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Posted by: djellison Nov 21 2007, 09:18 PM

Saw a new HiRISE image up of Gusev earlier and wondered if the 'chute had moved given the strong winds.

The top of the 'chute moved between Nov-22 and Dec-12 '06, and then back again May-10 '07. I'm not sure how much is lighting and brightness/contrast within photoshop - but I think the last image, the chute is possibly dirtier than the other.

Off to hunt at Meridiani now

Doug

Posted by: CosmicRocker Nov 23 2007, 04:03 AM

That is really interesting to see. Thanks for putting that together. Considering all the wind activity we have observed through Spirit's eyes, I'm surprised the parachute seems to have moved so little over such a large time interval. All I can guess is that the wind is consistently unidirectional, or that the 'chute is snagged on rocks. Did you find anything interesting at Meridiani?

Posted by: Phil Stooke Nov 23 2007, 04:19 AM

That is brilliant!

Phil

Posted by: Sunspot Nov 23 2007, 08:08 AM

There is a rather dramatic deterioration in image quality as time passes - is that due to the problems they have had with the camera, or atmospheric/distance to the target etc?

Posted by: djellison Nov 23 2007, 08:20 AM

There's only two of BS+HS at Meridiani and nothing's happened up there at all. Couldn't really see any changes at the HS either. I might see if the lander's deflated or had dust built up or something as well - but that's for another day

Not sure on the loss of quality - I agree it's not all as good as that first shot. It's all 1x1 binning - all the same product for each observation ( JP2 Grey scale map projected quicklook for IAS ).

Doug

Posted by: AndyG Nov 23 2007, 08:34 AM

While it certainly looks like the 'chute has moved, I suppose there's a chance that there's a process of deposition and uncovering by dust, too.

Andy

Posted by: djellison Nov 23 2007, 08:37 AM

The chance of one single patch of 'chute being covered to exactly the same shade and texture as the underlying soil, then being perfectly cleared again seems a bit of a long shot

Doug

Posted by: Juramike Nov 23 2007, 02:36 PM

That sequence brings Mars even closer to the imagination....

Would the "flap" sound of the parachute moving be at a higher pitch or lower pitch than here on Earth?

Posted by: nprev Nov 23 2007, 03:45 PM

Not sure if the freq would shift at all, Mike; can't see any obvious reason why it would. However, the volume of the sound would be MUCH lower because of the low density of the atmosphere (each flap slaps far fewer air molecules than a flap on Earth= less kinetic energy transmitted).

Posted by: kenny Nov 23 2007, 11:46 PM

We hear higher frequency "Donald Duck" speech from someone who has breathed Helium at normal atmospheric pressure - e.g. my daughter does it for a laugh out of those toy Helium balloons you can buy at the fair. That pitch change has nothing to do with pressure, which is constant, but with the composition of the gas mixture. Since the Martian atmosphere is different in composition as well as pressure from Earth's, might we also expect a different frequency and well as the lower sound volume which nprev mentions ?

Kenny

Posted by: nprev Nov 24 2007, 12:05 AM

That's a good point. Given that the Martian atmosphere is something like 95% CO2 (atomic weight around 44), 3% O2 (atomic weight 32) and ours is 78% N2 (atomic weight around 28) + 21% O2, there ought to be a difference, but I can't pin down the critical factor(s). The differences in the average atomic weight of the atmospheric mixtures should have an effect on amplitude (though not as much as the differential densities), but is there also a resonant component? Dunno.

EDIT: Aha! All hail Google; found http://www.newton.dep.anl.gov/askasci/chem99/chem99330.htm right away. Looks like pitch is a function of atomic weight, so my best guess is that sounds on Mars will actually have a lower pitch than their terrestrial equivalents, but of course not nearly as loud for the reason I cited earlier.

EDIT2: This is a more interesting question the more I think about it. As anybody who's ever traveled by air knows, things still sound the same when you're at cruising altitude, omitting the white noise from the engines & air friction, although the air density is a lot lower than normal (airliners are usually pressurized at the 10,000 ft equivalent level, or something like 75% of sea-level pressure). Therefore, pitch differences do seem to be almost completely dependent on gas mixtures...so, things are likely going to sound a bit different to us on every world, and also would have during some of Earth's previous geological eras when the O2 ratio was much higher...the dinosaurs probably had some pretty deep roars...neat!

Posted by: kenny Nov 24 2007, 09:37 AM

Interesting! I've been to 18,000 ft climbing and never noticed voice pitch changes, but volume certainly gets weaker. Incidentally I've always thought aircraft are pressurised to 8000 ft equivalent, but the precise figure doesn't much matter, your point is valid.

Posted by: Reckless Nov 24 2007, 09:53 AM

Very interesting comments, we really must have a microphone on Mars soon to hear the flapping of parachutes etc.
And of course the sound of dinosaurs on Mars would be super neat.

Roy

Posted by: ngunn Nov 24 2007, 03:04 PM

The molecular weight (and temperature) affect the speed of sound, and therefore the resonant frequencies in a cavity such as the human voice box or a tin whistle. This is what makes the helium-breather's voice go high. However there is no change in frequency of the sound once emitted, as a result of travel through the atmosphere - any atmosphere. The same number of wave-fronts must pass each observer in a given time interval, therefore sounds that are emitted by mechanical vibration such as a tuning fork will be heard at the same pitch on any airy planet. The only way to change the perceived frequency is via the doppler effect when relative motion is involved.

However it is the case that different atmospheres could attenuate different frequencies differently. That could affect the 'colour' of white noise such as might be produced by a rustling fabric. Any difference would be minimal at close range, increasing with distance. I have no details on this.

Posted by: fredk Nov 24 2007, 05:41 PM

I think you've nailed it there, ngunn. Given a source of some frequency, it must be heard at the same frequency barring Doppler shifts.

But it's still an open question what effects the different Martian air might have on the generation of sound to begin with. I'd expect that the dynamics of a large piece of flapping fabric would be quite different in Martian air than on earth, but it's a messy problem. Could there be resonances involved? Since there's less air resistance and the wind speeds are higher on Mars, I'd guess the generated pitches would tend to be higher. Just a guess.

Posted by: mike Nov 25 2007, 12:15 AM

The only way to really know is to actually go there, make the sound, and listen, but..

How does a person distinguish between 'white noise' and 'legitimate sound'?

Posted by: Shaka Nov 25 2007, 12:38 AM

How does a person actually go there, make the sound and listen, if his head is exploding?

Posted by: mike Nov 25 2007, 02:14 AM

It might be tough on Mars.. a robot could do it, and we could assume it hears what we would hear.. if our heads didn't explode.

Posted by: nprev Nov 25 2007, 03:12 AM

Yes, the whole 'exploding heads' issue is difficult to overcome via any current technology. Therefore, we'll sensibly leave our heads on or near the surface of the Earth for the time being & rely on microphones placed on future landers (interpolation may be necessary)...

Posted by: OWW Nov 27 2007, 09:36 AM

Interesting bit here:
http://space.newscientist.com/article/dn12968-mars-rover-spirit-gets-stuck-as-winter-approaches.html

HiRISE is sending back terabytes of data, but not as much as scientists had originally expected due to noise in some of its detector channels. The problem stems from errors in converting the electrical voltage in the camera's CCDs to digital data in the form of binary code.

Such 'analogue to digital conversion' has been used in space many times before. "But what's new about HiRISE is that it's a very complex camera, using 14 CCDs instead of just one typically, all operating at a very high rate and really pushing the limits of some of these devices," he says.

To reduce the noise, mission managers heat the camera from its normal operating temperature of about 18 to 32°C. But engineers do not want the camera's electronics to get too hot for fear they will become damaged, so they have to cool the camera down for about 30 minutes after each image.

"The downside is it takes longer for the camera to cool down between images," Herkenhoff says. "It means we can't take as many images as originally planned."

Recently, mission managers have been taking one large image during every two-hour orbit the spacecraft makes, so that "when the orbiter's on the back side of Mars, for that hour, there's plenty of time to cool down so we can take another big image on the next orbit", he says.

Engineers are also studying whether or not the camera can safely operate at higher temperatures, which would reduce the amount of time needed to cool it between images.

Posted by: edstrick Nov 27 2007, 10:12 AM

I *suspect* the spare transmission bandwidth is going to other instruments, *maybe* especially CRISM

Posted by: djellison Nov 27 2007, 10:55 AM

CRISM was a slightly bigger bandwidth user than HiRISE from the outset anyway.

Doug

Posted by: tedstryk Nov 27 2007, 12:06 PM

The wording is a bit confusing. The beginning sounds like they are sending back a smaller volume of data. However, the later part makes it sound like the smaller number of images is leading to them taking larger images (I assume longer ground tracks, not binning the data and more multispectral shots), which would mean that the actual transmission volume would not be reduced as much as the number of images would reflect.

Posted by: edstrick Nov 28 2007, 09:04 AM

There may be dynamic trade-off between instruments, though on very short terms <say one instrument has a "what's that" that needs to be investigated> it's undoutably impossible to plan new observations in a few days to fill some other instrument's bandwidth if that one goes off line.

Posted by: Oersted Nov 28 2007, 11:01 PM

The real question is, if there actually IS a sound of flapping canvas, if nobody is listening, isn't it?

Posted by: nnyspace Dec 7 2007, 11:57 PM

Well some could get a vacuum chamber and fill it to Martian pressure with Martian atmosphere composition, Just saying.

Posted by: djellison Dec 8 2007, 09:14 AM

The sound would travel through the metal of the chamber as well - it's not a totally honest test. I'm sure some was done for the Mars Mic sent with MPL though.

Doug

Posted by: ElkGroveDan Dec 8 2007, 04:49 PM

We'll just have to include a microphone and a flapping piece of cloth on the UMSF balloon.

Posted by: djellison Dec 8 2007, 05:23 PM

- That's GENIUS.

Doug

Posted by: atomoid Dec 13 2007, 05:30 AM

I'm a bit late to this 'sound' tangent, but i read http://www.sciencenews.org/articles/20070630/note17.asprecently with some tidbits some may find interesting: "...Mars' thin atmosphere would cause sound to fade away after traveling just a few meters. The atmosphere of Venus, almost entirely carbon dioxide, would muffle high-pitched sounds. Titan's frigid, nitrogen-based atmosphere would offer the best sound transmission across all frequencies..."

an older http://www.sciencenews.org/articles/20060708/note12.aspsays: "...The researchers' simulations suggest that the sound of a loud scream—which on Earth can be heard as much as 1,000 meters from its source—would travel only 16 m on Mars..."

another http://www.acoustics.org/press/151st/Hanford.html has a movie modeling sound wave propagation on Mars.

Posted by: ngunn Dec 13 2007, 10:24 AM

I have a problem with statements like this. They imply that the writer thinks that sound intensity decreases with distance from a source because of absorption, whereas the main effect is really the spherical spreading of wavefronts resulting in an inverse square law. Now it will be difficult to get much energy into the Martian air in the form of sound waves, either from a scream or anything else. There is after all little mass to set in motion. But once you've generated a Martian sound wave, however faint, its intensity should vary with distance in the same way as anywhere else - to a very good first approximation.

Posted by: dvandorn Dec 13 2007, 04:39 PM

I'm unsure of your logic there, ngunn. During Skylab, which IIRC was pressurized to about 5psia with pure oxygen, the crews found that they could not be heard by other crew members once they were more than about 5 meters away. Also, squawk boxes that were installed to provide comm through out Skylab weren't numerous enough, and even caution/warning klaxons became impossible to hear a very few meters away from one of them.

At lower pressures, sound attentuates faster. The pressure wave may move as far regardless, but its energy (i.e., ability to excite an eardrum) falls off faster. Thus, sound becomes inaudible at a much smaller distance. That's not speculation, it's empirical, observed fact.

-the other Doug

Posted by: fredk Dec 13 2007, 04:54 PM

Also, from the last of atomoid's links:

Also, you can find simulated sound samples for Titan, Venus, and Mars http://peppermintleafresearch.net/res_andi.html

Posted by: ngunn Dec 13 2007, 05:12 PM

I'm not questioning the second statement, or the empirical observation. If there is a hundred times less energy in a sound wave to start with then a one-over-r-squared decrease will certainly reduce it to the level of inaudibility over a shorter distance.

It's the first one. By 'attenuate' do you mean it gets absorbed and converted into heat energy or something? Does a thin atmosphere do this significantly more than a thick one? I doubt it, especially over a 'few metres'.

It's a question of acoustic impedence mis-match. Bang a drum and much less energy radiates off as sound in a very thin atmosphere (most of it continues ringing around within the drum). Likewise it's hard for that energy to cross back over into a denser medium like the air in a spacesuit (most of it reflects away again). Therefore it is harder both to produce and to hear sounds via the Martian atmosphere because of losses at both ends. That doesn't mean the atmosphere has the magical property of swallowing up sound energy.

Posted by: fredk Dec 13 2007, 05:25 PM

Also, http://peppermintleafresearch.net/Andi/figPlanets_Alpha.gif from the page I linked to above shows considerably higher absorption on Mars than Earth.

Posted by: ngunn Dec 13 2007, 09:00 PM

OK here's what I take from that diagram.

Consider a sound coming from a source of radius 1m. What is the relative intensity 10 metres away?

On Earth absorption is negligible over that distance, whereas on Mars absorption has reduced the intensity to about 1/e - a noticable difference indeed and more than I expected. However the drop in intensity due to geometrical spreading of the wavefront is by a full factor of 100 in both cases, so this effect would easily dominate over 'conversational' distances, even on Mars.

At what distance (on Mars) does absorption overtake geometrical spreading as the main factor reducing the intensity? Some back-of-envelope scribbles give me an answer of about 90 metres for this same 1m radius source.

Conclusion? Sound absorption might prevent you from hearing a rocket taking off a couple of kilometres away, but for closer human-scale sounds the main difficulty would be the impedence matching problem - getting the sound energy through the interface beween the thin Martian air and the denser medium in which your ear is presumably immersed.

Posted by: ngunn Dec 17 2007, 02:35 PM

Prompted by o'doug's and fredk's posts I decided to take another look at this. Here is my attempt at a graph: