The day the dynamo died; with no magnetic field, Mars lost ...



Libby Szarkowski

Geology 104

Homework #5

4/4/06

The paper that I found from the popular media was called The day the dynamo died; with no magnetic field, Mars lost its atmosphere and its chance of life, from New Scientist. This article summarized some points of a discussion held by members of the Mars Global Surveyor team in La Jolla, California in 2001. The main goal of this conference was to try and hypothesize some of the timing of the stages in the planet's decline (loss of the atmosphere). But unlike what the article stated as part of the title, the lack of life on Mars, the article only mentioned the lack of life once. I found this extremely odd and misleading to any reader who would have clearly expected that as part of the discussion. The article did however focus on the newly discovered data that impact craters from around 4 billion years ago do not contain magnetized rock while the surrounding older rock does have magnetized rock, and the fact that small magnetized patches of rock in Mars’s Southern Hemisphere may help keep some of the ionosphere from disappearing by the solar wind.

One problem with the article was its lack of understanding about what forms a magnetic field. The article remarks, “Global fields are generated by the dynamo effect of molten iron circulating in a planet's core” (Samuel, 2001). But this is in fact only partially correct; any planet with a liquid that can move, a material that can conduct an electric current, and an energy source to cause motion can have a planetary magnetic field. For example Jupiter and Saturn have metallic hydrogen as their source of the needed dynamo to create the magnetic field (Schubert et Al., 2004). This small over sight could bring people to assume that the larger gaseous planets beyond the Frost line (where not a lot of iron would be found) have no magnetic fields at all, when this would be impossible since all of their gases would be blown away in the Solar wind. This oversight is the exact opposite of the true facts, since the strongest magnetic field is actually found on Jupiter where the large quickly spinning metallic hydrogen core gives it a magnetic field 20000 times stronger than Earth’s. Clearly the article is made as a quick slight overview for some research done for Mars. That facts presented may be correct for the Mars but will not always translatable to the wider universe as a whole.

The article also mentioned localized magnetic fields that were found in Mars’s Southern Hemisphere. The article states that some of Mars’s magnetic field is preserved in the magnetic crustal rock, and that these patches can protect some of the ionosphere from the solar wind. In David J. Stevenson article he also supports that “[l]ocalized fields of up to Earth’s global field ([pic]10−4 T) are possible from permanently magnetized materials,” along with a mention that these fields are indeed common in the Southern Hemisphere of Mars (2003). When looking at some of David Mitchell (a researcher cited in the article) investigation on this topic though the situation was much more complicated than the article led the reader to believe. One of Mitchell’s articles states that the minimagnetospheres found in the Southern Hemisphere can cause populations of trapped ionospheric electrons to form and this may be the cause of what was found (Krymskii, AM. et Al., 2002). Mitchell’s research led me to believe that this was hardly an exact science at the time and much more research and a greater understanding was needed before greater assumptions could be made. The New Scientist article however gave none of this background and made the facts appear much more clear cut that they actually were.

I also looked at the rest of the article by Stevenson, to check on the facts about him saying that the lack of plate tectonics is the key to why he believes that Mars no longer has its magnetic field. Stevenson suggests three possibilities for why Mars’s dynamo at one time existed and then failed. His most developed theory, with the most data backing it up, is that the core cooled down to a point where conductive heat loss prevailed, but no new inner core was being formed. Stevenson’s second hypothesis was that Mars’s convective style changed. This could mean that it changes from a system of plate tectonics to a more inactive lid mode, which is seen in today’s observations. Stevenson suggests that this would cause a chain of events that would make the mantle and core stop cooling and turn off core convection and therefore the dynamo as well. But this model was not his only idea or even his favorite option. This may mean that indeed Stevenson did say that the lack of plate tectonics could have played a part in the loss of Mars’s magnetic field, but the reporter either did not understand or chose not to write about the other ideas Stevenson had.

Another set of facts that the New Scientist might have over simplified is the meteor craters being un-magnetized compared to the surrounding older magnetized rock. Carporzen and his peers published a paper saying that when looking from high altitudes, like what scientists view the craters on Mars from, the magnetic anomalies of the craters are not enough evidence to simply assume that the planet has no internally generated magnetic field at the time of impact. The impact of the meteor can cause reduced magnetic fields do to the pressure demagnetization caused by shock waves during impact when no significant magnetic field is present, but they can also be caused by the random orientation the magnetite grains (found in craters such as Vredefort in South Africa) canceling each other out when they are summed over the entire crater (Carporzen et Al., 2005). But other articles still believe that the magnetic field did fail around the 4 billion years ago mark which is about the time that these meteors hit (Stevenson, 2003).

Overall I believe that this article did a good job at explaining very complicated and still poorly understood facts to the general public. They focused on discussing Mars, so some of their data only pertains to Mars and not other planets our solar system, but the facts did need to be relatively simplified for such a short article. Their main audience was clearly not the leading scientists in the field, since they were at the conference, but the general public so I can understand why they would want to try and make fact appear much less complicated and more well know then perhaps they actually were.

References:

Carporzen L, Gilder SA, Hart RJ. (2005). Palaeomagnetism of the Vredefort meteorite

crater and implications for craters on Mars. Nature, 435(7039), 198-201.

Krymskii AM, Breus TK, Ness NF, Acuna MH, Connerney JEP, Crider DH, Mitchell

DL, and Bauer SJ. (2002). Structure of the magnetic field fluxes connected with

crustal magnetization and topside ionosphere at Mars. Journal of Geophysical Research, 107(A9), 0148-0227.

Samuel, Eugenie. (2001). The day the dynamo died; with no magnetic field, Mars lost its

atmosphere and its chance of life. New Scientist, 169.2277, 4. 

Schubert G, Chan KH, Liao XH, Zhang KK. (2004). Planetary dynamos: effects of

electrically conducting flows overlying turbulent regions of magnetic field

generation. ICARUS, 172 (2), 305-315.

Stevenson, David J. (2003). Planetary magnetic fields. Earth and Planetary Science

Letters, 208(1-2), 1-11.

My popular media article:

The day the dynamo died; with no magnetic field, Mars lost its atmosphere and its chance of life. (Brief Article). Eugenie Samuel. New Scientist 169.2277 (Feb 10, 2001): p4. 

[pic]Full Text :COPYRIGHT 2001 Reed Elsevier Business Publishing, Ltd. For more science news and comments see .

IT'S ONE of the central mysteries of the Solar System: why does a sizeable planet like Mars not have an atmosphere that could nurture and sustain life? That question may now have been answered. Scientists working with NASA's Mars Global Surveyor say the planet's atmosphere was blown away by the solar wind, following the demise of its magnetic field four billion years ago. This happened so soon after Mars's formation that it is unlikely complex life would have had time to evolve.

Every planet in the Solar System is buffeted by the solar wind, a stream of charged particles flowing from the Sun. Earth's magnetic field acts as a shield, deflecting the solar wind around and behind the planet. Researchers have long known that Mars does not have such a global magnetic field, but recent studies have indicated that the planet did once have one, and that its disappearance spelled the end for the atmosphere.

Last week, members of the Mars Global Surveyor team met in La Jolla, California, and one of their aims was to try and pin down the timing of the stages in the planet's decline. "The magnetic field is right at the heart of the most interesting questions concerning Mars--the climate, the atmosphere, even the life issue," said Bruce Jakosky of the University of Colorado at Boulder.

In 1999, David Mitchell of the University of California at Berkeley and Mario Acuna of NASA's Goddard Space Flight Center in Greenbelt, Maryland, found that the oldest terrain on Mars showed signs of having been magnetised by an ancient global field. But the terrain inside nearby impact craters was not magnetised, so the magnetic field must have switched off before the time of the impacts. The impacts date to about four billion years ago, only about half a billion years after Mars formed. "A lot of theorists were surprised," says Mitchell.

But did the disappearance of the field mean doom for the atmosphere? Since 1999, the Mars Global Surveyor has been taking all sorts of measurements, including the strength of the solar wind, the density of the ionosphere--the charged portion of Mars's residual atmosphere--and any remaining magnetic field.

Recently compiled maps show that in some areas of Mars's southern hemisphere the ionosphere extends above the 400-kilometre altitude of the probe, while elsewhere the solar wind has beaten it down closer to the surface. According to Mitchell, the areas where there is more ionosphere coincide with areas on the ground where remnants of Mars's global magnetic field are preserved in magnetised rocks. These rocks seem to be magnetised strongly enough to protect pockets of ionosphere from the solar wind.

This information confirms calculations made by Mitchell's colleague Janet Luhmarm that the solar wind stripped the atmosphere's nitrogen, carbon, oxygen and water during Mars's first two billion years. This does not rule out the possibility that some water is hidden underground. "The planet is not completely dead," says Victor Baker at the University of Arizona in Tucson.

So why did Mars's magnetic field fail? Global fields are generated by the dynamo effect of molten iron circulating in a planet's core. David Stevenson of Caltech thinks Mars's lack of plate tectonics may be key. On Earth, plate tectonics cool the mantle relative to the core, increasing the convection that keeps our planet's dynamo turning. "I certainly think if plate tectonics ceased on Earth, in 100 million years we wouldn't have a dynamo," says Stevenson. And with it might go our atmosphere.

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