Transit of Phobos from Mars

During a transit, Phobos can be seen from Mars as a large black disc rapidly moving across the face of the Sun.

A transit of Phobos from Mars usually lasts only thirty seconds or so, due to the moon's very rapid orbital period of approximately 7.6 hours.

At any given geographical location on the surface of Mars, there are two intervals in a Martian year when the shadow of Phobos or Deimos is passing through its latitude.

Because it orbits so close to Mars, Phobos cannot be seen north of 70.4°N or south of 70.4°S; observers at such latitudes will obviously not see transits, either.

Viewed from orbit, the penumbral shadow of Phobos can be seen to move rapidly over the Martian surface.

[1] This was detected only as a slight dimming of the ambient light; the Viking 1 Lander camera did not image the Sun.

Putting in the longitude/latitude/altitude coordinate values -310.8,10.9,0 into JPL Horizons[7] we see that the predicted time of mid-transit was 04:00:36 UTC, in perfect agreement within the error of determining the exact longitude and latitude of the center of the shadow.

For instance, a downtrack summing factor of 27 causes every 27 lines to be merged into one, resulting in an image 1600 pixels high.

Putting in the longitude/latitude/altitude coordinate values -124,14,0 into JPL Horizons gives an expected time of 20:49 for the transit, which is in good agreement once again.

Transits of Phobos in 2019 were detected as a transient drop in the solar array current data of the InSight lander.

[13] At the time, the data were only acquired at 30s intervals, and so the events were only recorded as single-sample dips of a few to a few tens of per cent.