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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?

uhoh
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SCH
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    https://mars.nasa.gov/msl/spacecraft/instruments/mahli/ – Organic Marble Oct 23 '19 at 14:15
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    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). – SCH Oct 23 '19 at 14:24
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    @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? – SCH Oct 23 '19 at 14:41
  • 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. – Uwe Oct 23 '19 at 16:49
  • 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. – Uwe Oct 23 '19 at 18:52
  • The classic optical microscope with a magnification of 100 or more has a very small distance between the front lens and the object, only some few millimeters. But MAHLI's minimum working distance is 21 mm. Some distance is needed for the illumination with the light sources. But MAHLI is not a microscopic camera only. It may focus from 21 mm to infinity. Images of very small details are possible as well as landscapes. Even 'selfies' of the rover are possible. – Uwe Oct 23 '19 at 21:45
  • @Uwe, interesting. So you're saying the short focal length outweighs the benefits of the higher magnification. Do you want to write this as an answer to my question? It seems reasonable. – SCH Oct 24 '19 at 07:20

1 Answers1

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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. But the weight budget of Curiosity did not allow both the camera and 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.

Uwe
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  • This answer presumes that the only thing worth looking at are geological samples. The "fungi on Mars" debate has been going on at least 15 years, if not 45, and to observe mycelia, entirely different techniques are needed. If you look at the Opportunity sol 1148 fungal photo, you can see the resolution is good, but quite inadequate to resolve fungal debates! – Linas May 08 '21 at 17:06
  • Great question! There is plenty of fine dust on Mars and a sieve can filter the smallest particles. The rovers are already drilling so they also produce dust. Adding a drop of water shouldn't be very hard to do. The lens can be protected with a lid against pollution and damage. I think the question still remains: why they are not using a microscope? – Joe Jobs Feb 15 '22 at 09:50