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#1 2017-03-08 18:37:14

Oldfart1939
Member
Registered: 2016-11-26
Posts: 484

Potatoes can grow on Mars.

According to the article below--Mark Watney was right. Potatoes CAN grow on Mars, verified by a CubeSat experiment.

https://phys.org/news/2017-03-indicator … -mars.html

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#2 2017-03-09 09:24:48

elderflower
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Registered: 2016-06-19
Posts: 331

Re: Potatoes can grow on Mars.

This might be misreported, or maybe I've misunderstood. It says that Mars air pressure is used. If that means seven milibars the water would have boiled out of the leaves and they could not have grown new tubers.

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#3 2017-03-09 11:31:50

RobertDyck
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From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 4,690
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Re: Potatoes can grow on Mars.

Triple point of water is 6.12 millibars. That means liquid water cannot exist below that pressure, but can exist at 7 millibars. Of course at Mars pressure, water freezes at 0°C and boils at 2°C, so won't stay liquid very long.

I do suspect it's misreported. The article mention soil taken from "Pampas de La Joya desert", and "salty soil with some help from fertilized Earth soil for both nutrition and structure". I suspect the intent was to show potatoes can grow in a greenhouse on Mars. Can they grow in Martian soil inside a pressurized greenhouse?

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#4 2017-03-09 16:26:41

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 2,394
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Re: Potatoes can grow on Mars.

That triple point pressure for water is water vapor pressure,  not total atmospheric pressure.  Expose a puddle of water to CO2 at even 10 mbar,  and it still bursts into vacuum boiling.  Air pressure does not stop that,  only water vapor pressure does. 

That's a standard steam tables value,  and that's what the pressure in those tables means:  vapor pressure.  If there is air,  then the water vapor pressure is a partial pressure of that air pressure. 

If the absolute humidity is 10%,  then to achieve 6 mbar water vapor pressure,  the total atmospheric pressure will be 60 mbar.  Which effect is probably why I have heard that the min feasible air pressure in a Mars greenhouse is near 100 mbar. 

GW


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#5 2017-03-09 17:09:59

RobertDyck
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From: Winnipeg, Canada
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Posts: 4,690
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Re: Potatoes can grow on Mars.

I was very careful not to read "vapour pressure", it's the actual triple point. That means liquid water can exist. Actual liquid water. On Mars. At that absolute pressure. Of course, as I said, the boiling temperature is extremely low at 7 mbar. It's so close to the triple point that freezing and boiling temperatures are almost the same. Then again, salt will lower the freezing temperature, and raise the boiling temperature at the same time. I suspect the rivulets seen dripping down the side of canyons on Mars are actually brine. Not just salt water, but brine.

Boiling point of water at vacuum levels

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#6 2017-03-09 18:07:04

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 2,394
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Re: Potatoes can grow on Mars.

RobertDyck:

Go look in any classical thermodynamics text that includes the Keenan and Keyes steam tables,  any units of measure (because that does not matter).  Consult the verbiage that defines the variables in the table.  You will find it to be water VAPOR pressure,  not total atmospheric pressure.

This is 19th century stuff,  and those interpretations and definitions have been known since then.  They were developed for steam engine equipment,  whether piston or turbine.  In equipment like that operating steady state,  all the air has been chased out by the volume of the steam (water vapor).  The pressures in the system CANNOT BE ANYTHING BUT WATER VAPOR,  because there quite literally is no air.  It was displaced,  before you seal up and raise pressures in the equipment.   

I've seen an awful lot of people (including scientists) misinterpret the meaning of the triple point pressure,  thinking it is atmosphere pressure,  instead of just water vapor pressure.  But they are wrong!  They make this mistake because their areas of expertise are not classical thermodynamics.  One of mine is;  I used to do this stuff for a living,  in a business where even small errors were fatal.

GW

Last edited by GW Johnson (2017-03-09 18:11:47)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#7 2017-03-09 23:07:11

3015
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Registered: 2017-01-08
Posts: 4

Re: Potatoes can grow on Mars.

GW, I think what you are saying about vapor pressure being the relevant pressure the triple point is true.

But if the partial pressure of water is below the vapor pressure, then either evaporation or boiling will occur depending on the total pressure. In your example of 10 mbar CO2, the water should evaporate rapidly rather than boil, because the total pressure is high enough.

If what you were saying was true, water would boil at normal temperatures on Earth on a dry day (unless I misunderstood your previous comments, in which case I apologize).

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#8 2017-03-09 23:19:26

Oldfart1939
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Registered: 2016-11-26
Posts: 484

Re: Potatoes can grow on Mars.

As the OP of this topic, the cubesat experiment was done to show that  zero gravity had no adverse effects, and the Atacama desert data was to indicate the soil type and salinity. Not that potatoes could be grown outside a pressurized greenhouse. Just making a case that crops could be grown there in greenhouses without too much chemical soil modification.

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#9 2017-03-09 23:42:09

3015
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Registered: 2017-01-08
Posts: 4

Re: Potatoes can grow on Mars.

Oldfart1939, isn't the cubesat on Earth?

I think you're right that they are not using Mars temperature/pressure. The article does state:

"Inside this hermetically sealed environment the CubeSat delivers nutrient rich water, controls the temperature for Mars day and night conditions and mimics Mars air pressure, oxygen and carbon dioxide levels."

However, I can't believe that could possibly be true. The oxygen levels would be orders of magnitude too low for aerobic respiration, and transpiration levels are inconveniently high for plants even at 10 kPa.

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#10 2017-03-10 05:47:15

RobertDyck
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From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 4,690
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Re: Potatoes can grow on Mars.

GW: I greatly respect your knowledge. And I truly enjoy debating with individuals who are experts in their field. It makes me feel that I actually know something. So thank you very much. However, I have been very careful to separate "vapour pressure" from "absolute pressure". And if you choose to challenge all established knowledge, then that throws us back into to stone age.

The table linked above is for absolute pressure, not vapour pressure. When it says "PSIA" that means "Pounds per Square Inch Absolute", as apposed to "PSIG" which means "Pounds per Square Inch Gauge". I'm sure you know the difference. "Gauge" means pressure difference from ambient, so pressure difference from the room where the measurement is taken. "Absolute" means pressure compared to absolute hard vacuum. Notice the table is not vapour pressure, and it is not "PSIG". The table is for "PSIA". The title says "Boiling Point of Water at Various Vacuum Levels". It's not vapour pressure. I know you would like to lecture me that it is actually vapour pressure. And I understand your point. I was very careful not to use any tables or graphs that have anything to do with vapour pressure. This is actual absolute pressure. That means real pressure.

And to be absolutely "anal retentive" (to use a Freudian term), the table has entries for 0.147 PSIA and 0.088 PSIA. A pressure of 7.00 mbar converts to 0.101526 PSI. Interpolating between the two entries in the table produces a boiling temperature of +1.59°C. However, since I used a Mars surface pressure of "7" with one significant digit, so rounded the boiling temperature to one significant digit. I'm sure you understand significant digits. It's basic high school science, and you use that every day in your engineering work.

To be more precise, Mars Pathfinder recorded pressures over 3 Mars sols (solar days). The low was 6.77, the high was 7.08 mbar. Using those values 6.77 = 0.098190 PSIA, and 7.08 = 0.102686 PSIA. Interpolating the table produces water boiling point of +0.100°C for the lower pressure, and +1.797°C for the higher pressure. Those pressures should be rounded further for significant figures; the lower pressure becomes +.100°C (no initial "0") while the higher pressure becomes +1.80°C.

So your assertion that liquid water cannot exist at Mars ambient pressure is not far off. Reality is clean water without salt can exist, but only for a very narrow temperature range. That range can be extended with salt, and Mars surface does have a lot of salt.

However, as Oldfart1939 pointed out, this discussion thread deals with the article. The reporter made a fundamental mistake. We do not believe the test was done at Mars ambient pressure, or Mars atmospheric gas. It was done in a simulation of a pressurized greenhouse. That means more pressure and more oxygen. The test was to show Mars soil could be used, without any extreme measures to amend salt.

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#11 2017-03-10 17:31:36

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 9,475

Re: Potatoes can grow on Mars.

What it shows is that we can grow food if its protected from the cold, fire and excess water.. We have known that we can grow food on board the ISS for a decade or so but we are not farming on the ISS to create an ecological world that is sustainable by there own hand.

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#12 2017-03-11 09:04:30

Oldfart1939
Member
Registered: 2016-11-26
Posts: 484

Re: Potatoes can grow on Mars.

What was in essence demonstrated is the Photosynthetic Reactions converting CO2 and H2O to carbohydrates can still work under extreme conditions. There are bacteria, notably Halobacterium Holobium that exist under pretty saline conditions. A simple reminder that bacteria are Plants!

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#13 2017-03-11 14:33:01

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 2,394
Website

Re: Potatoes can grow on Mars.

RobertDyck:

Somehow,  I was not able to express myself clearly about the water vapor pressure issue.  I apologize for that. 

The meaning of the steam tables and the associated phase and 3-D pressure-temperature-enthalpy diagrams in all thermodynamic texts since the 19th century is quite clear.  The pressure in the tables and diagrams is the equilibrium vapor pressure of the substance,  not that of any atmosphere which that substance might be exposed to.  Whether expressed as gage or absolute pressure makes no real difference,  although absolute is far more convenient to actually use. 

When you relate that to real pools of water exposed to actual ambient atmospheric conditions,  it can be quite complicated,  even here on Earth.  Strictly speaking,  the tables and diagrams are only valid for liquid and vapor phases in thermal equilibrium.  In actual practice,  the pool of water and whatever gaseous media are in contact with it,  do not have to be in actual equilibrium,  as long as it is understood that pool’s liquid temperature governs the vapor pressure it tries to produce,  not the temperature of the gaseous media in contact with that pool. 

There is also the completely-separate problem of just how much water vapor can exist in an atmosphere of gaseous media.  This shows up as a partial pressure of that water vapor in that atmosphere,  and for air is governed by standard psychrometric charts.  At typical Earthly conditions (near 59 F = 15 C) ,  this is in the neighborhood of 4% by volume,  or 4% of whatever total atmospheric pressure there is. 

It can,  and does,  vary quite a bit:   once the atmospheric temperature is low,  that maximum water vapor content is also quite low,  trending toward pretty much practically zero,  long before you hit -40 F = -40 C.  That is why it is often “too cold to snow” in Canadian winters,  the Arctic,  and the Antarctic.  The numbers might shift a little bit,  but pretty much the same behavior obtains,  in the Martian atmosphere,  which is ~97% CO2,  crudely. 

Whenever the vapor pressure associated with the liquid pool exceeds the vapor partial pressure in the atmosphere above that pool,  there is evaporation.  The evaporation rate depends in part on the difference in those partial pressures (basically a diffusion process),  and in part on how much heat energy is available to support the phase change.  Usually,  one or the other dominates,  but both can. 

We’ve all seen this with a water puddle on a dry sunny day versus a humid sunny day,  and at night versus day at any given humidity.  When the air is dry and/or the sun is shining,  evaporation is rapid.  When the air is humid and/or it is dark,  evaporation is slow.  But it does evaporate.

The other effect (actual boiling) happens when the vapor pressure implied by the pool temperature exceeds the total atmospheric pressure over the pool.  That’s when you see bubble formation,  and very high evaporation rates indeed!  The so-called Armstrong limit of vacuum death above ~63,000 feet in Earth’s atmosphere is based on that boiling effect.  At that altitude,  the vapor pressure of liquid water at body temperature (98.6 F = 37.0 C) equals ambient air pressure.  The moisture on lung surfaces quickly boils away.  If the blood pressure inside the body falls to ambient,  then the blood boils,  too. 

This boiling effect is quite distinct from the problem of vented versus pressure breathing,  which comes into play down around 40,000 to 45,000 feet in Earth’s atmosphere.  That is a diffusable oxygen concentration limit across lung tissues into the blood,  complicated by water vapor displacement of dry air,  inside the lungs.  You’re not very functional on a vented oxygen mask at 40-45,000 feet,  but you do not die of hypoxia.  Above that,  you do,  which is why pressure breathing is required above those altitudes.  This usually shows up as a pressure suit of some kind,  except for the briefest exposures. 

As for stable water on Mars:  the equilibrium vapor pressure for 0 C liquid water is 6.2 mbar.  If the Martian air is at or above that same 6.2 mbar,  no boiling occurs,  but evaporation can be quite rapid if the sun is shining,  because that “air” can only hold a small single-digit percentage of water vapor.  In other words,  the Martian atmospheric water vapor partial pressure is at maximum on a warm day a very small fraction of a single mbar,  for a difference on the order of 6 mbar to drive diffusion-driven evaporation.  Given some available heat,  evaporation is fast.  But it does not boil. 

On the other hand,  if the Martian air is below 6.2 mbar total atmosphere,  then the exposed water will boil,  and evaporate away very quickly,  faster if the sun is shining than if it is dark,  but fast regardless. 

The effect of salt concentrations is merely to shift the triple point (and the entire solid-liquid equilibrium line on the diagram) to lower temperatures.  Same basic thermo-physics apply otherwise. 

GW

Last edited by GW Johnson (2017-03-11 14:33:57)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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