The Day After Mars

 October 31, 1938 was the day after Martians encountered planet Earth, and everything was calm. Reports of the invasion were revealed to be part of a Halloween radio drama, the now famous broadcast based on H.G. Wells' scifi novel War of the Worlds. On Mars October 20, 2014 was calm too, the day after its close encounter with Comet Siding Spring. Not a hoax, this comet really did come within 86,700 miles or so of Mars, about 1/3 the Earth-Moon distance. Earth's spacecraft and rovers in Mars orbit and on the surface reported no ill effects though, and had a ringside seat as a visitor from the outer solar system passed by. Spanning over 2 degrees against stars of the constellation Ophiuchus, this colorful telescopic snapshot captures our view of Mars on the day after. Bluish star 51 Ophiuchi is at the upper right and the comet is just emerging from the Red Planet's bright glare.

Climate and Earth’s Energy Budget

Climate and Earth’s Energy Budget

by Rebecca Lindsey

The Earth’s climate is a solar powered system. Globally, over the course of the year, the Earth system—land surfaces, oceans, and atmosphere—absorbs an average of about 240 watts of solar power per square meter (one watt is one joule of energy every second). The absorbed sunlight drives photosynthesis, fuels evaporation, melts snow and ice, and warms the Earth system.

The setting sun, photographed from the International Space Station.

Solar power drives Earth’s climate. Energy from the Sun heats the surface, warms the atmosphere, and powers the ocean currents. (Astronaut photograph ISS015-E-10469, courtesy NASA/JSC Gateway to Astronaut Photography of Earth.)

The Sun doesn’t heat the Earth evenly. Because the Earth is a sphere, the Sun heats equatorial regions more than polar regions. The atmosphere and ocean work non-stop to even out solar heating imbalances through evaporation of surface water, convection, rainfall, winds, and ocean circulation. This coupled atmosphere and ocean circulation is known as Earth’s heat engine.

The climate’s heat engine must not only redistribute solar heat from the equator toward the poles, but also from the Earth’s surface and lower atmosphere back to space. Otherwise, Earth would endlessly heat up. Earth’s temperature doesn’t infinitely rise because the surface and the atmosphere are simultaneously radiating heat to space. This net flow of energy into and out of the Earth system is Earth’s energy budget.