The modularized equipment stowage assembly (MESA) was a drawer in the base of the lunar module descent stage which stored equipment and supplies.

The modularized equipment stowage assembly (MESA) is located on quad 4 of the descent stage. It is deployed by the extravehicular astronaut when the LM is on the lunar surface. The MESA contains fresh PLSS batteries and LiOH cartridges, a TV camera, tripod, cable, tools for obtaining lunar geological samples, containers in which to store the samples, and other scientific devices. It contains a folding work table that also serves as a bracket on which the equipment transfer bag, used to transfer the PLSS batteries and LiOH cartridges to the cabin, is hung.

Apollo Operations Handbook: Lunar Module volume 1, section 2.11.8

During Apollo 13, an explosion in the service module caused the command module to lose electrical power. This required using the environmental control system of the lunar module to remove carbon dioxide from the air. However, the LM contained only enough LiOH canisters to remove the CO$_2$ of two astronauts for two days, rather than three astronauts for four days. There were square LiOH canisters from CM, but they did not fit into the holes for the round LM LiOH canisters, and an adapter from materials on-hand was infamously made.

Did Apollo 13 have additional LiOH canisters outside in the MESA that could have been used to scrub the air instead of adapting the CM canisters?

LiOH canisters in the MESA astronaut taking canister out of the MESA

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This question is part of a series honoring the 50th anniversary of Apollo 13, "NASA's finest hour".

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    $\begingroup$ There were only two suit connections to the LM life support system but three astronauts needing them to depressurize the cabins. Much oxygen would be needed to pressurize LM and CM again. $\endgroup$ – Uwe Apr 14 '20 at 17:21
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    $\begingroup$ One or two of the crew could have retreated to the CM, sealing the docking tunnel, and effectively used the LM cabin as an airlock for an EVA. Venting the LM to space would also have been an effective method of clearing the CO2, but I don't know if there would have been enough oxygen available to repressurize and keep the three-man crew alive for the rest of the flight. $\endgroup$ – Russell Borogove Apr 14 '20 at 17:38
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    $\begingroup$ There was enough O2 to evacuate the LM twice for lunar EVAs. Closing the CM hatch would have helped conserve O2. Uwe is correct about two hose connections, but the two PLSS could have also been used. Apollo 9 did actually do a spacewalk (from the LM to the CM). $\endgroup$ – DrSheldon Apr 14 '20 at 18:03
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    $\begingroup$ @DrSheldon There was enough O2 to evac and repress, and there was enough O2 to keep three men alive for four days, but someone's gonna have to do the math to see if there was enough to do both. $\endgroup$ – Russell Borogove Apr 14 '20 at 18:23
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    $\begingroup$ @OrganicMarble: Exactly. Although there was a slot in the LM ECS that accepted the suit cartridges -- to allow the system to be used while changing the main cartridge -- their capacity was quite limited. So the cartridges out in the MESA would have been of limited utility. $\endgroup$ – DrSheldon Apr 15 '20 at 14:17

According to page 44 of the Apollo 13 stowage list, the LiOH on the MESA was part # SV718783-9, "Cartridge/Canister, PLSS LiOH". The LM environmental system used part # LSC330-122-3-12, "Canister, ECS LiOH". It's not the square-peg-in-a-round-hole situation that the CM canisters provided (part # ME901-0218-0061, "CO2 Absorber"), but the round pegs on the MESA were the wrong size for the LM ECS.

cartridge in the stowage list

It probably woudn't have been too hard to rig something to use the cartridges while installed in the PLSS packs, but that leads to the other problem: duration. The PLSS cartridges were only rated to provide four astronaut-hours of absorption. They're probably good for at least three times that if astronaut is relaxing rather than bouncing around on the Moon, but even using both cartridges already in the PLSS packs and both cartridges on the MESA, that's only another 16 hours of carbon dioxide absorption.

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    $\begingroup$ You are correct on the inadequate capacity. However, no adapter was needed. The lunar module had two slots: the larger "primary" slot held the larger canisters used for the LM cabin, and the smaller "secondary" slot fit the smaller cartridges used for the PLSS backpacks. $\endgroup$ – DrSheldon Apr 18 '20 at 3:02
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    $\begingroup$ This NASA page also confirms that "Additional LM canisters were hopelessly out of reach in the MESA." $\endgroup$ – DrSheldon Apr 18 '20 at 3:09

According to the Cortright Report - Apollo 13 Review Board (5-33), when the the LM was jettisoned, it had 124 hours of Oxygen remaining. That is probably plenty to have done an EVA, but was unnecessary since they found a more conservative plan by adapting for the CM LiOH cannister.

On the same page of the report, it shows the LM had 4.5 hours of electrical power and 5.5 hours of water. I do not know what an EVA would have meant to power or water.

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    $\begingroup$ Welcome to Space! You've done a nice job explaining the feasibility of a spacewalk. However, the core question is whether there was any LiOH out in the MESA. Try looking at this stowage manifest of the mission. $\endgroup$ – DrSheldon Apr 16 '20 at 1:33
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    $\begingroup$ You can add material to your answer by clicking the "edit" button under the text of your question. You may also find this image helpful: i.stack.imgur.com/p7tsT.png $\endgroup$ – DrSheldon Apr 16 '20 at 17:07
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    $\begingroup$ There was enough oxygen, but to reduce CO2 the LiOH was neccessary. $\endgroup$ – Uwe Apr 16 '20 at 18:44

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