The Solar System's Greatest Hits

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.

Comments

[Dave Mosher]

I had no idea a grad student was on the author list for one of the Mars impact studies. Cool!
Now the next question I'd like to have answered re: Mars - Did the impact whack off any atmosphere or oceans? Or did the impact prevent their formation entirely?
Oh, and agreed: Oliver Morton is definitely a clever cat.

[thomas]

I heard from a friend at work that within our solor system scienctist have found 3 new planets in the last 5 years does anyone know what he might have been hinting at

[David Chandler]

Yes, he's talking about icy worlds out beyond Pluto, also called Kuiper Belt Objects. One of the new icy objects out there (called Eris) is even bigger than Pluto, and there are a couple of others that are nearly as big.
But there's an ongoing controversy about what these objects should be called. Two years ago, the International Astronomical Union decided that these new "planets", and even Pluto itself, did not deserve to be called planets, and so suddenly the Solar System would have 8 planets instead of 9. Pluto and the others would be called Dwarf Planets. Then, just a couple of weeks ago, the IAU decided to call these dwarf planets Plutoids. (Ray Villard talked about that decision on his Cosmic Ray blog, at blogs.discovery.com/cosmic_ray ). But many astronomers, and lots of other people, think the IAU made a big mistake and are trying to get those decisions reversed. Watch this space, I'll talk about the whole issue soon on this blog.

[David Chandler]

To answer Dave's question, this impact would have preceded the formation of any ocean. However, it created the basin in which many people (me very emphatically included) believe that an ocean may have formed early on.
Personally, I suspect that most of the ocean is actually still there, frozen solid and covered with a layer of windblown sand and dust. The Phoenix mission may start to give us some hints of this.

[Renewable Electricity]

Thanks for this great post...