The issue continues with a commentary about Spaceguard, a survey of near-Earth asteroids that concluded that the chance of one of these striking the Earth is extremely small. Despite this, the Los Angeles Times is running a story this week that declares that the United States is unprepared for an impact by an asteroid. This press release from the Planetary Society contains additional information. (I believe it would be a waste of money to pursue this further.)

Finally, the issue contains three papers proposing explanations for the martian hemispheric dichotomy, which is the fact that the northern lowlands, the Borealis Basin, are on average four kilometers lower than the southern highlands. Using different approaches, the three papers provide evidence that the hemispheric dichotomy was caused by the impact of a dense asteroid with a diameter 25% to 40% of the diameter of Mars. Since the basin that exists today has an elliptical shape, it is proposed that the blow was not a vertical strike.

The evidence provided by these three papers strengthens the hypothesis of an impact origin, but Walter S. Kiefer points out in a News & Views article that an alternative explanation is that the Borealis Basin was formed by convection of the mantle. It is likely that the issue will not be resolved until we can directly examine the composition of the rocks from different parts of Mars.

References:

Andrews-Hanna JC, Zuber MT, Banerdt WB. 2008. The Borealis basin and the origin of the martian crustal dichotomy. Nature 453:1212–1215. DOI: 10.1038/nature07011.

Marinova MM, Aharonson O, Asphaug E. 2008. Mega-impact formation of the Mars hemispheric dichotomy. Nature 453:1216-1219. DOI: 10.1038/nature07070.

Nimmo F, Hart SD, Korycansky DG, Agnor CB. 2008. Implications of an impact origin for the martian hemispheric dichotomy. Nature 453:1220–1223. DOI: 10.1038/nature07025.

Much of the the 7 December 2007 issue of Science is devoted to the Hinode (“sunrise”) solar space telescope mission. (The web site for the mission is in Japanese, if you read that language.) Hinode was launched in September 2006 and since October 2006 has observed the sun from earth orbit. The initial observations have provide clues for solving the mystery of heating the sun’s corona.

In a perspective, Erdélyi and Fedun (p. 1572) explain that there are at least three fundamental questions to be answered.

Where is the energy generated? How does the generated energy propagate from the energy reservoir to the solar corona? How does the transported energy dissipate efficiently in the solar corona to maintain its multimillion-kelvin temperature?

It is now clear that the powerful magnetic fields in the solar atmosphere play a crucial role in heating the corona. In 1970, Hannes Alfvén was awarded the Nobel prize in physics for his predictions of magnetic waves, now called Alfvén waves, in the solar atmosphere. Many of Hindode’s observations are consistent with the presence of Alfvén waves, and the waves are sufficiently powerful to generate the solar wind and heat the corona.

Both the paper and the article make special note of the role of amateur astronomers in making regular planetary observations. Amateur astronomers around the world can coordinate to make series of continuous observations of a planet over many weeks. The advancement of optical and image processing technologies has led to astonishing improvements in the quality of the images that can be obtained with relatively modest equipment. Dr. Sayanagi writes:

This coverage from around the world nicely complements the more powerful, but less flexible capabilities of the large ground- and space-based telescopes.

For example, see the website of Chrisopher Go, who is one of the contributing authors to the paper. Mr. Go has a second website devoted to Jupiter’s Red Spot Junior.