Note: The MAHLI images below were acquired when both the two groups of white light LEDs and the ultraviolet LEDs were off.

When looking for the damage on the wheels of Curiosity on the raw images i came upon the image below. (in fact, it is part of an image )

thin piece of material

Part of an image captured by Curiosity on sol 1729 (June 17, 2017)

What is that thin piece of material, doesn't it look translucent ?

Could it be gypsum, or even foliated marble or is it just a piece of plastic from Curiosity ?

Edit: Below another image of the object with surroundings in the sunshine.
Click once or twice on the image for magnification.

object with surrounding

Another edit: Another image below to show "the ridge" to the left that also has the same "soft" appearance.

ridge to the left

Update: More ridges magnified.

More ridges

  • 1
    $\begingroup$ Is it something on the soil at all, or a scrape dug into the soil? Not sure. $\endgroup$ Jan 12, 2020 at 18:15
  • 3
    $\begingroup$ doesn't look translucent to me $\endgroup$
    – user20636
    Jan 12, 2020 at 18:46
  • 1
    $\begingroup$ I don't understand the lighting. Unless it was a cloudy day on Mars, shouldn't we see shadows? Or is the camera hovering over the spot and this is all in a shadow? $\endgroup$
    – uhoh
    Jan 13, 2020 at 2:19
  • 2
    $\begingroup$ @uhoh - these look like detached ridges of playa clay that roll via wind creep until they (neck and) bend. $\endgroup$
    – amI
    Jan 14, 2020 at 2:13
  • 2
    $\begingroup$ As to your comment about it possibly being montmorillonite (answer by @Oscar_Lanzi), montmorillonite is a clay, with a Mohs hardness of 1 to 2, which is very soft. I very much doubt that such a soft material would stand so proud for millennia. From the 3rd & 4th pictures it looks like a obdurate vein, possibly gypsum of silica. It's difficult to know from just photographs. I know cross posting of questions is frowned upon, but some of the geologists on SE Earth Science might be about to provide a better opinion. $\endgroup$
    – Fred
    Jan 15, 2020 at 5:08

2 Answers 2


As noted in the answer by Michael, a precise chemical composition is not known at this time.

We can, however, consider the overall mineral composition on the surface of the Gale Crater. This diagram provided by Curiosity's CheMin analyzer (source) shows that the composition varies with depth inside the crater:

enter image description here

The lower part of the crater contains large amounts of magnetite, $\text{Fe}_3\text{O}_4$ and mafic minerals, the latter being silicates rich in magnesium, iron and calcium (magnesium+ferrum+ic). Under weathering conditions with water and an oxidizing atmosphere, such as might have existed in earlier Martian history, the magnetite would be oxidized to hematite ($\text{Fe}_2\text{O}_3$), which is seen in the upper regions in preference over magnetite. The mafic minerals would be decomposed, a process known also on Earth (where the most common igneous rocks exposed on the surface, such as granite, are felsic rather than mafic). This weathering can lead to various products such as more iron oxides (which again would be oxidized to hematite), lighter silicates such as feldspar and clay, silica ($\text{SiO}_2$), and perhaps salts of the calcium and magnesium depending on other materials and environmental conditions. Thus the composition difference between lower and upper regions appears to be due to the upper regions being weathered while the lower regions, perhaps protected until the atmosphere became thinner, is more "preserved".

In the presence of sulfur-bearing materials the weathering of mafic minerals could indeed form calcium sulfate, therefore the gypsum ($\text{CaSO}_4\cdot 2\text{H}_2\text{O}$) or anhydrite ($\text{CaSO}_4$, no included water); both of these minerals may appear translucent. Like hematite and the lighter silicates, calcium sulfate is concentrated in the weathered upper regions and not in the mafic/magnetite-rich lower regions. Thus gypsum or anhydrite is one possibility for the material in question, most likely if the image is from the upper part of the crater.

  • 1
    $\begingroup$ It appears that the upper right circle (sebina) is from sol 1500, while the piece of material on the image is from sol 1729, so it could indeed be gypsum. Couldn't it also be montmorillonite ? en.wikipedia.org/wiki/Montmorillonite $\endgroup$
    – Cornelis
    Jan 14, 2020 at 22:20
  • $\begingroup$ Exactly. So many minerals are present that even if we exclude the ones confined to the lower layer we can't know one answer. Gypsum is merely possible in the upper layer. $\endgroup$ Jan 14, 2020 at 23:09
  • $\begingroup$ The large orange areas in the top four samples are the largest abundances of calcium sulfate. Only a few this slivers are the darker orange of fluorapatite. $\endgroup$ Jan 14, 2020 at 23:32

Mudstone, although the precise chemical composition is unknown at this time (at least to the extent I was able to find it out).

From the Planetary Society, it is identified as being mudstone. Unfortunately, from wikipedia, it is clear that mudstone is a very general type of mineral and can be composed of many different things, as long as it was once primarily clay.

  • 1
    $\begingroup$ Usefull links, but the fact that the object in question lies upon mudstone, doesn't mean that the object itself is made of mudstone ! $\endgroup$
    – Cornelis
    Jan 14, 2020 at 9:34

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