars?

This is important because the helicopter now heading for Mars can’t fly unless you know what the density of the atmosphere is. The question is complicated because this density varies not just with altitude but also with season (the atmosphere freezes at the poles) and the temperature.

I looked up the density of Mars’ atmosphere on wikipedia – it isn’t there. On Google – it isn’t there. On a property table for the atmosphere of Mars – it isn’t there.

So how would you measure it? I can think of four methods, none of which is satisfactory.

1. Evacuate a chamber and measure the change in weight. This is very inaccurate because the weight of the chamber is going to be very much more than the weight of the air. We’re losing three significant digits in accuracy this way, quite apart from the cost of the pump.

2. Drop a feather and time how long it takes to land. This gives the drag. You wouldn’t use an actual feather of course but a feather-like object that falls at a uniform speed. The drag is the velocity squared on 2 (known) times the air density (unknown) times the drag coefficient (also unknown). The inaccuracy comes from the drag coefficient being a function of the flow regime (Reynolds number) and other factors such as the viscosity of the atmosphere.

3. The third way is to make up a chamber on Earth and put in it a gas of the same composition, pressure and temperature and measure the density. But that’s a simulation, not a direct measurement.

4. Measure the speed of sound. But like dropping a feather this is also an indirect measure.

Bing says : The highest atmospheric density on Mars is equal to the density found 35 km above the Earth’s surface. The atmosphere of Mars has been losing mass to space throughout history, and the leakage of gases still continues today. The atmosphere of Mars is colder than Earth’s.

Tamb said:

mollwollfumble said:

How would you measure the density of the atmosphere of Mars?

This is important because the helicopter now heading for Mars can’t fly unless you know what the density of the atmosphere is. The question is complicated because this density varies not just with altitude but also with season (the atmosphere freezes at the poles) and the temperature.

I looked up the density of Mars’ atmosphere on wikipedia – it isn’t there. On Google – it isn’t there. On a property table for the atmosphere of Mars – it isn’t there.

So how would you measure it? I can think of four methods, none of which is satisfactory.

1. Evacuate a chamber and measure the change in weight. This is very inaccurate because the weight of the chamber is going to be very much more than the weight of the air. We’re losing three significant digits in accuracy this way, quite apart from the cost of the pump.

2. Drop a feather and time how long it takes to land. This gives the drag. You wouldn’t use an actual feather of course but a feather-like object that falls at a uniform speed. The drag is the velocity squared on 2 (known) times the air density (unknown) times the drag coefficient (also unknown). The inaccuracy comes from the drag coefficient being a function of the flow regime (Reynolds number) and other factors such as the viscosity of the atmosphere.

3. The third way is to make up a chamber on Earth and put in it a gas of the same composition, pressure and temperature and measure the density. But that’s a simulation, not a direct measurement.

4. Measure the speed of sound. But like dropping a feather this is also an indirect measure.

Bing says : The highest atmospheric density on Mars is equal to the density found 35 km above the Earth’s surface. The atmosphere of Mars has been losing mass to space throughout history, and the leakage of gases still continues today. The atmosphere of Mars is colder than Earth’s.

> Bing says : The highest atmospheric density on Mars is equal to the density found 35 km above the Earth’s surface.

Yeah, saw that, which is pretty bleedin useless.

I had not seen the equation for density from pressure and temperature, for Mars. I know the one for Earth, which ought to be very different.

dv said:

The density can be easily computed from the temperature, pressure and composition, all of which are well known now.

Is that really ideal?

sibeen said:

dv said:

The density can be easily computed from the temperature, pressure and composition, all of which are well known now.

Is that really ideal?

LOL

sibeen said:

dv said:

The density can be easily computed from the temperature, pressure and composition, all of which are well known now.

Is that really ideal?

Even the non-ideality can be allowed fr

sibeen said:

dv said:

The density can be easily computed from the temperature, pressure and composition, all of which are well known now.

Is that really ideal?

Thank you, that was my point. By “known” we mean “measurable”. They vary a lot.