It may be in its 14th year on Mars but Opportunity still has some surprises to show us—like this, a series of images captured on May 3, 2017 showing the Sun as seen from Mars. But that’s not the special part: see the change in brightness along the Sun’s edge near the end? That was a brief transit of Phobos, the largest (and nearest) of Mars’ two moons!
Can’t see it very well? It’s quick, I know—so check out a cropped and enlarged version below:
I rotated the image above 90 degrees counterclockwise for another angle on the transit. The images were captured with Opportunity’s right panorama camera on mission sol 4719.
Unfortunately in the public raw images that were posted to the MER site the Sun is close to the edge of the frame, so there’s not more detail than this.
I’m going to guess that some of the bright streaking coming from the Sun is internal glare in the camera due to its being off-center, but also it could be from dust in the atmosphere of Mars too, especially considering Phobos seems to be cutting a shadow through some of it. The few persistent bright spots in the top images are hot pixels in the camera, with a couple of flashes of radiation noise.
This isn’t the first time Phobos has transited the Sun in rover images, but it’s always cool to see when it does. Mission scientists know when these events occur so they plan observations for them.
As a moon Phobos is pretty small—only about 16 miles wide—but it orbits Mars at such a low altitude (5,840 miles) that its silhouette is quite visible when it passes in front of the Sun…or skims along its edge, in this case. Add to that Phobos orbits Mars once every 7 hours and 40 minutes and there is often an…opportunity…for a transit event! (I’m here all week folks.)
Thanks to Emily Lakdawalla for the tip on what these images show.