Monday, June 23, 2014
Wednesday, March 19, 2014
Tuesday, March 11, 2014
Every obituary of Professor Bruce Murray that I have come across focuses on his administrative career, so here is one about his seminal scientific contributions.
Murray, together with Caltech colleagues Kenneth Watson and Harrison Brown, predicted that water ice accumulates in permanently cold craters on the Moon. He and his colleagues realized that, unlike other volatiles, H2O can be retained on the Moon by a combination of hopping and trapping of molecules. Molecules hop along ballistic trajectories at thermal speeds until they reach one of the cold traps near the lunar poles. Detailed quantitative studies of this process were published in two papers in 1961. The work received little attention initially; after all the Apollo samples were bone dry. Today it is a main focus of the lunar exploration program. Many lines of evidence suggest that there is indeed water in cold dark craters near the lunar poles. The same theory also applies to Mercury, where ice deposits have been identified.
In 1966, Prof. Murray made a fundamental prediction about water ice on Mars, together with physicist Robert Leigthon, otherwise known as co-author of the Feynman Lectures. Leighton and Murray realized that the presence of water vapor in the martian atmosphere implies that ice can persist just a few centimeters below the surface, where it is protected from daily temperature variations. This study gained traction also only slowly, but was eventually worked out in increasing detail and served as motivation for sending a neutron spectrometer in orbit around Mars, which indeed identified near-surface ice in great abundance. It was also the primary motivation behind the Phoenix Mars Lander, which in 2008 ultimately touched the ice that Leighton and Murray had predicted 42 years earlier.
Murray's scientific contributions are manifold, and, of course, they include major exploration discoveries such as canyon systems on Mars, the cratered surface of Mercury, and the atmospheric circulation of Venus. The two theoretical contributions described above are not only his most cited, but they will continue to be highly relevant for the exploration of the Moon, Mars, and Mercury for many more decades to come.
I have refereed way over a hundred scientific papers for various journals. Here are some of those that I found most memorable and impressive:
- Serina Diniega et al. A new dry hypothesis for the formation of martian linear gullies. Icarus 224, 526-537 (2013).
- Wei-Jay Ni and Herve Capart. Groundwater drainage and recharge by networks of irregular channels. J. Geophys. Res. F 111, F02014 (2006).
- Farhang Radjai and Stephane Roux. Turbulentlike fluctuations in quasistatic flow of granular media. Phys. Rev. Lett. 89, 064302 (2002).
Wednesday, August 5, 2009
P. Chylek. Uncertainty over weakening circulation. Physics Today, March 2007.