Planet Four Talk

A Change In The Wind

  • msc386 by msc386

    This image is interesting. Maybe not to others, perhaps, but I think it is. This image was marked has showing seven fans,
    one blotch, and three 'features'. Normally, fans tend to weather-vane in the same direction, indicating that they formed at
    approximately the same; but here, there is a slight but perceptible angular difference from one fan to the next, suggesting
    that wind direction was shifting as the fans were being formed in that same wind. Apparent density of these 7 fans would
    indicatie a slow, steady wind, changing direction slowly over time.

    Thoughts and opinions are welcome.


  • p.titchin by p.titchin in response to msc386's comment.

    Hi, msc, I spot the small differences, but I think it is sometimes hard to interpret them, and the time scales, as direction of venting can also be influenced by the slope and terrain, and on differently facing slopes, by katabatic winds.There are lots of extreme examples of this on the slopes Inca City. eg, have a look at the HiRISE greyscale image for APF0000ze3. ~ Pete


  • Kitharode by Kitharode moderator in response to msc386's comment.

    Multi-directional fans are very mysterious indeed. Have a look at this

    The image comes from wassock's discussion 'Bendy Winds' here; Enjoy 😃


  • PaulMetcalfe by PaulMetcalfe

    The first thing I want to say is that I am not disagreeing with current multi direction fan thinking and I would not apply what I am putting forward to all fans.

    However, the fans as depicted by picture Image APF0001m5f have intrigued me. They appear to have all burst to life at the same time or within a short while of each other and all appear to have the same split directions. This can be and most probably is just wind shift although it strikes me as odd.

    On the picture the origin of the right hand fan of each group seems to overlay the left hand origin or formed last.

    Although a mass of gas under the surface does not have to have a direction of flow, I have given it one just for this example (the left fan in the groups).

    The gas builds in pressure and breaks out at the weakest points (possibly, where a natural vent already exists) and if the velocity of the escaping gas and particles exceeds that of the wind it may flow in a direction different to the wind. As the under surface pressure decreases, possibly rapidly, it would be like activating a switch where the flow will be with the higher velocity wind (the right fan in each group).

    Just a thought.


  • Kitharode by Kitharode moderator

    And a very interesting thought it is, Paul.

    I'd suggest that the gas under the icelayer travels in whatever direction it can to get to the vent to escape, be this up, down, or sideways, so the under ice process will not have an effect on the direction of fans. When the gas and dust reach the vent however, it may well be pointed in a direction by the vent itself if the vent is 'leaning' that way, or if it is on a slope.

    Wassock has asked questions similar to yours (somewhere) and I'll post his discussion here if I can find it. There's an article (again somewhere) that discusses 'vent angles' and fan formation. Again, I'll post it if I find it.

    Nice idea, Paul. I hope you get some feedback on this. 😃


  • mschwamb by mschwamb scientist, translator in response to PaulMetcalfe's comment.

    An interesting idea Paul.

    One thing you might think about is Ithaca. Ithaca is very flat so topography is basically taken out of the equation. If you're right and the fans are more controlled by the direction and angle the gas is escaping than I'd be surprised to see all the fans in the same directions because I'd think that the gas would break out in different directions from different weak spots. Perhaps something to think about? It might be some evidence to support the wind blown hypothesis, perhaps not. I haven't thought too much about this, but I wanted join the discussion.




  • PaulMetcalfe by PaulMetcalfe in response to mschwamb's comment.

    Hi Meg,

    I am still busy on other things and hoped to respond sooner. I find Ithaca very interesting as I think I mentioned many months back but I have not studied it closely. I believe I read somewhere that is a very stormy area but don't quote me on that. My last comment on this region was to do with the apparently green rough texture areas. I think I called it 'Veggy lookalike'.

    Anyway, I do agree that gas will normally find its own escape routes in many directions under normal circumstances. I have tried to piece together possible unusual circumstances where it might not and have not posted them because they are essentially 'could be' rather than scientifically researched. Also it is based on possibly non-existent sub surface features. However, if you think I should stick them on the board, I will.

    It may give everyone a bit of a giggle or food for thought.



  • p.titchin by p.titchin

    High Paul.= More thoughts on venting! When we consider venting directions, if we postulate a high venting pressure from a vent pointing in a direction differing from the wind direction prevailing at the time of that initial high pressure release, then- if there is a wind blowing at the time, - once the high pressure deposits have been fired into the atmosphere ( whatever the direction of venting), due to the thinner atmosphere, they will be carried by the wind, and the fan will lay out in the direction of the wind. My own thoughts are that this explains the many fans we see with a 'pool' or 'globular' shape at the venting point, ( these thick bases being formed from the heavier dust which is not carried far by the wind , the rest of the dust being carried further and going on to form the classic fan). The dust on Mars in the thinner atmosphere stays 'in the air' longer than on earth, ( witness the length of the dust storms), and so I think that if there is wind, regardless of venting angle or venting pressure, if a classic fan forms, it is likely to indicate the wind direction. With the huge night time drop in temperature, I would expect sublimation and venting to stop or slow dramatically during the Martian night, and wind directions may change during the night.. With the new day, sublimation will increase, and a new fan may form .Depending on the new day's wind direction, the new day's fan direction may be the same, or similar, or may differ hugely. These are only my views, but they seem to explain many of the fan shapes and direction changes we see.(when severe slopes are not involved with a venting episode).. ~Pete


  • PaulMetcalfe by PaulMetcalfe in response to p.titchin's comment.

    Hi Pete,
    I like what you say because if I am reading you correctly it tallies with a surface wind thought I consider highly plausible to give the remarkable multiple fan effect where the similarities between groups of fans is uncanny.

    If a group of vents is constantly active (either continuously or during a specific period) I believe the following could apply.

    1/ The gas would be invisible at low pressure (not carrying particles) but would still dump particles close to the vent.

    2/ At a higher pressure it would carry particles further away causing a fan. If the wind has changed between low and higher pressure a new fan direction will show.

    3/ They could even switch on and off depending on time of day etc. and thus when they switch on and the wind has shifted you get a new fan direction..

    Anyway, I will give everyone a laugh on a separate post with my other less probable thoughts.



  • PaulMetcalfe by PaulMetcalfe in response to p.titchin's comment.

    PLEASE NOTE - The following is pure non scientific speculation on what might cause a mass of gas to RUSH in one direction on an alien world. I know very little about gas mechanics so it is quite possible that events listed below may have a completely different effect on a gas other than move it.

    I will use the term cavern but this can be small or gigantic and an open space or porous material. It can be deep or near surface.

    As loathe as I am to post these thoughts on what cannot be seen, I decided - what the heck - It'll keep them entertained, so prepare to strain those laughter muscles.

    1/ Confined sub surface space or cavern with narrow under surface outlet such as fissures/tunnels to other sub surface caverns/tunnels etc.

    Imagine a cavernous space under the true surface permanently enclosed by solid matter (Rock or hard ice). The cavern is gradually filled with gas from another cavern or from a non-return source. The gas can flow into one other cavern or to a vent through a narrow fissure that allows less gas through than is filling the cavern. Thus the pressure builds and although the gas does not necessarily rush to that fissure (it cannot), it may have a mommentum toward that outlet. Eventually the pressure is enough to break through blocked vents/fissures or weak spots in the cavern roof. The initial breakout, if (BIG IF) all breakouts are at the same time may be with that momentum. It may only be seconds of venting in that direction before it vents with the wind direction and probably only seconds again before the cavern loses pressure and the vents block up again.

    2/ Vacumn event or nearby drastic pressure drop

    Imagine the same cavern with larger sub surface fissure outlets to other caverns or tunnels. For some reason one of those destinations becomes vacuous or more likely has a big pressure drop probably due to a massive surface venting event. It could be possible for the gas in the cavern to rush toward that exit. However, the weak spots in that cavern roof will not be opened by gas pressure but by suction from the distant 'vacumn event'. The gas can then exit those weak spots (to surface) at high velocity in the direction of the fissure it was drawn to. With this event it could be possible that the multi fan vents are already venting with the wind. This event turns them briefly to another direction before ruturning to normal. With this I might expect to see a major venting event within a kilometre or two of the multiple fans.

    3/ Explosive event or nearby drastic pressure increase

    Similar to above but the gas gets a big push instead of a big suck. The ground vents can in this version be opened by the pressure and/or the explosive event. Of course this event would NOT need to be powerful enough to do anything other than give the gas a big push. With this event it could be possible that the multi fan vents are already venting with the wind. This event turns them briefly to another direction before ruturning to normal.

    4/ Blocked 'Nozzle' event (highly improbable but possible for this to effect all vents the same)

    If you think all the above is silly then this takes the biscuit and is basically reversing the directions previously assumed. Gas and particles escape a vent and blow with the wind. However particles start to clog the vent in the direction of the wind and the gas is deflected by the block and if it has a velocity higher than the wind may take a new direction. This of course could repeat many times giving an arc of fans. However I think I would be expecting the tail end of additional fans to be bending with the wind.

    5/ Tunnel event (Also improbable but possible)

    This is really (1) above without a cavern. The tunnels would all have to be running in the same direction from one source to vents. Or at least the vent end of the tunnels would need to be running in the same direction.

    Okay class you can stop laughing now.



  • p.titchin by p.titchin in response to PaulMetcalfe's comment.

    High again Paul,
    (1) your para 5 is close to what seems to be the consensus of what is occurring over much of the south polar cap. The gas collects under the ice as the sunlight passes through the translucent ice, and warms the dark surface. It then seems to find a weakness in the ice layer and vents, carrying surface material with it. This vent can be the point of escape for gas for some distance in all directions, etching out small surface gullies, and so, channels (your ' tunnels') form leading to the vent. This is thought to be the mechanism by which our fascinating south polar 'spiders' form, with radial legs ( surface ditches under the ice sheet), leading to the deeper central 'body' at the vent. The time scale is not known, but the process could well be occurring over quite a long period of time, eventually forming "spider networks".

    ( 2.) Your para 3

    I would expect that there must initially be a build up of pressure to force the vent open, and provide the gas velocity to etch the surface and carry the debris out of the vent and into the atmosphere. Where low pressure leakage occurs, there would be no force to cause venting jets.

    (3) Your para 4

    I'm sure debris could often block a vent as pressure falls, then with further sublimation, the pressure would increase until either the vent cleared, or a new vent was forced.The latter occurring could be why, after spiders have formed, vents and fans appear in the spider legs rather than the central body. The point to remember is that the direction of gas flow is from the legs, inward to the centre. Not as you might at first assume, from the centre outwards. Hope these thoughts help. ~Pete.


  • PaulMetcalfe by PaulMetcalfe in response to p.titchin's comment.

    Hi Pete,

    It was interesting that Paragraph 5 fitted into current theory. When you explain your interpretation as basically converging tunnels, I can see the sense of that whereas my Paragraph 5 was not referring to tunnels converging but running parallel and hence why I regarded it as improbable.

    The converging point I can totally agree with. I am not disagreeing with current thoughts but I would like to know if it is absolutely certain that the gas in a spider leg is flowing toward the centre.

    The reason I ask is that I can visualise Boulders and then Spiders (not all by the way) being formed both from flow away from a vent and secondly from converging legs. It would be interesting to get other views on this.

    Let us assume that there are several natural vents from sub surface and over time their venting maybe creates a boulder at the vent that leads to spider legs developing (I touched on this months ago). The flow will be from centre out.

    Over more time the legs from those spiders converge. At that point of convergence there is no vent and the flow is to the centre of convergence. However, that convergence point now becomes a surface vent fed from many legs and in time can create its own boulders and/or spiders. It would essentially be Spiders breeding spiders.

    You would have some spiders where the flow is away from centre (Mother spiders if you like) and some where it is toward centre (Children spiders).

    Although this has nothing really to do with multi fans, I thought it a worthwhile point to examine. I haven't looked for any evidence of this but I may keep an eye out for it.



  • Kitharode by Kitharode moderator in response to PaulMetcalfe's comment.

    Hi Paul. You've got some good stuff going on here and I'm enjoying your enquiry very much. Pete is doing a grand job with his contributions and he seems to have covered everything I might have offered up. Keep it up, y'all. 😃

    Your ideas seem to be concerned with what I call 'true' spiders (central body with legs). Is this the case, or are you discussing vents and fans for all 'species' of spider channels?


  • p.titchin by p.titchin

    Hi Paul,- yes, I think the view is that the gas flows from the periphery of the area of collected sublimation gasses when a vent is forced, and flows to the vent and thus escapes. However!! (a big 'however' that may give you pause for thought with some of your ideas) it is believed by some that the build up of pressure before venting occurs actually lifts the ice sheet, and then, with the rapid drop in pressure that occurs upon venting, the ice layer suddenly drops , increasing the pressure and maybe forcing gases back down the spider legs away from the 'body'. I hope that gives you some more intriguing thoughts to pursue! ~ Pete 😃


  • PaulMetcalfe by PaulMetcalfe in response to Kitharode's comment.

    Hi Kith,

    There has been a slight detour from multi fans to spiders and as of yet I am not connecting the two. My Para 5 assumed parallel tunnels/gullies for an improbable multi fan event. Pete mentioned them as converging which ties in with part of my Spider thoughts and hence the detour.

    The Spiders I refer to are I suppose 'true' spiders. As you may recall I have a not generally accepted view that some Spiders come into existence from sub surface rather than sub ice vents. If they do exist, they would flow centre out. However, Spiders created by sub ice vents could be many times more numerous due to converging spider legs or other non spider tunnels converging. That is just my opinion.


  • PaulMetcalfe by PaulMetcalfe in response to p.titchin's comment.

    Hi Pete,

    Yes, I think that is a very valid belief that the ice sheet gets lifted and collapses again. It seems logical to me that if the ice is say thick and heavy it is more likely to heave up weakening and opening a vent. It collapses with the vent expelling gas pushing gas down the legs. I think that would be viable whether the gas was travelling toward or away from the centre. In either case the legs could also weaken and vent due to either excess pressure down the leg or a clash of opposing gas flows.


  • wassock by wassock moderator

    Course there is also the heretics view which runs thus:

    There is no evidence for a causal relationship between spiders and fans or any sign of the spiders being made bigger. There are plenty of examples of places with spiders and no fans, and areas of fans without attendant spiders. Where spiders and fans co exist there seems to be little rhyme nor reason to where the fans form in relation to the spider. It would be great if there was always a fan at the middle of the spider but there isn't. In some areas, see the Heart..... under HiRise, there seems to be more fans on the edge of the spider which sort of goes against the mainline theory. In the heart area therecare lots of spiders all over withe islands of fan producers. The island spiders are well defined but the surrounding ones appear sort of worn out.
    However the fans make the spiders is the best theory we have at the moment and does fit the observations (mostly ?).

    My problem with multi directional fans and a pressure build up between eruptions, is that this requires the sealing and rupturing of vents in concert over a wide area. But the groups of fans all seem to be proportionate to one another. There may be a large number of 2 lobed fans over an area of various sizes, but if one lobe is twice the size of the other for one vent then the same proportions apply across the whole field.

    The thread Kitharode refers to above is here

    The multi fans in this area have formed to show the same pattern in each of the 4 years we have pics for. AND the different fan directions seem to form in the same order each year. AND some of the directional fans grow over the year across the whole area and maintain the directionality. Please read the thread if you want more detail.

    Hi folks I'm back 😃


  • iansmith123 by iansmith123

    Hi – I have looked at this discussion after classifying Image APF0002gw0 which is also Ithaca and looks to have fans deposited in two stages with two distinct arms. If the wind theory holds then this would look to suggest the fans were formed at times with two different prevailing wind directions. These observations are from a totally amateur’s viewpoint but pose a similar question to msc386.


  • Kitharode by Kitharode moderator in response to iansmith123's comment.

    Hi There. First off I'd like to say that it's good to know some of these old discussions are being read and added to. At present it doesn't matter whether you are an amateur or a professional; until we get some definitive results all viewpoints are valid. That's part of the fun of citizen science, don't you think? 😃

    There's a related discussion called 'Bendy Winds' which you might like to browse. (Link here). In that discussion my feeling is that 'slope' plays a major part in the fan formations.

    Good luck with your investigations. Cheers.