In October 2014, NASA had a unique opportunity to observe some of the changes on Mars as comet C/2013 A1 (Siding Spring) flew past the Red Planet. According to a recent news release about the event, "the effect was temporary but profound."
Team member Jared Espley has been quoted as saying, "Comet Siding Spring plunged the magnetic field around Mars into chaos....We think the encounter blew away part of Mars' upper atmosphere, much like a strong solar storm would."
At the closest approach point, the two entities were separated by a distance of about 87,000 miles.
See images of the Red Planet's moons:
NASA: Comet's flyby of Mars had 'profound' effect on its magnetic field
March 23, 2008 - NASA's Mars Reconnaissance Orbiter took two images of the larger of Mars' two moons, Phobos, within 10 minutes of each other on March 23, 2008. This is the second, taken from a distance of about 5,800 kilometers (about 3,600 miles). It is presented in color by combining data from the camera's blue-green, red, and near-infrared channels. The illuminated part of Phobos seen in the images is about 21 kilometers (13 miles) across. The most prominent feature in the images is the large crater Stickney in the lower right. With a diameter of 9 kilometers (5.6 miles), it is the largest feature on Phobos. The color data accentuate details not apparent in black-and-white images. For example, materials near the rim of Stickney appear bluer than the rest of Phobos. Based on analogy with materials on our own moon, this could mean this surface is fresher, and therefore younger, than other parts of Phobos. A series of troughs and crater chains is obvious on other parts of the moon.
The command module, now free of the larger Phobos mission rocket, begins a close approach to Phobos. With an average diameter of less than 12 miles, irregularly shaped Phobos has a very weak gravitational field making it relatively easy for the command module to come very close without being drawn all the way to its surface. The goal is to come close enough to permit space suited astrogeologists equipped with personal manned maneuvering units (MMUs) to act as mini spaceships themselves and descend to the surface. On the surface of Mars to the right can be seen Elysium Planitia and the volcano Albor Tholus.
An artwork of the terrestrial planet Mars set against the backdrop of the Milky Way. Mars's two moons, Phobos and Deimos, are seen passing in front. In this view of Mars we can see Valles Marineris in the middle - a vast chasm compared to which the US's Grand Canyon is little but a scratch. While off to the left we can see the Tharsis rise with its four giant volcanoes, the largest of which (Olympus Mons) is the most massive volcano in the known Solar System.
This panel illustrates the transit of the martian moon Phobos across the Sun. (Photo by: Universal History Archive/UIG via Getty Images)
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Siding Spring's coma, which is the gassy haze surrounding the nucleus, reportedly "washed over the planet for several hours."
As such, the news release continues, "Mars was flooded with an invisible tide of charged particles from the coma, and the powerful magnetic field around the comet temporarily merged with – and overwhelmed – the planet's own weak one."
NASA's observations were made with the help of its MAVEN spacecraft.