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I've noticed this discoloration / stains / burn marks along the edges of the Quest Joint Airlock thermal cover on the hatch before, but they seem even more pronounced and contrasting with the cover's natural color now, during the last US EVA-24 of the Expedition 38 crew (Rick Mastracchio as EV1, Mike Hopkins as EV2, and Koichi Wakata as Canadarm2 operator and support inside the station). For example, this is one of photographs taken during EVA 24 as the two spacewalkers egress:

   enter image description here

   Rick Mastracchio as EV1 (red stripes) and Mike Hopkins as EV2 (no stripes) leaving the airlock during US EVA-24 (Credit: NASA)

The thermal cover is visibly discolored along its edge, gradually pronounced towards the outermost edge. I'm not sure if this is some sort of oxidation, chemical burn of thermal insulation materials as the airlock gases escape, discoloration of some materials due to exposure to the Sun as it remains open during the EVAs, or something else?

What is the cause of discoloration along the edges of the Quest Joint Airlock thermal cover?

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OK, I found an answer on my own in a caption to the NASA Image of the Day #1740 (11 August, 2010):

enter image description here

Into the Light

Reflecting on his experience as he emerged from the craft into the daylight on the Expedition 24 mission's second spacewalk, astronaut Doug Wheelock said "the colors of the Earth just explode at you as you exit toward the planet. Notice what looks like scorch marks on the hatch thermal cover, the effect of vacuum and atomic oxygen on the threads and thread sealant used on the thermal cover. The 'smell' of space follows suit, I’ve heard it described like burnt cake or cookies, or like the smell of an extinguished match."

Wheelock and fellow astronaut Tracy Caldwell Dyson completed the second of three spacewalks to remove a failed ammonia coolant pump module on the station's S1 truss on Wednesday, Aug. 11.

Image Credit: NASA

Emphasis is mine. So this is indeed due to oxidation of the sealant materials used on the thermal cover, caused by monatomic oxygen coming from the tenuous atmosphere at ISS orbital altitude that the hatch is exposed to when it remains open during Extravehicular Activity (EVA), some of them over 7 hours long, and also when closed along its edge where the seal isn't hermetically tight with the rim of the airlock. At ISS orbital altitude between 330 km and 435 km (205 - 270 mi), 90% of atmosphere is monatomic oxygen due to solar UV radiation breaking bivalent molecular bonds of diatomic oxygen:

$$\require{mhchem}\ce{O_2} + \text{UV 116 kcal} \to \ce{2O} $$

The more pronounced oxidation is on the Space Station's ram-facing side of the thermal cover, where interaction with the atmosphere is the greatest. Kudos to Mark Adler for this explanation!

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    $\begingroup$ The atomic oxygen ($\mathrm{O}$, not $\mathrm{O}_2$) is from Earth, not the airlock. 90% of the atmosphere at the ISS altitude is atomic oxygen. It is made atomic by Solar UV. The side that's burnt is the ram direction relative the atmosphere below. It has nothing to do with where the Sun shines. $\endgroup$ – Mark Adler Dec 30 '13 at 1:07

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