Mars Once, On the Waterfront

A Face that Sticks in Your Mind

Why is the crust of Mars up to 30 kilometers thinner on one half of the planet than the other, one side of the planet rugged terrain, the other plains? Twenty-five years ago a couple US scientists theorized that a collision had impacted the planet in a way that caused the dichotomy between the two hemispheres. However geological tools couldn't validate the theory, which also wasn't the only possible explanation.

Mars' mantle, like Earth's, shifts over time, and an alternate theory was that the 30km difference was due to upwards shift of the mantle. Overturn from magma ocean melting could have also produced the differences. Then some scientists thought that an impact of great magnitude would create different features from those found, or would simply obliterate all evidence.

Last week Nature (subscription) published studies by three research groups who used new modeling techniques to provide evidence for the collision theory. Jeffrey Andrews-Hanna, Maria T. Zuber, and Bruce Banerdt's team from MIT estimated that >4 billion years had passed since the Mars dichotomy formed, and that in the intervening time geological activity had obscured evidence from the original event. They programmed specific assumptions about gravity and terrain into their model to account for changes such as activity from Mars' Tharsis volcanic range.

The group then determined the original boundaries of the dichotomy, which happened to match their measurements of the elliptical area formed by the theorized impact. The huge elliptical area formed in the event is 10,600 by 8,500 kilometers (6,586 X 5,281 miles) covers about 20% of the planet and is bigger than than largest country on Earth -- Russia's width is ~5,000 miles. At about the same time as the Mars collision a similar event occurred on Earth which threw off the moon and lots of debris. It was a violent time in the solar system.

Margarita M. Marinova et al., from California Institute of Technology and University of California (UC), Santa Cruz, used modeling to determine the type of impact that would create the unique geology, The team calculated that an object 1,600-2,700 km wide hit the planet with about 3 X 1029 Joules of energy. Scientists believe that the collision not only created a giant crater and changed the planet's crust, but that it was responsible for some of the other features of Mars. F. Nimmo and team, also from UC Santa Cruz, produced a third study to round out current understanding of the possible impact.

What's The Problem on the Water Front?

In other exciting Mars news, robots earlier in the month discovered what looked like it could be ice. Scientist programmed robots had taunted us for years, foraying around the planet then duly reporting back no signs of water. Last weekend the "NASA Phoenix Mars Lander" scooped up some of the icy soil for analysis. By vaporizing it in an oven analyzing the gases emitted, and by determining the minerals in the clumps of icy soil retrieved by the robot, scientists will try to ascertain what the substance is, whether it was at some time liquid, and how it formed. The lab tried to run this experiment a few weeks ago, but it went awry when the robot deposited the soil into the oven but the oven reported back that the soil wasn't there. Scientists were planning to process the soil sample differently or use a different oven in order to complete the analysis.


Acronym Required previously wrote about Mars in "Mars Global Surveyor Bites the Dust".

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