26 January 2015
This week the analysis journal Science printed a particular challenge on the preliminary outcomes of the Rosetta/Philae mission. Together with it got here a number of high-resolution and coloration photos of the comet 67P Churyumov-Gerasimenko. The photographs alone are fairly astounding. The floor is numerous, with proof of fracturing, faint jets, and a mottling of colours. It’s clearly a fancy and dynamic physique. Whereas the photographs are interesting, different information is telling an interesting story. One in every of these tales issues the formation of the comet’s magnetosphere.
Comets like 67P don’t have a powerful intrinsic magnetic subject, so by themselves they don’t have a magnetosphere to guard themselves in opposition to photo voltaic wind. It’s icy floor is due to this fact bombarded by ions and magnetic fields. However because the comet approaches the Solar, volatiles resembling water chic to supply a skinny ambiance. Among the water vapor is then ionized by ultraviolet daylight, and these ions work together with the magnetic fields of interplanetary area. Because the ambiance of the comet will get thicker, and extra of it’s ionized it turns into conductive. The photo voltaic wind accelerates a number of the atmospheric ions at excessive pace, and the ensuing present induces a magnetic subject across the comet. Ultimately the magnetic subject is powerful sufficient that it pushes again in opposition to the photo voltaic wind, making a magnetosphere.
At the least that’s what we’ve thought. Earlier fly-bys of comets resembling Halley have discovered that comets don’t have robust intrinsic magnetic fields, and we’ve seen extra energetic comets have an effect on the encompassing magnetic surroundings. However now outcomes from Rosetta have confirmed this mannequin by straight observing it occur.
The crew noticed each impartial and ionized water within the comets ambiance, in addition to the magnetic fields within the area. They collected information because the comet was greater than 3.6 AU to when it made its closest strategy to the Solar at about 1.25 AU. They discovered an preliminary faint ambiance penetrated by the photo voltaic wind, then watched because the ambiance thickened to the purpose the place it began resisting the wind by way of an induced magnetosphere. In addition they discovered the ambiance is erratically distributed concerning the comet (as totally different floor areas produced kind of volatiles) and that even a faint ambiance interacts extra strongly with the photo voltaic wind than initially supposed.
Because of this missions like Rosetta are essential. Observing the complicated interplay of photo voltaic wind and cometary ambiance can solely be completed on web site. It doesn’t present up in photos. So benefit from the flood of recent coloration comet photos, however do not forget that we are able to’t see the colours of the wind.