Транзит Фобоса над Солнцем с Марса

Transit of a Moon from Mars

A transit of Phobos across the Sun as seen from Mars takes place when Phobos passes directly between the Sun and a point on the surface of Mars, obscuring a large part of the Sun's disc for an observer on Mars. During a transit, Phobos can be seen from Mars as a large black disc rapidly moving across the face of the Sun. At the same time, the shadow (antumbra) of Phobos moves across the Martian surface. The event could also be regarded as a particularly quick and shallow annular solar eclipse by Phobos.

Viewed from orbit, the penumbral shadow of Phobos can be seen to move rapidly over the Martian surface. This shadow on the Martian surface has been photographed on many occasions by Mars Global Surveyor.

In the 1970s, the Viking 1 Lander and Orbiter photographed the shadow as well. The Lander detected the penumbral shadow of Phobos passing across it. This was detected only as a slight dimming of the ambient light; the Viking 1 Lander camera did not image the Sun. The shadow took about 20 seconds to pass over the Lander, moving at about 2 km/s. The shadow was simultaneously imaged from the Viking 1 Orbiter, which permitted locating the position of the lander in the orbiter pictures.

Many dozens of other images of the shadow exist, but these are much lower resolution (by a factor of 27/4). Three such images are shown in the 1 November 1999 NASA press release. The timestamps printed on the photos in the NASA press release do not correspond to the actual time that the shadow was imaged, rather they represent the "image start time" of a vertically much larger original image. Mars Global Surveyor orbits Mars in a Sun synchronous polar orbit with orbital period 117.65 minutes, moving from south pole to north pole, and continuously points its camera straight down. The result is an image in the form of a very long thin vertical strip, where the pixels in the top part of the image are imaged nearly one hour after those in the bottom part of the image. In principle the image could be as much as 43200 pixels high, but "downtrack summing" is used to merge adjacent lines. For instance, a downtrack summing factor of 27 causes every 27 lines to be merged into one, resulting in an image 1600 pixels high. Thus to determine the actual time that the Phobos shadow was imaged, it is necessary to locate the original image and measure how many pixels from the bottom of the image the shadow is found and add the corresponding offset to the image start time. For example, we examine the image labeled with the timestamp 1 September 1999 20:13:05 (UTC). We have the original images M07 00166 (red) and M07 00167 (blue), part of the gallery of MOC Global Map Images, Subphase M07. The shadow is situated at roughly 14°N 236°W. In this case the image start time is 20:13:04.69 UTC, the line integration time is 80.48 milliseconds, and the downtrack summing factor is 27. The shadow is about 8 pixels high, centered at 993 pixels from the bottom of the original 1600 pixel high image. We add (993 × 0.08048 × 27) = 2157.75 seconds = 35 minutes 57.75 seconds to get a time of 20:49:02.4 UTC for the center of the shadow. 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. On 1 September 1999 the distance between the Earth and Mars was 9.9 light minutes.

Transits of Phobos in 2019 were detected as a transient drop in the solar array current data of the InSight lander. 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. During the transits of spring 2020, all instruments onboard InSight recorded with full sampling rate and a slight tilt of the ultra sensitive seismometer was observed, as well as a drop in solar irradiation and a 2 K drop in surface temperature. The tilt of the seismometer was caused from contraction of the ground due to the temperature drop; everywhere but in the shadow the thermal shielding around the seismometer.