Can the Mars lander's/rover's solar panels be cleaned?

Question

A comment by SF. regarding the InSight Mars lander caught my attention:

I really, really wonder why they didn't just include a small compartment with a duster brush grippable by the robotic arm.

Dust on the solar panels is a major issue for Mars landers without an RTG, and InSight does have a robotic arm. So adding a brush of some sort (either fixed to the arm or graspable) sounds like a good idea. It hasn't been done and (AFAIK) no other lander had this either, ever.

What could be the reason for not including a brush? Simple oversight or are there issues that are worse than having dust on the solar panels?

(Secondary question: Would a fan work in the thin atmosphere to blow away most of the dust?)

Answer 1

Reasons not to provide mechanical means to clean solar panels on Mars:

1. and this is the primary reason: Wind on Mars occasionally blows the dust away. This means dust is not a major issue, but a minor one. Spirit and Opportunity functioned for years despite not having dust removal equipment.
2. Mechanical operations are expensive: you've just added a series of operations that have to be designed and tested to avoid damaging the solar panels, and to avoid interference with the experiments. You've also added weight and complexity which eat into your budget.
3. Dry dust is abrasive. It's all to easy to scratch the solar panels.
4. Mars dust is tiny, more like cigarette smoke than the particles we recognize as dust. Any imperfections in the brush will result in dust remaining on the solar panels.
5. Based on my own experiments, a fan doesn't work well: as soon as you start blowing the dust away, there'll be a cloud around the rover that will be sucked back in by the fan.

NASA is looking into technologies that can clean a solar panel. This has been identified as an enabling technology for crewed missions to Mars. Evidently, NASA does not see this as a necessity for robotic missions up to and including InSight.
These 2 options are being explored:

1. Electrostatic cleaning: this leverages the static charge of dust particles. By creating an EM field, the dust can be moved to the edge of the panel.

The system takes advantage of the fact that most dust particles, particularly in dry environments, have an electric charge. A transparent electrode material such as indium tin oxide delivers an alternating current to the top surface of the panel. As it swings between being positively and negatively charged, it creates an electric field that repels positively and negatively charged particles. The electric field also helps to impart a charge to uncharged dust particles, allowing them to be quickly repelled as they come in contact with the panel. The researchers have designed the system so that the electric field works its way from one side of the solar panel to the other, gradually moving the dust along until it falls off.

...typically it only needs to be on between two and five minutes a day,

2012 paper on initial tests with a prototype panel showed promising results.

2. Vibration: this removes the dust by shaking the entire panel.

Using vibrations is simpler and requires fewer modifications to the solar panel, he says. But it does not remove fine particles as well as the electrical field approach.

Answer 2

Answering the "what would work?" part of the question:

I would use an electret haired brush which barely touches the surface of the solar panels.

Electret filters are very effective in collecting fine particles. The brush is mounted on a simple wiper. Once it collected the brown powder it vibrates downstream to shed the dust of, a bit like a dog after a swim.

Answer 3

Supplementary answer, which is also somewhat complementary to both @Hobbes's extensive/exhaustive answer and @LouisvanRijn's intriguing answer and shows that their answers were spot-on!

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From Forbes.com's Mars Opportunity And Spirit Rovers Could Have Lived Practically Forever With One Tiny Change:

One option would be to install some mechanism such as wiper blades to remove any Martian dust that accumulates. Here on Earth, windshield wipers are so commonplace that it seems like the obvious solution to such a problem. But Mars is very different from Earth in two important ways when it comes to dust accumulating on a surface like a solar panel.

• Martian dust is extremely small and fine-grained. Atmospheric dust on Mars is approximately three microns in diameter, and adheres via electrostatic forces. You can't just "brush it off" like you would on Earth; much dust would remain.
• There is no fluid to help on Mars. Water won't remain liquid on Mars, and you can only bring a limited amount of any fluid with you. Dry wiping motions would harm the panel surfaces, making them an inferior solution.

There are better options, though. Wiper blades are heavy, complex, subject to damage at the temperature extremes that Mars experiences, and are easily broken. A second solution would be to have articulated (tilt-able) panels, where you could simply orient them vertically. Since Mars normally has low-level winds that blow continuously, any dust accumulation could be blown off by the natural environment of Mars. In fact, there are "natural" cleaning events that occur on Mars, which we didn't know about when we designed these rovers; the winds occasionally, overnight, will blow large amounts of dust off of the rovers, upping their power output. It was only when Spirit didn't get one for an entire Martian year that it wound up dying.