Starship Asterisk*

by **henk21cm** » Sat Jun 14, 2008 10:19 am

apodman wrote: Minor correction: Looking at the phase diagram, find the letter "n" in the word "melting". The orange area covered by this label represents liquid water above the triple-point pressure but below the triple-point temperature.

Higher pressure means a lower freezing/melting point.

Yes, you are correct. The slope of the solid-liquid equilibrium line is slightly negative: about -0.1 K/MPa, until about 210 MPa (i.e. 2100 bar). See http://www.lsbu.ac.uk/water/explan2.html#Pmelt and find out that the line is not straight, but slightly curved. For Martian conditions neglible. So i do not expect liquid water under natural conditions at the landing site of Phoenix. Pressures and temperatures are far beyond the possibilities as indicated in the phase diagram.

The first image of http://www.lsbu.ac.uk/water/phase.html shows a nice piece of linking. By clicking on one of the lines in the phase diagram you are transfered to the appropriate section.

by **apodman** » Fri Jun 13, 2008 9:13 pm

henk21cm wrote:Liquid water can only exist above the triple point temperature and above the triple point pressure.

Minor correction: Looking at the phase diagram, find the letter "n" in the word "melting". The orange area covered by this label represents liquid water above the triple-point pressure but below the triple-point temperature.

Higher pressure means a lower freezing/melting point.

by **Nerull** » Fri Jun 13, 2008 3:12 pm

astrolabe wrote:Hello All,

Some thoughts........

1) Ice was melted by the thruster from a larger, shallower area at first and then, as the heat became more concentrated the closer it got, a deeper, more pin-pointed melting occurred which forced water out of the cone and onto the surrounding area to spread and refreeze.

You wouldn't get water. Ice on the martian surface goes directly to gas form, like dry ice (frozen CO2) does on Earth.

by **henk21cm** » Fri Jun 13, 2008 11:11 am

cgb wrote:Ummm... for water ice to melt at 0 degrees C doesn't the atmospheric pressure have to be 1 Atm? How would the very low pressure of the Martian atmosphere affect the melting point of water ice?

Not much. Liquid water can only exist above the triple point temperature and above the triple point pressure (610 Pa). As you can see, ice does not have to be heated up to 0 C to be converted into water vapour: from red to blue in the image (recognizing colours is not my strongest asset). This process is called sublimation and is not shown in the image.

cgb wrote:And can this thin atmosphere conduct heat very well?

Conductivity of gasses becomes an issue at very low pressures, when a gas is so thin, it is called a Knudsen gas. Of the order of μPa. Heat transfer through the Martian atmosphere is mainly by convection.

The pressure at Mars is about 10 mbar. For a spacecraft such an atmosphere is 'thick soup'. It would tumble out of orbit rather soon.

by **cgb** » Fri Jun 13, 2008 9:32 am

Ummm... for water ice to melt at 0 degrees C doesn't the atmospheric pressure have to be 1 Atm? How would the very low pressure of the Martian atmosphere affect the melting point of water ice? And can this thin atmosphere conduct heat very well? Just wondering.

by **apodman** » Fri Jun 13, 2008 1:55 am

When a visitor from further down the Orion Spur arrives to investigate Mars an eon from now, ours will not be litter. Ours will be artifacts. We will be Martians.

by **astrolabe** » Fri Jun 13, 2008 12:07 am

Hello All,

Some thoughts........

1) Ice was melted by the thruster from a larger, shallower area at first and then, as the heat became more concentrated the closer it got, a deeper, more pin-pointed melting occurred which forced water out of the cone and onto the surrounding area to spread and refreeze.

2) The ice is very thin on top of a silica layer (the wavy edge is interesting).

3) PICK UP THAT SPRING AND PUT IT IN YOUR POCKET!!!!!! It would be an unprecedented gesture to not litter someone's/something's extraterrestrial home and set the tone for future visits. In which case, if we get domiciled there, maybe we'll take out more than we bring in. And the younger generations should watch us do it, too.

by **Sputnick** » Thu Jun 12, 2008 7:42 pm

June 12 Mars Lander photo .. http://antwrp.gsfc.nasa.gov/apod/ap080612.html

Those cones in the ice (or perhaps it's rock, but probably ice) are, if the photo is showing proper orientations, angled the wrong way to have been melted by the landing rockets' flames. I suspect they are vents .. and perhaps it's my imagination but is that a little hole at the base of one of the vents? The description says the cones are below the rockets, but that must be coincidence unless the picture was pieced together wrong.

by **iamlucky13** » Thu Jun 12, 2008 7:37 pm

Arramon, I think I watched the same program. Discovery channel?

Yes, the jets cut off about 1 meter above the ground. I don't think the jets they showed in the program up close were actually burning though. I think that was just an inert liquid being pumped through injectors to test the pulsed modulation system. Anyway, the caption is suggesting that the ice we think we see there isn't unmelted. Those two shallow holes are likely caused by the thrusters.

The don't want to fire the jets directly on the surface because the debris kicked up by the backblast could damage something, especially now that none of it is in it's protected landing positions. Also, there's probably not much fuel left, and there's no instrument designed to observe whatever might be kicked up. Basically all you would get out of it is a before and after photo. Not much return for the effort and risk when they can just wait a few more days and probably find the same thing with the shovel a few feet away.

by **Arramon** » Thu Jun 12, 2008 6:07 pm

The jets fired only up to a point above the surface, and the legs attached to the lander were built to sustain a medium jolt from landing after the jets were cut-off just above the site. I'm thinking about a 3-4 foot drop maybe?

Saw a program last night showing the different testing phases, and it showed what the jets looked like (about 12 pulsating jets that weren't one continual blast) and how the legs stood up to test landing drops on surfaces much harder than what the martian soil looks like.

Get that oven going and give me my cookies dammit! =b

hold the martian blueberries.... =/

by **HappyAmateur** » Thu Jun 12, 2008 5:18 pm

...I'm assuming, though, that thruster rockets are somewhat hotter than propane torches and were firing for more than a second or two (else the lander would have gone splat-crunch, yes?) I'm fine with the idea that not all the ice would necessarily have melted -- it just seems odd to me that there would be such a large patch of unmelted ice right under a thruster. (Perhaps it has to do with the extremely cold (by terrestrial standards) temperatures? -- I know that glacier ice melts more slowly than "normal" ice, but I don't know how much of that has to do with temperature and how much with pressure.)

by **Nerull** » Thu Jun 12, 2008 5:12 pm

Take a large block of ice, and a propane torch.

Pass the torch over the ice quickly.

Did the entire block just melt?

by **HappyAmateur** » Thu Jun 12, 2008 5:07 pm

Okay, my stupid question of the day...granted, I was raised in the pulp era of science fiction, but I don't think the melting temperature of water ice has changed much since my day -- if the lander was firing rockets all the way to touchdown, how can there be that much solid ice on the surface directly under it? (If they've invented cool fire, I want to know about that, too -- my cat's favorite perch is on top of my gas stove, and there's a disaster of some sort in the making...)

by **Nerull** » Thu Jun 12, 2008 5:01 pm

I doubt they want to contaminate the area more with hydrazine byproducts any more than they already have.

by **Bill Geraci** » Thu Jun 12, 2008 3:32 pm

That's just so, of course! I guess I was just thinking of a way they could tell, visually, in a short time. Maybe after they've gotten their sample(s) they could blow this to see how't behaves....

by **orin stepanek** » Thu Jun 12, 2008 2:25 pm

http://apod.nasa.gov/apod/ap080612.html

Right now they are looking for material in the ice that may be conductive to life. The material they scoop up does go into an oven and I believe that the contents are analyzed. So in a way they do melt the ice. 8)
Orin

by **Bill Geraci** » Thu Jun 12, 2008 2:08 pm

Actually, why not *try* and melt what *may be* ice? ;-)

I note with interest the shinny patch under the Mars polar lander. Today's (08/06/12) APOD says that the hole in that shiny bit is under the landing jet of the lander. I (obviously) have no idea what I'm talking about, but, it occurs to me: why not turn on that jet (gently!--not enough to flip the lander over!) and see if that deforms / melts the hole more? As another test of whether it's ice. (Needless to say, this assumes they can turn that jet back on in a controlled way and that there's fuel....)

Whadaya think?

Starship Asterisk*

Why not melt ice? (APOD 12 Jun 2008)

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Phase diagram of water: melting line has a negative slope

Re: 06/12/08 Mars Lander Photo

06/12/08 Mars Lander Photo

June 12 Mars lander photo

Probably not...

Why didn't ice melt in the first place?

Yes, but I'm impatient....

Why not melt ice? (APOD 12 Jun 2008)