Sunday, February 16, 2014

Recent Visit to Mars


I had an opportunity to dig up old data & analysis on the Mars interior for a recent meeting. It was fun on many levels.

First, there’s the fascinating Mars interior with its huge transition zone. Is it drenched with water? Does it show evidence of compositional as well as structural changes? And Mars D double-prime (aka the core-mantle-boundary) is fun to ponder. A single measurement of the core radius will tell us whether there is a transition to perovskite at the bottom of the Martian mantle. A phase transition right at a huge chemical and (likely) thermal boundary layer is interesting—because it will either amplify or dampen instabilities—and therefore help govern dynamics. 

A single data point for the position of the Mars core-mantle boundary will tell us a huge amount about the thermal, mechanical, and chemical history of Mars.


Left: Earth interior density vs. pressure on a log-log scale, so the important parts (for Mars) are emphasized. Right: my preliminary Mars interior density model, based on Earth's, but with 1. low velocity zone removed and 2. increased density of mantle material, due to SNC meteorite constraints suggesting there's more iron in the Mars interior. Depending on the size of the core (which depends on sulfur content to fit mass and moment of inertia constraints), there is either a perovskite layer at the base of the mantle (small, iron-rich core) or not (larger, sulfur-containing core).
 The Mars transition zone is interesting also. On Earth, the transition zone is a relatively small sliver of the mantle, but it's interesting because the steep seismic velocity profile shows that the mineral assemblage is changing throughout this zone. Is it purely a structural evolution (SiO4 tetrahedra to SiO6 octahedra within the garnet) or is it compositional also (e.g. Fe changing? Mg/Si ratio changing?). On Mars, this mineral assemblage makes up the bulk of the lower mantle--and therefore a significant portion of the Mars interior. A measurement of the depth-dependent velocity of the transition zone will also tell us a lot about Earth and Mars.

A mineralogical model of the silicate mantle of the Earth (and Mars).
The minerals that make the transition zone: silicate spinels and garnets happen to be able to store a lot of water in their structures. Up to a percent or so. That's the rock equivalent of a wet sponge. So--if water can get down to the martian mantle, or if it was there and never left, there is the possibility that the Mars interior may store a huge amount of water relative to its surface, and relative to the amount the Earth's interior might store. This could be important for rheology, partial melting, and electrical conductivity. Water softens everything.

Then, there was digging through the old pictures of data collection at the synchrotron. My colleagues Dan and Wendy are there (were they both grad students then?) and a smiling Abby showing off her big baby-bump at the synchrotron. I think I might have been the first pregnant user at the APS, and people wearing ties came to discuss it with me and with the PI of the beamline. See I’m the only one wearing a dosimeter? They checked and rechecked every corner of the hutch with Geiger counters before my time started. I also worked while pregnant at the NSLS. The floor manager met with me before my time and said: it’s safe to go almost everywhere most anytime. Just don’t go near this area during insertion. (I forgot the exact, just remember the gist).

Dan Shim, Wendy Panero, and Abby Kavner  late 1998
The measurement was a high pressure and temperature phase stability and equation of state for FeS, the sulfur-rich endmember of our best guess of Martian core composition—likely along the iron-iron sulfide join. I analyzed the data, developed a model of the Martian interior, and wrote a draft of the paper during maternity while I was nursing my son.

The icing on the cake was that revisiting the science, the data, the history this week coincided with my son’s 15 birthday. So in parallel, I was able to reflect on 15 years as a mom-scientist. He’s thriving as he should, and my mom-heart overflows. My science-heart also pounds, but in different ways than it did 15 years ago. I am still a striver, but my definition of thriving is changing.

2 comments:

  1. Recent gravity field displays from years of satellite data show Mars and Earth share a common pattern, yet we see so much variation in the visual surface appearance. On Earth the GRACE results show the 5+ mag. earthquakes follow the satellite data temporal shear zones, and soon someone should be publishing estimates of local zones for Mars similar activity. Can we presume the stresses are activating some portion of Mars as well? Mineral or structural displays even at the surface possibly?
    Your work is inspiring and I will be waiting your estimates of the single location InSight lander results.

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  2. This is amazing - here are how Earth looks from Mars from NASA and others http://earthspacecircle.blogspot.com/p/earth-from-space.html

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