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Projects: Observing Earth

Here, There, and Not Quite Everywhere

by Charlene M. Anderson

Methane drizzle on Titan
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Methane drizzle on Titan
These images yielded the observation of morning drizzle near the bright patch called Xanadu (yellow box) on Titan. The two columns represent two observations of Titan in infrared wavelengths performed at different observatories (left, the Very Large Telescope, Chile, from February 28, 2005; right, Keck, Hawaii, from April 17, 2006). The dark regions represent local concentrations of solid and liquid methane in tropospheric clouds and drizzle above the surface. The drizzle appears to be localized to the part of Titan where it is morning (the left sides of the globes); by 10:30 a.m. local time, the drizzle has disappeared. Credit: Máté Ádámkovics / UC Berkeley

Some of the earthly phenomena that satellites monitor from space also occur on other worlds. Here’s a brief look at those mentioned in this article.

Precipitation—It’s just a light drizzle, but it falls all the time on Titan, Saturn’s largest moon. The temperature there is about –150 degrees Celsius (about –240 degrees Fahrenheit), so the rain that falls is liquid methane. At depths within the gas giant planets, deeper than we can see, other strange species of rain also may be falling.

Lightning—Thunderstorms big enough to swallow Earth rage around Jupiter, generating lightning 10 times as powerful as any strike on our planet. From radio signals detected by passing spacecraft, we know that lightning occurs on Venus, Saturn, Uranus, and Neptune as well.

Ozone (O3)—Nasty-smelling ozone occurs on other worlds, but not in quantities large enough to form an atmospheric layer that can block ultraviolet (UV) light from the Sun—and protect organic things from that damaging radiation. Mars has traces of ozone in its atmosphere, but not enough to shield its surface from UV rays. The top layers of soil are effectively sterilized.

Nitrogen dioxide (NO2)—This noxious creation of human industry is not found on other worlds. Earth alone must cope with it.

Sea level—On Earth, mean sea level is the reference point for measuring height and depth on the surface. On Jupiter’s moon Europa, the entire world beneath the ice is covered by an ocean. Determining sea level there is a problem future explorers would like to solve.

Surface temperature—Our benign climate is perfect for life as we know it, but Earth is a neighborhood anomaly. Temperatures on other worlds range from about 480 degrees Celsius (almost 900 degrees Fahrenheit) on the surface of Venus to around –230 degrees Celsius (–380 degrees Fahrenheit) on Pluto. Even a minor change in average temperature can tip the balance, positively or negatively, for terrestrial life-forms.

Planetary surface—Some worlds in our solar system may have no solid surface at all. On these gas giants, the gases just get denser and denser with depth until, for example at Jupiter, the lightest element in the universe—hydrogen—is compressed into liquid metal.

Fire—Should we ever detect fire on another world, we will have found a signature of life. To start the chemical reaction we call fire, an oxidizer is necessary. On Earth, we would run out of the oxygen we breathe unless plants continually replenished it. Without plant life, oxygen would react with minerals and metals to get locked away in rocks.

Land use—As far as we know, Earth is the only planet with life-forms that modify its atmosphere and surface. For most of humanity’s history, these changes were invisible from space. Now our “eyes in the sky” look down on cultivated fields, diverted rivers, and city lights at night, all dramatic demonstrations of our species’ power over the planet.

—Charlene M. Anderson, Associate Director of The Planetary Society