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Emily Lakdawalla • Oct 14, 2014

Phobos over Mars

Today the Mars Orbiter Mission released a nice four-image animation of teeny dark Phobos crossing Mars' huge orange disk. I made this animated GIF by taking screen-captures of the four different frames of a video that they released to Facebook -- not the cleanest path for acquiring spacecraft data. But this is the best I've been able to find on the Internet. Given the range at which the image sequence was taken (66,275 kilometers, very similar to this global view) the original four frames should've been able to contain all of Mars within their 2048-pixel-square frame, so we're seeing a subsampled view of the original data. Still, it's neat to see Phobos in motion.

Phobos crossing Mars
Phobos crossing Mars An animation of four images of Mars taken by Mars Orbiter mission, showing Mars' moon Phobos crossing the disk at upper right. The images were taken from a distance of 66,275 kilometers in October, 2014.Image: ISRO

Mars Orbiter Mission joins a long line of Mars missions that have produced images of Mars and Phobos together. When Mars and its moon are in the same frame, you learn very quickly how tiny Phobos is, and also how dark it is. Like most of the solar system's small bodies, it has a very low albedo, reflecting only about 7% of the light that strikes it. Actually Mars is not bright either, but with a global albedo of 15% it's more than twice as reflective as Phobos, and when you adjust an image to show Mars bright, Phobos remains pretty dark.

The image in my library that's most similar to the Mars Orbiter Mission one is this, from Rosetta:

Phobos over Mars from Rosetta
Phobos over Mars from Rosetta As Mars loomed in Rosetta's forward view, it caught Phobos in the act of transiting the planet. You may need to enlarge the view to see Phobos' tiny dark speck.Image: ESA / MPS / UPD / LAM / IAA / RSSD / INTA / UPM / DASP / IDA / processed by Emily Lakdawalla

That one actually contains not only Phobos but also Phobos' shadow:

Phobos over Mars (labeled)
Phobos over Mars (labeled) A view of Phobos over Mars as seen by Rosetta, with various key features labeled.Image: ESA / MPS / UPD / LAM / IAA / RSSD / INTA / UPM / DASP / IDA / processed by Emily Lakdawalla

...which makes me think that the Mars Orbiter Mission animation should also contain Phobos' shadow, but at the resolution and quality that I was able to grab it from Facebook, I don't see it.

Mars Express has gotten some great grayscale pictures of Phobos over Mars. Unlike Mars Reconnaissance Orbiter and Mars Odyssey, Mars Express has a distant, elliptical orbit that allows it to get pretty close to Phobos for a dozen orbits every five months, making for gorgeous photographs.

Phobos over Mars from Mars Express
Phobos over Mars from Mars Express The High Resolution Stereo Camera (HRSC) on ESA's Mars Express' spacecraft caught Phobos over Mars' limb on March 26, 2010. The waviness of Mars in the background is a by-product of the line-scanning nature of HRSC.Image: ESA / DLR / FU Berlin (G. Neukum)

But the loveliest color images of Phobos over Mars that I've seen come from a somewhat surprising source: they're Russian. We usually think of the Russian Mars missions as failures. But Phobos 2 got some fantastic images of Phobos before it failed. Here are a few, processed by Ted Stryk.

Phobos over Mars from Phobos 2
Phobos over Mars from Phobos 2 The moon Phobos, 25 kilometers in diameter, is much darker than bright, dusty Mars, as seen in this image captured by the Phobos 2 spacecraft on March 25, 1989. Phobos 2 took a total of 13 color sets of images of Phobos before contact was lost on March 27, 1989.Image: Russian Academy of Sciences / Ted Stryk
Phobos crossing Mars' limb from Phobos 2
Phobos crossing Mars' limb from Phobos 2 The moon Phobos, 25 kilometers in diameter, is much darker than bright, dusty Mars, as seen in this image captured by the Phobos 2 spacecraft on February 28, 1989. Phobos 2 took a total of 13 color sets of images of Phobos before contact was lost on March 27, 1989.Image: Russian Academy of Sciences / Ted Stryk
Phobos just off Mars' limb from Phobos 2
Phobos just off Mars' limb from Phobos 2 This view of Mars and its moon Phobos, 25 kilometers in diameter, was captured by the Phobos 2 spacecraft on February 28, 1989. Phobos 2 took a total of 13 color sets of images of Phobos before contact was lost on March 27, 1989.Image: Russian Academy of Sciences / Ted Stryk

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