# Mercury's magnetic field and definition of "active" **URL:** \ **Category:** Factual Questions\ **Created:** [January 31, 2008, 5:53pm UTC](https://boards.straightdope.com/t/mercurys-magnetic-field-and-definition-of-active/435736 "2008-01-31T17:53:03Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Sailboat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sailboat/32/461_2.png) [@Sailboat](https://boards.straightdope.com/u/Sailboat)\ **Post date:** [January 31, 2008, 5:53pm UTC](https://boards.straightdope.com/t/mercurys-magnetic-field-and-definition-of-active/435736/1 "2008-01-31T17:53:03Z")
For the second time in two days, the Washington Post has described Mercury’s magnetic field in a way that I believe is flat out wrong. Before I write them a letter, I’d like the SDMB folks to vet my claim. The errors (as I perceive them): [QUOTE=NASA Photos Reveal Mercury Is Shrinking, By SETH BORENSTEIN, The Associated Press, Wednesday, January 30, 2008] Mercury is the only solar system planet other than Earth to have a magnetosphere. [/QUOTE] [QUOTE=Messenger’s Pictures From Mercury Surprise Scientists, By Marc Kaufman, Washington Post Staff Writer, Thursday, January 31, 2008; Page A02] Among the mysteries researchers hope to unravel is where and how Mercury was formed and the nature of the magnetic fields around it. Earth is the only other planet with such an active magnetosphere. [/QUOTE] Surely both of these quotes are in error. The first one is an AP wire article, so I can’t totally blame the Post, but [Jupiter’s giant, highly active magnetosphere](http://www.windows.ucar.edu/tour/link=/jupiter/upper_atmosphere.html) is pretty well-known, being discussed in movies and numerous educational programs. And surely its famous radio noises, aurora, and impact on Jupiter’s moons qualify it as “active”…more active in any sense that Mercury’s magnetosphere is…right? I’m also pretty sure the other outer planets, the gas giants, have powerful magnetospheres as well. In fact, if you look [here](http://www.windows.ucar.edu/tour/link=/mercury/Magnetosphere/magsphere_overview.html), you’ll see the likely source of the misunderstanding: [QUOTE=Windows to the Universe] Mercury is the only inner planet other than the Earth that has a significant magnetic field (220 nT). [/QUOTE] Looks like the reporters missed the significance of the words “the only _inner_ planet other than the Earth”. But is it possible that Mercury’s proximity to the sun causes so much material to strike it’s magnetosphere that that’s what they’re talking about? Could that be the definition of “active” from the second article? Or something else? Or are they just plain wrong? Thanks for opinions before I potentially make a fool of myself scientifically. Sailboat ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [January 31, 2008, 6:05pm UTC](https://boards.straightdope.com/t/mercurys-magnetic-field-and-definition-of-active/435736/2 "2008-01-31T18:05:46Z")
Straight from Messenger: [Mercury’s Internal Magnetic Field](http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?gallery_id=2&image_id=152) > [@](#): > > MESSENGER saw an internal magnetic field that is well described by the field from a dipole nearly aligned with the planet’s spin axis (dipole tilt ~ 10°). This geometry is similar to that observed by Mariner 10 during its first flyby. The field strength is weaker by about one third than that detected by Mariner 10 during its third (and last) flyby, owing primarily to the difference in trajectories (Mariner 10 flow directly over the magnetic pole where the field strength is greatest). When corrected for our best estimate for the external field, the MESSENGER observations and the two Mariner 10 passes are consistent with very similar solutions for the mean planetary magnetic dipole. The dipolar field is consistent with an active electrical dynamo in which the magnetic field is produced by electrical currents flowing in an outer core of molten metal. The cores of venus and mars are quiescent, leaving only remnant fields. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [January 31, 2008, 7:06pm UTC](https://boards.straightdope.com/t/mercurys-magnetic-field-and-definition-of-active/435736/3 "2008-01-31T19:06:41Z")
There seems to be little question that [Jupiter](http://en.wikipedia.org/wiki/Jupiter's_magnetosphere), [Saturn](http://www.windows.ucar.edu/tour/link=/saturn/upper_atmosphere.html), and [Uranus](http://www.windows.ucar.edu/tour/link=/uranus/upper_atmosphere.html) have significant magnetospheres. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [January 31, 2008, 7:14pm UTC](https://boards.straightdope.com/t/mercurys-magnetic-field-and-definition-of-active/435736/4 "2008-01-31T19:14:32Z")
Also [Neptune](http://www.windows.ucar.edu/tour/link=/neptune/upper_atmosphere.html&edu=elem). [Pluto](http://www.windows.ucar.edu/tour/link=/pluto/magnetosphere.html) not so much (and is it even a planet?). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 31, 2008, 7:30pm UTC](https://boards.straightdope.com/t/mercurys-magnetic-field-and-definition-of-active/435736/5 "2008-01-31T19:30:25Z")
This is the problem with using the single word “planet” for objects as different as Earth and Jupiter. In an ideal world, we’d have completely separate words for “big rocky orb orbiting a star”, “big non-fusing gassy orb orbiting a star”, and “big icy orb orbiting a star”. This way, we’d also manage to avoid the whole debate about Pluto: Mercury and Earth are the only rockballs with active magnetism, Ceres is the smallest of the rockballs, Jupiter is about as big as a gasball can get, and Pluto is one of the largest of the iceballs.