Large impact craters often have pits on their floors. Some of these (with raised rims) are later impacts, but some are thought to originate immediately after the crater forms when slurries of molten and broken rocks occupy the crater floor.
Bakhuysen Crater, located in Noachis Terra, is thought to be the largest crater that possesses (and has preserved) these pitted materials.
Coarsely crystalline gray hematite is an iron oxide (Fe2O3) initially discovered from orbit by the instrument TES (Thermal Emission Spectrometer). TES has detected gray hematite in this area of Capri Chasma, one of several large depressions that make up the Valles Marineris canyon system.
This HiRISE image shows light-toned units beneath darker mantles. At the Opportunity landing site in Meridiani Planum, the same gray hematite is found in millimeter size globules that have weathered out of the sulfate outcrop and become concentrated along upper soils.
It is likely that the same scenario is taking place here in Capri with the hematite grains forming in the light-toned sulfates and then eroding out and concentrating in the darker mantle soils.
Earlier this year, the CTX camera team saw a crater containing a dark spot on the dusty slopes of the Pavonis Mons volcano. We took a closer look at this feature with HiRISE and found this unusual geologic feature.
The dark spot turned out to be a "skylight," an opening to an underground cavern, that is 35 meters (115 feet) across. Caves often form in volcanic regions like this when lava flows solidify on top, but keep flowing underneath their solid crust. These, now underground, rivers of lava can then drain away leaving the tube they flowed through empty. We can use the shadow cast on the floor of the pit to calculate that it is about 20 meters (65 feet) deep.
The origin of the larger hole that this pit is within is still obscure. You can see areas where material on the walls has slid into the pit. How much of the missing material has disappeared via the pit into the underground cavern?
What Lies Beneath
Universe Today | Jason Major | 2011 Aug 18
Instead of an Ant Lion, how about giant termites They live underground and have tunnels.
Re: HiRISE Updates (2011 Aug 17)
Posted: Fri Aug 19, 2011 5:16 am
by bystander
I found it humorous that BMAONE23 mentioned ant lions and you beneath, and then UT printed that article with that title. Ahh, "Great minds ..." I'm sure there's some around here somewhere.
Mars crater with hole in base
Posted: Tue Aug 23, 2011 8:28 pm
by JohnD
New HiRise pic shows a very symmetrical and new looking crater, with a hole in the base into a cavern about 25 meters deep. http://hirise.lpl.arizona.edu/ESP_023531_1840
The blurb discusses lava tubes as a possible cause of caverns.
But a perfect crater on top of a lava tube?
Has there been an impact that by chnace has gone through into a tube cavern?
John
Re: Mars crater with hole in base
Posted: Tue Aug 23, 2011 8:56 pm
by rstevenson
Although they initially call it a crater in the description, they go on to say...
The origin of the larger hole that this pit is within is still obscure. ... Later this year, HiRISE will acquire a second image to create a stereo pair. Seeing this feature in stereo will help us unravel the mystery of its formation.
So it may not be a crater from a hit, but rather a crater from a failure of the lava tube material. In other words, crater-like.
Rob
Re: Mars crater with hole in base
Posted: Tue Aug 23, 2011 9:15 pm
by JohnD
YEs, but speculation as much as my impact on a lava tube!
The 'crateresque formation' would have to be in very loose material to form what appears to be a conical pit, like the top chamber of an eggcup as all the sand runs out. Little sign of dust/sand/marbles inside the cavern.
"Pit craters" caused by collapse typically have vertical or even overhanging sides.
Wait 'n' see? Hirise promises a further pass soon.
JOhn
Re: Mars crater with hole in base
Posted: Tue Aug 23, 2011 9:21 pm
by BMAONE23
Maybe they will catch the Ant Lion poking it's head out through the hole
Re: HiRISE Updates (2011 Aug 17)
Posted: Tue Aug 23, 2011 11:31 pm
by Beyond
bystander wrote:I found it humorous that BMAONE23 mentioned ant lions and you beneath, and then UT printed that article with that title. Ahh, "Great minds ..." I'm sure there's some around here somewhere.
Yes, but the problem is; where are they hiding?
Re: HiRISE Updates (2011 Aug 17)
Posted: Wed Aug 24, 2011 7:10 am
by JohnD
My thnakls to bystander for merging mione with this thread.
I did look for, but didn't see, any prior posts to mine about this.
As it is in 'Sky photography', "new, spectacular, or mysterious sky images", I forgive myself for not looking there for this image there!
And 'ant lions'? Yes, analogous, but irrational, captain. Would that people did not indulge their whimsy quite so much. Serious Qs present themselves, for instance, what is the material of the scenery around? Is it loose dust, shale or scree, that is unstable, and so would collapse into a cavern? The heat of an impact into soft stuff would still have fused it into a solid crater, and as I said, pit craters aren't conical - or rather they are but upside down, with overhanging edges. Much to speculate about without saying, Ooooooo, it looks like an ant lion's lair!
John
Re: HiRISE Updates (2011 Aug 17)
Posted: Wed Aug 24, 2011 5:08 pm
by BMAONE23
It does resemble that particular type of Pit Fall Trap though which, although is dug by the critter in a top to bottom fashion, leaves a conical style crater (of sorts)
Perhaps a Dune Worm used his spice to fold space and tunneled under the crater
Re: HiRISE Updates (2011 Aug 17)
Posted: Thu Aug 25, 2011 4:59 pm
by BMAONE23
In all seriousness though, The hole does resemble the sounding hole in a Flute. I wonder if, when the wind blows, this hole whistles? Like blowing over the lip of a soda bottle
Re: HiRISE Updates (2011 Aug 17)
Posted: Thu Aug 25, 2011 5:42 pm
by JohnD
In all seriousness, you can work it out.
If the crater and cavity below approximate to a Helmholtz resonator, then the resonant frequency is v/2Pi x SQR(A/VoL).
v=speed of sound in the ambient gas - 879km/hr or 546mph on Mars, or 244m/sec
A=area of the neck (hole in base?) - No scale on pic. Previous holes were 150m across, so 17500m^2? See: http://apod.nasa.gov/apod/ap070928.html
Vo=Static volume of the cavity - complete guess Same hole "at least" 78m deep, so 1750000m^3?
L=Length of the neck (in this case nil, or 1 to avoid a zero)
(See: http://en.wikipedia.org/wiki/Helmholtz_resonance)
So, Resonant frequency = (very approx) = 4 Hertz
It would be a very bass flute indeed, and inaudible to the human ear!
JOhn
Re: HiRISE Updates (2011 Aug 17)
Posted: Thu Aug 25, 2011 11:29 pm
by rstevenson
JohnD wrote:A=area of the neck (hole in base?) - No scale on pic. Previous holes were 150m across, so 17500m^2? See: http://apod.nasa.gov/apod/ap070928.html
Vo=Static volume of the cavity - complete guess Same hole "at least" 78m deep, so 1750000m^3?
From the description...
The dark spot turned out to be a "skylight," ... that is 35 meters (115 feet) across. ... We can use the shadow cast on the floor of the pit to calculate that it is about 20 meters (65 feet) deep.
Rob
Re: HiRISE Updates (2011 Aug 17)
Posted: Fri Aug 26, 2011 8:35 am
by JohnD
rstevenson,
You seem to be reading a different APOD description to me!
The blurb attached to http://apod.nasa.gov/apod/ap070928.html reads, in part, " .... this mysterious dark pit, about 150 meters across, ...... though the bottom, at least 78 meters below, is still not visible. ...... "
Where did 35m and 20m come from?
But your maths is as good as mine - what note would such a smaller hole sing?
John
Re: HiRISE Updates (2011 Aug 17)
Posted: Fri Aug 26, 2011 5:20 pm
by rstevenson
JohnD wrote:rstevenson,
You seem to be reading a different APOD description to me!
Very much NOT the same pit!
OK ; v/2Pi x SQR(A/VoL) for this one.
v=speed of sound in the ambient gas - 879km/hr or 546mph on Mars, or 244m/sec
A=area of the neck (hole in base?) - No scale on pic. Previous holes were 35m across, so 55m^2?
Vo=Static volume of the cavity - 20m deep, so at least 1100m^3?
L=Length of the neck (in this case nil, or 1 to avoid a zero)
So, Resonant frequency = (very approx) 8 Hertz
Still a very bass flute, and inaudible to the human ear!
If the chamber were bigger then it might become audible.
John
Re: HiRISE Updates (2011 Aug 17)
Posted: Fri Aug 26, 2011 11:17 pm
by rstevenson
So now we have 4 Hertz and 8 Hertz -- I feel a tune coming on!
Rob
Re: HiRISE Updates (2011 Aug 17)
Posted: Fri Aug 26, 2011 11:49 pm
by Beyond
I didn't think Hertz had a company song. But they probably rent two-tone cars
Re: HiRISE Updates (2011 Aug 17)
Posted: Sat Aug 27, 2011 5:41 am
by Sam
rstevenson wrote:So now we have 4 Hertz and 8 Hertz -- I feel a tune coming on!
They live underground and have tunnels.
