Sky's Up Summer 2016 - Page 28

Auroras The presence of an atmosphere allows many comparisons. Earth, Mars, Jupiter, Saturn, Uranus and Neptune all exhibit auroras. Most auroras are a result of the interaction of the solar wind with a planet’s magnetic field. The exception is Mars because it lacks a field. The magnetic field provides a pathway for solar wind particles to enter the planet’s atmosphere near the planet’s magnetic poles. The auroral oval structure is common. The direct collision of the solar wind at the top of the Martian atmosphere leads to auroral glow that is much less defined over Mars. These images show the auroral ovals of Earth, Jupiter and Saturn. Storms Earth COURTESY OF NASA / UC Berkeley Earth COURTESY OF NASA Jupiter COURTESY OF NASA/JPL Storms are seen around the Solar System — on every planet and satellite with a substantial atmosphere. Surface wetting and flooding from rain is familiar on Earth and also seen on Titan. Convection storms, some including lightning, are seen on Earth, Jupiter, Saturn and Titan. Cyclonic storms are observed on Earth, Mars, Jupiter, Saturn and Neptune. Spiraling cyclones are found throughout the Solar System. Mars Saturn COURTESY OF ESA/Hubble Jupiter COURTESY OF NASA/ESA, and John Clarke (Boston University) Atmospheric circulation George Hadley published a theory explaining the circulation of Earth’s atmosphere in 1735. He suggested that convection cells spanning from Earth’s equatorial zone to its poles carried heat to higher latitudes. Now called Hadley cells, this circulation would make the poles warmer and cool the temperate latitudes as the air flowed back to the equator. It turned out that he oversimplified Earth’s circulation pattern (there COURTESY OF ESA, Venus Express, VIRTIS, INAF-IASF, Obs. de Paris-LESIA are two large convection patterns and plenty of complications as well in each hemisphere) but he got it right for Venus. Atmospheric circulation can get interesting at planetary geographic poles. A polar vortex on Earth plays a role in the Antarctic ozone hole, that is now repairing itself thanks to the withdrawal of chlorofluorocarbons COURTESY OF NOAA (CFCs) from production and dispersal. But our spacecraft have shown us very interesting polar vortices around the Solar System. Clockwise from top left, the polar vortices on Venus, Titan, Saturn and Earth are similar phenomena but exhibit very different appearances. 28 Neptune COURTESY OF NASA/JPL Saturn COURTESY OF NASA/JPL COURTESY OF NASA Dust Storms / Dust Devils Titan COURTESY OF NASA/JPL-Caltech/SSI Saturn COURTESY OF NASA/JPL-Caltech/SSI Sky’s Up Wind carries dust on planets and satellites with atmospheres and hard surfaces. While Mars can suffer planetwide dust storms (one of which was under way when NASA’s Mariner 9 arrived at the planet in 1971), smaller dust storms there are more common and frequent, and they can look eerily familiar. We also see local effects that combine to make familiar dust devils in unfamiliar places. A dust storm on Mars (top right) shows flow very similar to dust blowing off the Sahara over the Atlantic Ocean (top left). Dust devils on Earth and on Mars can extend high into the atmosphere and track across the ground for long distances. No one knows if Titan has dust devils. They cannot be seen easily from above through its hazy atmosphere. Sky’s Up Earth COURTESY OF NASA/GSFC Earth COURTESY OF NASA/JPL Mars COURTESY OF NASA/JPL/Malin Space Science Systems/MGS Mars COURTESY OF NASA/JPL-Caltech/University of Arizona 29