Why are some Martian terranes strongly magnetic and some non-magnetic? by Michael Purucker

SLIDES & TRANSCRIPT
Wednesday, March 13

Why are some Martian terranes strongly magnetic and some non-magnetic
Michael Purucker (purucker@geomag.gsfc.nasa.gov)

Slide 1: Introduction-1

Good afternoon. My title is something of a misnomer and I wanted to clarify what I meant. I am describing the large-scale contrast in measured magnetic fields seen from the Mars Global Surveyor spacecraft orbiting Mars. I don't mean to imply that there are no small-scale magnetizations. But the contrast between the measured magnetic fields in the strongly magnetic and so-called non-magnetic regions is a factor of 300, much larger than contrasts that would be measured over Earth from satellite. Terrestrial contrasts are more typically a factor of 10.

As a counterpoint to the rest of this talk and as an example of the potential importance of small-scale magnetizations, consider the Martian surface fields expected at the surface by Mars Netlander. Targeted to the northern lowlands, the Netlanders will probably land in places where the magnetic fields are less than 20 nT at the closest MGS approach (120 km). You can show that surface fields of up to 2300 nT could exist for a 2-D magnetization distribution (stripes) that are 60 km wide and buried at a depth of 2 km (under a blanket of sediment). Contrast this with the surface fields expected in the more magnetic south, where surface fields probably exceed the earth's surface field (30000 to 60000 nT).

Slide 2: Introduction-2

Strongly magnetic crust at Mars is restricted to a broad sinuous region, extending southward from the crustal dichotomy for several thousand kilometers. There are two large terranes with no large magnitude magnetic fields, one centered at Noachis Terra in the south and one centered on the northern lowlands. Old impact craters such as Hellas and Isidis show up because of the absence of large scale magnetic fields. The southern terrane here is enigmatic because the age of the rocks at the surface is little different from those in the adjacent, strongly magnetic region. And the boundaries of this terrane in the south do no correspond to any obvious geologic or structural feature. I’m going to start from the premise that the old terranes south of the dichotomy boundary were all magnetized in the field of the primordial dynamo and ask how those magnetizations could have subsequently been erased. This figure illustrates the radial component of the magnetic field at 120 km altitude. It is a composite map made from the global maps of Cain, Arkani-Hamed, and Purucker and others. Compositing the maps has the effect of minimizing small-scale fields. Just as a reminder, the magnetic fields associated with the crust at Mars are more than an order of magnitude larger than those associated with the terrestrial crust. The reason for this remains baffling. Map references: Purucker M. et al. (2000) Geophysical Research Letters, 27, 2449-2452. Arkani-Hamed, J. (2001), Journal of Geophysical Research-Planets, 106, 23197-23208. Cain, J. et al. (in press), Journal of Geophysical Research-Planets.

Slide 3: Southern weakly magnetic region

Here is another view of the southern weakly magnetic region, using the same data set as in the previous figure. The boundaries of the southern non-magnetic region can be characterized as gradational between Argyre and Hellas and to the west but in the South polar region the boundary is relatively sharp, such as might characterize a fault block We'll look at the profiles in this region to see if they support the fault interpretation. Notice the presence of magnetic terranes immediately north of Argyre and Hellas, and to the east of Hellas, in contrast to what would be expected from the Hood, Richmond, and Halekas model of impact demagnetization (2002) of the entire region.

Slide 4: Motivating questions

Now that I've given you some perspective, Here are the questions that motivate this talk.

Slide 5: Improvements in ability to represent the field

Our ability to represent the Martian magnetic field has significantly improved in the past year. The close-in AB2 profile data was released by the MGS MAG team of Acuna, Connerney, and others. It supercedes the AB quicklook data. However, degrees 1-50 of the spherical harmonic representation of the planetary magnetic field don't change significantly. The changes affect largely the higher degrees. The mapping orbit data will improve our ability to map planetary scale external fields, and assess their ability to probe the interior conductivity. A new global model to degree 90 utilizing the AB2 profile data, supplemented by additional data, was prepared by Cain and his students. In my estimation, terms to degree 50 can be used to characterize the global field. Terms in excess of that are useful for representing small-scale detail on the individual profiles, but should not be used to characterize regions between widely spaced profiles.

Slide 6: Unrecognized impact or thermal events-1

Assuming, as I’ve previously mentioned, that the terranes south of the dichotomy boundary were magnetized in the field of the primordial dynamo, we can ask how these terranes were demagnetized. Obviously, the Hellas and Argyre impacts were effective in demagnetizing the terranes within the crater rim. Are there unrecognized impact or thermal events in this region? I'm showing the stretched MOLA topography on the left and the magnetic field on the right. Notice that there is a suggestion of one or more roughly circular depressions in the area between Argyre and Hellas.

Slide 7: Unrecognized impact or thermal events-2

These circular depressions are outlined here. The larger, more circular feature, is some 900 km in diameter. The smaller, less defined and more elliptical feature, is about 800 km in long dimension. Smaller hidden craters in this region have been recognized by Herb Frey and co-workers. The minimum size required to demagnetize the terrane seems to be 200-300 km in diameter. Another possibility, which I'll mention but not discuss, is that volcanic activity here may have erased the magnetizations. The compilation of Sakimoto (2002) assembled of volcanic features will be very useful in assessing this possibility.

Slide 8: Cimmeria region magnetizations

Let's look at another example of large-scale magnetic fields, in this case, in Terra Cimmeria in the southern highlands. This is the area of the most intense magnetic fields encountered at Mars, in excess of 1500 nT at 80+ km. The largest crater in this image, Copernicus, is some 300 km in diameter developed in the early Noachian. You can see from the profiles in the upper right that it has effectively demagnetized the Martian crust. Note also that the boundary between the positive and negative radial fields (shown in red and blue, respectively) is effectively a great circle arc, as indicated on the gnomonic projection in the lower right.

Slide 9: Fault boundaries in Cimmeria

Finally, let's return to the question of whether the eastern boundary of this most magnetic terrane is a fault. We show here the calibrated profile data from the three components and also a color and shaded relief view of the Martian topography here. The inferred fault would be located in the center of the image, running North-South, approximately coincident with the largest gap in the data. The radial component seems consistent with a fault but the presence of a possible through-going negative (blue) magnetic feature in the theta component means that further work and modeling will be required.

Slide 10: Conclusions

I've listed some tentative conclusions here but this is intended as a living document so I'd like to encourage any of you with an interest in this subject to comment, either with text or figures or a separate talk. I will work with you to include your comments and criticisms in this document. Thank you.