First LED left on another planet⁺ by humans?

July 20, 1976, Mars, Viking 1 lander.

In the article "Viking gas chromatograph–mass spectrometer" by Rushneck et al, Review of Scientific Instruments 49:817-834 (1978), section G (pp. 828-9) describes the GCMS's soil loader and pyrolyzer subassembly, which accepts a pulverized soil sample, loads it into an oven, and then seals and heats the oven.

As can be seen from Fig. 18, all mechanical functions are driven by two permanent magnet, incremental stepping motors: one drives the loader and carriage, and the other drives the clamp. ... Carriage and clamp positions are verified by signals from light emitting diode-phototransistor pairs on the carriage and clamp. These signals are fed into the GCMS computer and are used for verification of the proper function of the mechanism.

This paragraph also directs the reader to further details in ref. 94,

Encoder Assy P/N 550050, 550052, [available from] Beckman Instruments Inc, Advanced Technology Operations, 1630 S. State College Blvd., Anaheim, CA 92806

but that may be a dead end by now.

The Vega 1 balloon that entered the atmosphere of Venus on June 11, 1985, had LEDs on the anemometer:

The diameter of the rotating anemometer was 25 cm. The rotor was mounted on ball bearings, and rotation was monitored by a coded disk and two sets of light-emitting diode (LED) light sources and solid-state detectors.

Source: VEGA Balloon System and Instrumentation - Kremnev, et al.

July 20, 1976, Mars, Viking 1 lander.

(You're really not going to like this one.)

The lander's cameras included an array of 12 photodiodes to measure various things. One reference even plots each photodiode's spectral sensitivity; the abstract of another paper gives enough evidence for their existence.

But every photodiode also acts as a (rather inefficient) LED. QED!

May 25, 2008, Phoenix Mars Lander. (Surely there's an earlier lunar example?)

The Robotic Arm Camera took an image of the Robotic Arm scoop using its red LED (Light-Emitting Diode) lamp.