The biggest wallop ever suffered by any planet during the violent, turbulent early days of the solar system was when something as big as Mars slammed into the Earth with such force that the entire crust melted and splattered off into space. Some of it fell back, and much of the rest globbed together and became the moon. That's something scientists only managed to figure out after analyzing the rocks brought back from the moon by the Apollo astronauts.
The second biggest hit, it seems, was one suffered by Mars, when it got clobbered by something bigger than Pluto. That idea, which has been kicking around since the mid-80s, was not taken very seriously until this week, but now suddenly gets catapulted into being the likeliest explanation for the strange lopsided surface of the red planet.
The new evidence for a colossal impact that left a deep gouge on Mars that's bigger than Eurasia came out in a trio of papers published this week in the journal Nature. One of the papers used recent spacecraft data to reconstruct the outline of the giant basin on Mars -- the largest impact remnant ever seen anywhere. The other two papers used computer modelling to figure out exactly how big the impacting body must have been, and how fast and at what angle it struck Mars.
These are major, significant papers that apparently solve one of the biggest remaining mysteries about our planetary system, the riddle of why Mars has two such radically different hemispheres. These papers, though they will undoubtedly be debated for a few years as is the norm for any significant shift in scientific understanding, will likely stand as among the most important advances in planetary astronomy in recent decades. (See these stories in the New York Times, Christian Science Monitor, Discovery Channel and National Geographic, for example)
One of those papers, an analysis of the size and direction of arrival of the impacting body, was done by a group at Caltech. And the lead author of the paper, Margarita Marinova, is a graduate student there. Kudos to Marinova for an important and groundbreaking piece of research.
Marinova, a graduate student in Caltech's Division of Geological and Planetary Sciences, found that the impactor must have been between 1,600 and 2,700 km. across, producing a 100 billion gigaton blast. "This size range of impacts only occurred early in solar system history," Marinova says. It all happened about 4 billion years ago, around the same time as the moon-forming crash. Those early days of the solar system were very violent times, when the planetesimals from which the planets were built were still in flux and there was a whole lot of crashing going on.
Sometimes student work in science is thought of, especially by outsiders, as sort of practice work, a kind of second-class level of research. But most scientists know very well that this example, of truly significant new findings, is actually pretty typical. Much of the cutting-edge work in science, in fact, is the product of student research projects.
It's nice to be reminded of that from time to time, as in this case.
By the way, in that same issue of Nature there's a whole package of articles related to impacts on Earth, timed to the fact that this Monday, the 30th, will be the 100th anniversary of the biggest terrestrial impact known to have been witnessed by people -- the Tunguska blast over Siberia in 1908. Among that collection of articles is one that I wrote -- a sort of oral history of how our understanding of near-Earth asteroids and the danger they pose has evolved over the last few decades. It's accompanied by a very clever cartoon, depicting some of the pioneers in that field (who were the subjects of my interviews). I take no credit for the illustration -- the idea came from my friend and Nature's news and features editor, Oliver Morton, and was quite brilliantly executed by the artist David Parkins.











Determining, and reproducing accurately,
the true colors of Mars is a surprisingly complicated thing to do, and
sometimes there are no clear-cut "right" answers. But some students from Florida are helping to get us a little closer.


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