A variety of media outlets are reporting on a series of new papers that strengthen the case that the peculiar differences between the northern and southern hemispheres of Mars (e.g. see this topographic map of Mars, or this web page) are due to a very large impact by a proto-planetary object of order 1000 miles in diameter.
Kenneth Chang discusses this in an article in the NYT, although he awkwardly calls the impactor a "meteor." Clara Moskowitz at Space.com also discusses the story, as does JR Minkle at SciAm news (see if you can spot the mistake in the image caption accompanying the SciAm article).
[Image shown is a color-coded map of the elevation of Mars, from the Mars Orbiter Laser Altimeter, showing the low, smooth, northern hemisphere and the higher, cratered, south. Note that even recently (1999) this data was used to argue against an external cause for the north/south dichotomy.]