Letters from the June 9, 2007, issue of Science News

Safe passage

I have to ask you to remove the subtitle “Dangerous Bridge” under the photograph of the exit ramp from the New Jamarat Bridge in Saudi Arabia (“Formula for Panic: Crowd-motion findings may prevent stampedes,” SN: 4/7/07, p. 213). There has never been an accident on that ramp, and the bridge is now being overhauled to make it safer.

Dirk Helbing
Dresden University of Technology
Dresden, Germany

Mars evasion?

I find it interesting that even the scientists studying Mars can’t accept that our local star can have a major impact on climate (“No Escape: There’s global warming on Mars too,” SN: 4/7/07, p. 214). I am still waiting for the global-warming-crisis mongers to explain how carbon dioxide causes global warming.

Donald R. Laster Jr.
West Long Branch, N.J.

So now we are asked to believe that the global warming on Mars has released billions of tons of carbon dioxide from the melting ice caps, but on Earth, carbon dioxide from humans is causing the warming. If Mars is not warmed from an external influence (the sun), what else could be causing Mars’ warming?

Jack Hayden
Eagle River, Alaska

The article says that Mars’ global warming is caused by albedo variations. What about the billions of tons of carbon dioxide?

Roger Gordon
Nazareth, Pa.

Mars’ atmosphere is already 95 percent carbon dioxide. The added gas from the polar ice caps didn’t significantly affect that proportion.—S. Perkins

I have a question about the two Mars-polar images in the article. While the later image is darker overall, the white polar cap actually looks slightly larger than in the earlier image. Can you please explain how this comports with global warming?

James Parkinson
Glendale, Calif.

The images were composites of satellite photos taken over several months. So, the sections showing the polar ice cap might not have been taken at the same time of the Martian year, says Paul Geissler of the U.S. Geological Survey. He adds that long-term observations indicate that the polar cap is shrinking and that Mars’ southern hemisphere is darkening.—S. Perkins