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Good magnification microscopes seem both light and cheap. As far as I could find out, there hasn't ever been one on any Mars rovers. The closest I could find is the Mars Hand Lens Imager (MAHLI) which only has a resolution of ~14 µm/pixel, closer to a hand lens (hence the name).

I can think of two reasons:

  1. There is a technical limitation: fragility, reliability, ...
  2. It isn't expected to teach us anything interesting.

Number 1. seems unlikely to me: there are plenty of cameras on the rovers, which don't seem fundamentally different from microscopes. Number 2. would also surprise me: there are a lot on earth you can learn from looking at rocks or soil under a microscope, why would it be different on Mars?

Since NASA scientists are actually pretty smart, I assume I'm missing something, so:

Why is there no microscope on the Mars 2020 rover?

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    $\begingroup$ mars.nasa.gov/msl/spacecraft/instruments/mahli $\endgroup$ – Organic Marble Oct 23 '19 at 14:15
  • $\begingroup$ organic-marble, I had seen this, but with an image resolution of ~14 µm/pixel, it is closer to a hand glass than a microscope (hence the name: Mars Hand Lens Imager). $\endgroup$ – SCH Oct 23 '19 at 14:24
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    $\begingroup$ @Uwe, edited to add a reference to MAHLI. Did you mean that the question should justify why it is not as good as having a microscope? Shouldn't it be the job of an answer to explain why the MAHLI is as good as having a microscope? $\endgroup$ – SCH Oct 23 '19 at 14:41
  • $\begingroup$ Is a finer image resolution than ~14 µm/pixel useful with completly unprepared samples? No cutting, grinding, etching and polishing of probes possible. No special microscope lighting. $\endgroup$ – Uwe Oct 23 '19 at 16:49
  • $\begingroup$ There is a microscope on the Mars 2020 rover: "Mars 2020's 7-foot-long robotic arm can move a lot like yours. It has a shoulder, elbow and wrist "joints" for maximum flexibility. The arm lets the rover work as a human geologist would: by holding and using science tools with its "hand" or turret. The rover's own "hand tools" extract cores from rocks, takes microscopic images and analyzes the elemental composition and mineral makeup of Martian rocks and soil. " From this NASA page. $\endgroup$ – Uwe Oct 23 '19 at 18:52
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The MAHLI camera of Curiosity may focus from infinity down to only 18.3 mm working distance. At minimal distance the resolution is 13.9 µm per pixel. It may image objects of some tens of meters in size down to only 22 by 17 mm. The Sherloc camera of the Mars 2020 Rover has a similar minimal object size of 23 by 15 mm.

A microscope with a magnification of 100 or more has a very small depth of field requiring very flat surfaces to get a sharp image. Gelogical samples on Earth require a lot of preparation (cutting, grinding, polishing, etching) before imaging under a microscope. To view samples by transmitted light you need to cut and polish samples so thin (about 30 micrometres) to be transparent. There is no rover capable of sample preparation for microscopy. Some preparation methods would require liquid water. A microscope would need another camera like MAHLI to find the spots where to use the microscope.

The distance of the front lens to the object is very small for a microscope, only some few millimeters. The lens may be damaged or polluted by dust very easily. A microscope has a fixed magnification, another ocular lens or objective lens is neccessary to change it.

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