That’s not Mars!
The image above taken by HiRISE isn’t of the South Pole of Mars or any region on Mars for that matter. It’s an interloper from the Oort cloud (reservoir of the long period comets) coming in for a close encounter to Mars on its way into the inner Solar System for its closet approach to the Sun. This icy planetesimal originated in the Oort cloud and was perturbed onto an orbit that has slowly brought it into the inner Solar System and on a path that brings the comet close to Mars. So close in fact (87,000 miles away from Mars) that this is closer than any comet has come to Earth since the dawn of modern astronomy. This provided a rare opportunity to study this icy remnant of planet formation up close and personal with the flotilla of spacecraft orbiting Mars.
HiRISE, is the highest resolution camera sent to to the Red Planet, and the images you see on Planet Four come from it. HiRISE is designed to taken observations staring below at Mars. It’s a push broom camera so it’s using the motion of the spacecraft (Mars Reconnaissance Orbiter, MRO) its aboard to create the image. To observe a comet requires a whole different way of observing using MRO to point and slew to target the comet. This was no easy feat but the HiRISE team accomplished it, taking images of the comet several days before and shortly before cloest encounter. During the closest part MRO was behind Mars to shield it and its instruments (including HiRISE) from the large amounts of dust entering the Martian atmosphere and could possibly damage or destroy the onboard instruments. This is likely the best optical image of Comet C/2013 A1 Siding Spring, we’ll have. It may look blurry and span only a few pixels, but observations like this will significantly constrain the size of the nucleus. Congratulations to everyone involved for making these challenging observations successful.
This is the only the second comet imaged by HiRISE. HiRISE has tried this previously observing Comet ISON, a sun-grazing comet that broke up shortly before or during its encounter with the Sun. HiRISE imaged ISON’s nucleus and was able to put the best size constraints on the comet (better than the limits from the Hubble Space Telescope), that placed it around 1 km or smaller. With that size, ISON would be predicted not to survive matching the observations. The cool thing about both Siding Spring and ISON is that these comets were discovered with at least a year’s notice before their closest approach giving astronomers and planetary scientists time to apply for telescope time and mobilize resources (including spacecraft orbiting Mars!) to observe these elusive objects.
You can read more about these HiRISE observations here and here. If you’re interested in hearing more about observations like this get undertaken by HiRISE and the team behind the camera check out this Planet Four Live Chat where we had discussing the preparations for the Comet ISON imaging with Kristin Block and Christian Schaller. For a summary of all the Comet Siding Spring observations taken by the spacecraft orbiting Mars check out this blog by the Planetary Society’s Emily Lakdawalla.