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NASA has just released a photo showing a part of the InSight Mars lander wet, like a cardboard box that came in contact with water (see the picture). What is that liquid?

The Instrument Deployment Camera (IDC), located on the robotic arm of NASA's InSight lander, took this picture of the Martian surface on Nov. 26, 2018, the same day the spacecraft touched down on the Red Planet. (Source)

Detail:

This is the object that looks like a wet cardboard box.

Regarding the theory of reflections advanced by somebody in a long answer,

I have to mention that the same image shows numerous reflections (see the picture below). None of them looks like a wet cardboard box and besides this I have never seen a similar case, when a reflection created the optical illusion of a wet cardboard box.

Evident reflections.

Using two more pictures posted by NASA, I made a gif that you can see below. It is evident that the theory of reflections, so insistently advanced by a user, is wrong. It can not explain why there is no change in the shape and size of what appears an area of cardboard that came in contact with a liquid. It is evident from the animation, which stretches across Sol 0 and Sol 1 (Day 0 and 1 on Mars, since landing), that there are numerous reflections and shadows that vary from photo to photo while nothing changes regarding the wet area.

Animation made of 3 pictures taken by one of the on board camera of the InSight Mars Lander on November 26 and 27, 2018. (Source)

Update, December 1, 2018:

Animation made of all available pictures taken by one of the on board camera of the InSight Mars Lander during 5 martian days. (Source)

Still no change in the size and shape of the wet area.
There is an evident shadow above the wet area that changes. It is generated by the black object, in the middle of the image, which moves.

tl;dr: The wet appearance of the soft-sided hexagonal box-like structure is an artifact of reflection and interpretation. The object itself is a cover over the vacuum vessel of the seismometer Seismic Experiment for Interior Structure (SEIS).

It is called an RWEB or "Remote Warm Enclosure Box (RWEB) consisting of two Mylar sheets with a two-centimeter gap that will fill with Martian atmospheric gas (mainly CO2) which will be mostly motionless and provide an additional thermal insulation layer due to the low thermal conductivity of carbon dioxide." See below.

About the wet appearance:

1. A dark but transparent Kapton layer has been applied on top of the shiny cover, giving it a wet varnish look.


2. Wet surfaces have high reflectivity at glancing angles due to strong Fresnel reflection at those angles for low index of refraction materials. The photo in the question is viewing at a high incident angle.


3. It's possible younger viewers haven't varnished anything and so are unfamiliar with what wet varnish looks like. Wet varnish has that "Kapton on Mars" look. ;-) The point being that while it may not look wet to some, to others, it does.

^†See comments below the question for context.

From Scientific American's NASA’s Next Mars Lander Zooms toward Laun

Close-up view of the Seismic Experiment for Interior Structure (SEIS), a seismometer provided by France’s Centre National d'Études Spatiales (CNES). Getting this delicate, precision instrument on track for flight caused InSight’s launch to be delayed from 2016 to next year. Credit: Barbara David

You can see in the pictures below that behind the seismometer there is a white dome. After the seismometer is placed on the surface of Mars, the robotic arm will pick this dome up and put it over the seismometer. An accordion-like ring will drop down from the dome so that it is tall enough to cover and protect the seismometer from direct exposure to wind and dust and some of the heating/cooling from day/night cycle.

below: The page https://www.seis-insight.eu/en/public-2/seis-instrument/rweb shows several more paper-like views:

below: From Spaceflight 101's InSight Instrument Overview "Wind & Thermal Shield – Image: CNES/InSight Project"

The LVL Sensor Package itself is further protected by a Remote Warm Enclosure Box (RWEB) consisting of two Mylar sheets with a two-centimeter gap that will fill with Martian atmospheric gas (mainly CO2) which will be mostly motionless and provide an additional thermal insulation layer due to the low thermal conductivity of carbon dioxide. Internally, the RWEB is also coated with VDA while the external face uses Kapton to prevent overheating during the deployment phase.

The vacuum sphere is a third layer of thermal protection for the VBB sensors and titanium washers provide isolation of the sensors from thermal variations on the sphere itself.

below x2: Click for full size! From Spaceflight 101's Photos: InSight Mars Lander Undergoes Assembly & Testing

Before the application of the Kapton?:

Your "wet cardboard" theory is nonsense. Wet cardboard is not glossy or reflective, unlike the object shown in the photos.

This is a cropped version of your first image. No liquid in the world can make that kind of pattern. That means it is a reflection after all, and the larger panel on the right also shows a reflection.

As uhoh already showed in his answer, the object we're looking at is covered in kapton. Kapton is a glossy plastic film which is eminently capable of reflecting light.

And some more rolls of Kapton, with recognizable reflections:

Further to @uhoh's "The wet appearance of the soft-sided hexagonal box-like structure is an artifact of reflection and interpretation." and @hobbes' "Wet cardboard is not glossy or reflective, unlike the object shown in the photos."

With today's (7th Decemeer 2018) release of images from the last few days there are a number of alternate views of the SEIS thermal cover. Here I've highlighted in green the panel in question, but taken from the opposite direction:

-- from here there is no horizon reflection on that panel (the camera angle is too high to see the sky), so no dark/light border to be seen.

I've created another animation demonstrating the same effect - the horizon reflection creates a dark/light border that could be interpreted as a wet patch, but as the camera moves, so the reflection moves - in one frame, circled in sky blue, it passes between two of the holes, curving just under the leftmost hole with the reflection of the arm to the left; in the other frame the reflection of the sky has moved up above the tape line, and the robot arm reflection straddles the central hole:

Here is a closeup of the reflection of the horizon (or wet border), as the camera moves over a 5 minutes. I've adjusted the images to keep the reflection of the horizon (or wet border) steady. If this was a wet border, the gap between it and the row of tape would remain constant, whereas in reality it shrinks. I've circled a mark on the surface (visible in all the photos) that is conveniently on the reflection of the horizon (or wet border) - it's moving means it can't be a wet patch:

In this animation of the first and third image, taken roughly a day apart, the arm has been moved. I've circled (in blue) the reflection of the claw in the dusty Kapton covered box.

Here's a rough idea of the layout of the area in question cgi

the sky blue line shows what I believe to be the horizon, the green line the shaped plate on the manipulator arm, and red the arm itself.

Just what is reflected in the left panel is harder to determine, without knowing the geometry - if it's showing to the right of the camera, the deep blue is TWINS, and the yellow more arm or part of RISE. If it shows to the left, then deep blue is HP$^3$, yellow may be the SEIS wind shield.

For those who beleive "it can not be the reflection of the horizon as long as it does not start exactly in the point where the horizon touches the hexagonal hat":