Why is there no dust on the Lunar Lander's footpads?

[Edwardwb1001]

A straightforward question but one to which I have not yet received a straightforward answer.  Photos of Apollo 11's landing pads whilst resting on the moon show them to completely dust free, the gold foil covering them shimmering and gleaming - with not a speck of lunar dust or soil present.

Even if the LM's exhaust did not produce a strong enough blast to form a crater, and even if the landing site was quite hard and rocky, there surely would have been a certain amount of dust/soil which would have billowed up to float down and settle on the abovementioned landing pads?

Other landers show decidedly dusty footpads whilst resting on the surface of other worlds. The 2008 Phoenix Mars lander of 2008 shows a completely dust-covered footpad in photos taken.  Why would there be this discrepancy?

[Grashtel]

A straightforward question but one to which I have not yet received a straightforward answer.  Photos of Apollo 11's landing pads whilst resting on the moon show them to completely dust free, the gold foil covering them shimmering and gleaming - with not a speck of lunar dust or soil present.

Even if the LM's exhaust did not produce a strong enough blast to form a crater, and even if the landing site was quite hard and rocky, there surely would have been a certain amount of dust/soil which would have billowed up to float down and settle on the abovementioned landing pads?

Other landers show decidedly dusty footpads whilst resting on the surface of other worlds. The 2008 Phoenix Mars lander of 2008 shows a completely dust-covered footpad in photos taken.  Why would there be this discrepancy?

The dust would have billowed up and floated down in what?  Remember that the Moon, unlike Mars, doesn't have an atmosphere so there is nothing to cause the dust to billow or for it to float down in, instead it flies away on a ballistic trajectory going a long way before landing.

[Nowhere Man]

Even if the LM's exhaust did not produce a strong enough blast to form a crater, and even if the landing site was quite hard and rocky, there surely would have been a certain amount of dust/soil which would have billowed up to float down and settle on the above mentioned landing pads?

Other landers show decidedly dusty footpads whilst resting on the surface of other worlds. The 2008 Phoenix Mars lander of 2008 shows a completely dust-covered footpad in photos taken.  Why would there be this discrepancy?

Guiding question:  What is it that allows dust to "billow up" and "float down?"  What is one difference between the Moon and Mars?

Curse you Grashtel for beating me to the punch.

Fred

[cjameshuff]

http://www.lpi.usra.edu/resources/apollo/frame/?AS11-40-5926
http://www.lpi.usra.edu/resources/apollo/frame/?AS11-40-5925

Both images clearly show dust in the little wrinkles of aluminized mylar. Not a lot, but given the airless environment, that's normal. The atmosphere of Mars is quite thin, but it's still enough for dust storms. The moon has nothing of the sort to keep dust suspended. This might even be stuff kicked up by the contact probes rather than the rocket exhaust.

[JayUtah]

Even if the LM's exhaust did not produce a strong enough blast to form a crater, and even if the landing site was quite hard and rocky, there surely would have...

Your expectation; your burden of proof.

The exhaust gas velocity was about 2,000 meters per second.  The "dust" was entrained in a gale approximately 200 times faster than the most ferocious hurricane ever seen on Earth.  Throw a handful of gravel at a pie plate and see how much stays.

...would have billowed up to float down...

Nope.

Why would there be this discrepancy?

Try to work out which one has an atmosphere to support aerosolization and which does not.

[Glom]

As mentioned, your talk of dust billowing and floating despite the lack of an atmosphere calls into question how informed your expectations are.

[Edwardwb1001]

If dust 'flies away on a ballistic trajectory', and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels, and not flying away on a ballistic trajectory or travelling a far distance before landing. It falls virtually straight down, albeit in slow motion.

And Glom, Grashtel speaks of dust 'landing'. Does this not indicate that it did indeed 'float down', even at a distant point? Hence dust does float down - even on the moon. Actually, Glom, you imply from your comment that 'floating', cannot take place on the moon, due to the lack of atmosphere. I said 'float down'. I think you are confusing gravity with atmosphere.  How informed are you? Not very much, it seems. If objects do not float down on the moon (although Grashtel states that they eventually do), why did the astronauts not fly away after each bouncing step on the moon?

[Trebor]

And Glom, Grashtel speaks of dust 'landing'. Does this not indicate that it did indeed 'float down', even at a distant point?

No, it indicates it hits the ground.
Please stop with the word games.

[DataCable]

then how would you explain video footage of dust flying up behind the lunar rover's wheels

Because it was propelled up by the rover's wheels.

and not flying away on a ballistic trajectory

Do you know what "ballistic trajectory" means?

[Valis]

If dust 'flies away on a ballistic trajectory', and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels, and not flying away on a ballistic trajectory or travelling a far distance before landing. It falls virtually straight down, albeit in slow motion.

It all depends on the point where a grain of "dust" (regolith) leaves the wheel. This point defines the horizontal and vertical velocities for the particle. Some particles have mostly vertical velocity, so they'll land near the place they were picked up; others leave the wheel earlier, and travel further. I think the main point is that there is no dust cloud left behind, thus there can't be an atmosphere.

And Glom, Grashtel speaks of dust 'landing'. Does this not indicate that it did indeed 'float down', even at a distant point? Hence dust does float down - even on the moon.

No. "Floating" implicates a buoyancy of some sort, and you need a medium (air, water, etc.) for that. "Landing" means just touching down on the ground. And would you say that a rock floats down here on Earth when dropped? The little buoyancy the atmosphere gives to the rock is pretty much negligible here. However, dust does float on Earth, and float down eventually.

Actually, Glom, you imply from your comment that 'floating', cannot take place on the moon, due to the lack of atmosphere. I said 'float down'. I think you are confusing gravity with atmosphere.

No, it's you who's mixing things up. The dust doesn't float down on Moon, it falls down. 

If objects do not float down on the moon (although Grashtel states that they eventually do), why did the astronauts not fly away after each bouncing step on the moon?

Once again, objects don't float (down or in any other direction) without an atmosphere. The motion is strictly defined by the initial velocity vector and gravity.

[Jason Thompson]

If dust 'flies away on a ballistic trajectory', and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels, and not flying away on a ballistic trajectory or travelling a far distance before landing.

Do you know what a ballistic trajectory is? Or why you can't look at a cloud of fine particles and deduce the behaviour of the individual particles from watching the behaviour of the cloud?

And Glom, Grashtel speaks of dust 'landing'. Does this not indicate that it did indeed 'float down', even at a distant point? Hence dust does float down - even on the moon.

No, it does not float. Floating requires something for it to float in. The dust falls.

I said 'float down'. I think you are confusing gravity with atmosphere.  How informed are you? Not very much, it seems.

More so than you if you can't tell the difference between floating and falling.

Can we assume you accept the explanation about the dust being blown away by the rather large and powerful rocket engine next to the footpads, since you declined to even mention it in your response?

[smartcooky]

If dust 'flies away on a ballistic trajectory', and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels, and not flying away on a ballistic trajectory or travelling a far distance before landing. It falls virtually straight down, albeit in slow motion.

That is exactly what I would expect in a vacuum UNLESS that dust is thrown upwards (in which case, the ballistic trajectory comes into play)

But that isn't what is happening. The lunar dust is simply being carried upwards on the Lunar Rover's wheels and falling off.

And its NOT falling off in slow-motion (which implies manipulation of the video playback speed, which is the angle you are trying to approach it from). The dust falls at 1.63 m/s. It doesn't matter in a vacuum whether its a dust particle weighing a microgram or a rock weighing a kilogram; BOTH will fall at the same rate because there is no air-resistance.

[Echnaton]

If dust 'flies away on a ballistic trajectory', and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels, and not flying away on a ballistic trajectory or travelling a far distance before landing. It falls virtually straight down, albeit in slow motion.

One of the observed features of motion of the dirt (regolith) ejected by the LR tires is driving is that the dirt falls forward from where it was picked up.  This may appear to be odd or unexpected to some observers but is a perfectly normal motion.  It can be seen in certain clips where the LR brakes quickly and the dirt falls into the back of the wheel.  IIRC, it is most visible in the lunar grand prix but, sorry, I don't have any reference clips.

Some people may have a intuitive notion that a turning wheel should eject the material in the opposite direction of travel of the vehicle.  Seeing cars driving in snow (maintaining traction and not spinning the wheels) with rooster tails coming off the back tires it may look this way, but it is not the case.  The snow travels in the direction of travel of the car, but at a slower speed making it fall behind the tire that kicked it up, but ahead of where it was ejected.  By fixing the observational reference on the ground rather when observing the movement of the snow it becomes apparent that the snow is moving forward in a nearly parabolic arc.  A little thought to imagine the rotation of the tire yields the conclusion that a rolling tire that has traction can only move forward or be still with reference to the ground.   The surface of an ideal circular tire would be still to the ground at the line of contact and move at twice the speed of the vehicle at the line opposite of contact.  Actual tires are not geometrically circular, and make contact over a significant area, but the concept remains.

So on the LR, we should expect the material ejected while driving with traction to move forward of the point which it was ejected.  While this may seem counter intuitive to some, it is an expected result that can easily be observed on earth.  There are certain times that the LR does spin its wheels and send a nice rooster tail out the back but that is also no different than spinning your wheels in a car.  If you don't have any snow handy to try this, you can also see the same effect while four wheeling on a beach.  We do more of that in Houston.   

[sts60]

Hi, Edwardwb1001.  Welcome to the board.

If dust 'flies away on a ballistic trajectory', and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels, and not flying away on a ballistic trajectory or travelling a far distance before landing. It falls virtually straight down, albeit in slow motion.

(Bolding mine.)

You might also want to consider the difference in speeds of the two means of propelling lunar regolith.  As Jay pointed out above, the stuff getting blown out from under the LM at landing is getting pushed by a 2,000 m/s exhaust gas.  I just did a rough estimate in my head for tangential speed of the LM tires and got, order of magnitude, about 5 m/s.  So even a stationary LRV spinning its wheels can't throw surface material nearly as hard as the LM's exhaust.

The exhaust gas velocity was about 2,000 meters per second.  The "dust" was entrained in a gale approximately 200 times faster than the most ferocious hurricane ever seen on Earth...

Jay, I think you mean "approximately 20 times faster".  Hurricane Camille was clocked up to around 190 mph, or about 85 m/s, so if we round up to account for some prehistoric monster storm that's roughly 100 m/s.

Anyway, [ETA: forgot to finish this], the difference in speeds imparted to the lunar material is striking - over two orders of magnitude. 

[JayUtah]

Jay, I think you mean "approximately 20 times faster".

Yes.