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Mariners 2 through 9, and the Soviet spacecraft of that era, sometimes suffered from leaks that impaired or disabled parts of their scientific payloads.
What kinds of sensors couldn't work in vacuum? Why not just let the spacecraft body gradually vent the air that it happened to contain?

Cameras, magnetometers, radiometers, spectrometers, and their supporting electronics surely tolerate vacuum.

(I'd eventually noticed this over the first hundred pages of Robotic Exploration of the Solar System, Part I, but the book's index is too sparse to retrace my steps.)

A partial, perhaps simplistic answer: the Zond 2 was reportedly pressurized

to provide a laboratory-like environment for the internal equipment in order to simplify the design and testing of various systems as well as provide easier thermal control.

The same report gives an example of this impairment, for Zond 1.

The pressurized orbital compartment was leaking and all its gas would be lost within a week, severely compromising the ability of its equipment to operate.

But even the article from which these quotes come doesn't say how depressurization would hurt the payload.

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This was done to simplify thermal management. In a pressurized container, you can use air cooling. In a vacuum, you have to use heat pipes or liquid cooling to transfer heat to a radiator.

This also explains the failures: as the air leaked out, heat transfer was impaired to the point were components would overheat.

As the space race progressed, companies gained experience and confidence building systems that could operate in a vacuum, so the early pressurized systems were phased out.

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