I was looking at a panoramic picture of one of the rovers on the edge of a crater and I noticed a lot of dust covering the solar arrays. I know that the mission team had a tough Winter trying to keep the robots from freezing, conserving battery power as much as possible… all that because the panels weren´t producing enough electricity.
All that a very forseeable situation, so why didn´t they add a little robotic broom to sweep the dust away?, all things considered that would be a tribial thing to do.
In addition to Squink’s good reply, I did hear on a National Public Radio program once that another reason is the mass of any sort of cleaning system. Whether the cleaning method is wiping or shaking, the apparatus would result in some other critical scientific apparatus would need to be removed. How small could you make a wiper motor and blade? It would still probably run to a kilogram or two. They decided to take the chance and it worked out.
While weight may be a factor, another issue is reliability; a wiper motor would be just one more thing to break, or short out, or otherwise cause problems. Besides, wind would probably blow free laying dust off of the panel in due time. You have to realize, these rovers were intended to operate for a nominal mission duration of 90 sols (Martian days) with a maximum duration of somewhere around 5-6 months. To date, they’ve kept operating for almost four and a half years, not only beyond mission and design lifespan, but beyond any expectation that some significant component would fail, seize up, or be damaged.
Part of the power loss during the Martian winter, too, was that large clouds of suspended dust were blocking sunlight. Once those winds died down, maximum power ranges were still in the low end of nominal (300-600 watt-hours per sol), which means even the basic expected degradation of the solar array and battery has been less than predicted.
While this is an impressive display of technical prowess on the part of both the rover designers and the mission operators, it is hardly unprecedented; the Voyager program spacecraft, already obsolescent Mariner-era platforms when launched in the late 'Seventies, essentially fulfilled all of the goals of the original Planetary Grand Tour mission (except for the Pluto fly-by) despite being funded only for Jupiter-Saturn passes. Putting a couple of guys on the Moon, well that’s all right, but by any rational measure NASA has made much better scientific and exploratory progress for time, risk, and cost with unmanned probes.
Perhaps Stranger on a Train can further elucidate, but IIRC from NPR, the dust build up on the solar panels was actually the main consideration in the life-span of the rovers. That is, engineers expected that the thing to kill the rovers in 90 days was dust build-up on the solar panels but they still couldn’t engineer an effective dust removal system and 90 days was good enough for the mission criteria.
The Phoenix probe is at a high enough latitude that it will experience a very long night, as Scandinavia does in winter. (Mars’ tilt to its orbital plane is very nearly the same as Earth’s.) The night will be even longer than that, since the Martian year is twice as long. Controllers plan to try contacting the probe when the Martian spring comes, but they’re pessimistc that the batteries will be able to hold out that long, so we could be looking at the mission lasting only a few months.
Additionally, the lander will likely be encased in ice during the winter, so having reasonably functional instruments, even if the batteries last, is highly unlikely. Nominal mission is 90 days, but current best guess is that the lander will function until around Christmas time. The rovers really set a ridiculously high bar for outlasting primary science phase!
If you use a liquid to squirt the panels, you contaminate the area and possibly sensors too. Remember the first time we were on Mars, they said life existed because of a test. Later they said oops it was a bit of contamination, and then maybe something else.
