Wednesday, June 15, 2011
I think I've stated before on this blog that I love reading studies about planets. However, when looking back through the blog archive I was surprised to find that I hadn't written any posts about Mars. All that changes today.
The current theory of planetary formation holds that planets form out of the protoplanetary disk of material left over from the formation of a star. The dust and gas in this disk are rotating around the star, and through accretion (coagulation of particles) larger and larger bodies form. When enough particles have come together they form planetesimals (100m to 10km across). The larger ones, which have more gravity, may even perturb the motions of nearby planetesimals, attracting them towards themselves in a process called gravitational focusing. Eventually one planetesimal will outpace all of the others in its orbit and become a larger body known as a planetary embryo. The terrestrial planets are thought to have formed through the collisions between large planetary embryos of diameters between 1,000 and 5,000 km. For Earth, the last collision was the one that formed the Moon approximately 50-150 million years after the birth of the Solar System.
Mars is the fourth planet from the Sun, at a distance of 1.5 astronomical units (AU). Its radius is about half that of the Earth with a mass about 10% that of the Earth. It has highly cratered highlands in the southern hemisphere with relatively smooth lowlands in the northern hemisphere. The structure of the craters in conjunction with areas that appear to have experienced erosions and degradations suggest a combination of aeolian (dust storms, wind streaks, etc.), fluvial (water), and volcanic processes in the planet's history. The crust of the planet has been estimated to be about 25-70 km thick. Below that is a silicate mantle about 1300-1800 km thick. The iron-rich metallic core has a radius of 1500-2000 km.
You add all of this information together an an interesting question comes up: Why is Mars so small?
Model simulations can explain the mass and dynamical parameters of Earth and Venus, but they fall short in explaining the size of Mars. A new(ish) paper in the journal Nature explores one explanation for the planet's diminutive size, that the planetary embryo did not collide and merge with that many other planetary embryos. A "stranded planetary embryo" origin. Now how in the world do you go about figuring that one out? I mean, you don't exactly have the planet's baby pictures.
To assess the formation of Mars the researchers had to know the planet's accretion timescale. How and how fast did it come together. So they used the 182Hf–182W decay system in shergottite-nakhlite-chassignite (SNC) meteorites.
I'm going to try to make a complex subject sound simple in a short amount of space. Chemically and mineralogically, chondrites (stony meteorites) are a good resource for testing the early composition of a planet. The timescales, mechanisms, and chemical differentiation of planets can be figured out by quantifying the radioactive decay of short-lived isotopes. Hafnium (Hf) 182 decays into tungsten (W) 182 in a half-life of nine million years. It doesn't sound like it but this is actually a relatively rapid decay process, and it means that almost all of 182Hf will disappear in 50 million years. Both elements are refractory or non-volatile, and so remain relatively constant in meteorites. They are also lithophile elements which are known to stay in the mantle when the core of Mars formed. All this makes the 182Hf–182W decay system is ideal for dating a planet's core formation.
By chemically testing 30 chondrites and another 20 Martian meteorites, the scientists are able to measure the excess abundance of 182W relative to other non-radiogenic isotopes of W (the tungsten isotopic composition) as well as the Hf/W ratio in the Martian mantle. They also measured the relationships between hafnium (Hf), thorium (Th), and tungsten (W) and generated a hafnium-thorium ratio (Th/Hf). Because Th and W have very similar chemical behaviors the researchers were able to calculate how long it took Mars to develop into a planet. They found that Mars accreted very rapidly and reached about half of its present size in about 1.8 million years or less. This is very rapid formation and consistent with their stranded planetary embryo hypothesis.
There are some other implications for these results. 26Al is known from meteorites and has a half-life of 700,000 years. Thermal modelling shows that planets accreting in under 2.5 million years incorporate enough 26Al for radioactive decay to induce silicate melting. If the time estimates of this study are correct then that would mean "Mars would have reached [about 69%] of its present size by that time and the heat generated from 26Al decay alone would have been sufficient to establish a magma ocean." Whoa! Additionally, this evidence of a quickly forming Mars could help to explain the similarities between the xenon (Xe) content of the Martian atmosphere and Earth's atmosphere, what is referred to as the "missing xenon problem." On both planets, Xe is not very abundant compared to the concentrations of other noble gases or to Xe concentrations in space. The authors suggest that part of the atmosphere of Earth was inherited from an earlier generation of planetary embryos that had their own atmospheres. "Earth may have inherited its missing Xe problem from the atmosphere of a Mars-like planetary embryo, possibly the impactor that also formed the Moon. This idea is consistent with the time when Earth became retentive for Xe, which is estimated to be, 100Myr after the birth of the Solar System and may correspond to the time of the Moon-forming giant impact."
This is the type of paper that reminds me of how cool I think planetary science is while also reminding me why I became an ecologist rather than a chemist. Overall, very interesting results!
You can read the paper for yourself here:
N. Dauphas, and A. Pourmand. (2011) Hf–W–Th evidence for rapid growth of Mars and its status as a planetary embryo. Nature, 2011; 473 (7348): 489. (DOI: 10.1038/nature10077)
Also, check out these stories:
(image via nasa.gov)