Lunacy: Finding New Moons?

Everyone knows our Moon: lovers stare at it, wolves howl at it, and ESA recently sent SMART-1 to study it. Once it has arrived at the Moon (expected to be in January 2005), SMART-1 will perform a scientific study of the Moon’s composition, particularly any hints of its water-ice history if deposited there by colliding comets. The spacecraft will search for signs of water-ice in craters near the Moon’s poles, as well as provide data on the still uncertain origin of the Moon and reconstruct its evolution by mapping and the surface distribution of minerals and key chemical elements. But there are over a hundred other moons in our Solar System, each a world in its own right.

Scientists would like to know the origin of the atmospheric patches imaged on Saturn’s moon, Titan, as imaged by Hubble. Image Credit: Hubble Space Telescope/UA Smith

A moon is a natural body that travels around a planet. Moons are a by-product of planetary formation and can range in size from small asteroid-sized bodies of a few kilometers in diameter to several thousand kilometers, larger even than the planets Mercury and Pluto.

Landing on another moon

One such large moon is Titan, the target for ESA’s daring Huygens mission that in 2005 will become the first spacecraft ever to land on a moon of another planet. Next summer, NASA’s Cassini spacecraft, launched in 1997, is scheduled to go into orbit around Saturn and its moons for four years. The piggybacking Huygens probe is scheduled to plunge into the hazy Titan atmosphere and land on the moon’s surface. The Huygens probe is geared primarily towards sampling the atmosphere. The probe is equipped to take measurements and record images for up to a half an hour on the surface. But the probe has no legs, so when it sets down on Titan’s surface its orientation will be random.

Titan is slightly bigger than the planet Mercury, and is only called a moon because it orbits the giant planet Saturn rather than the Sun. Titan, which is about 50 percent larger than the Earth’s moon, is the only satellite in the solar system with a dense atmosphere.

The haze is much thicker than Earth’s worst city smog. It was impenetrable to cameras aboard the Pioneer and Voyager spacecraft that flew by the Saturn system in the late 1970s and early 1980s. But the smog-shrouded atmosphere of Titan has been parted by Earth-based radar to reveal the first evidence of liquid hydrocarbon lakes on its surface. The chemical composition of its environment resembles that of early Earth but it is far colder and lacks liquid water. Scientists think Titan may have carbon- and nitrogen-containing molecules accumulated on its surface. And these primitive precursors to life might be brought even further towards life’s door if liquid water makes an occasional appearance. The by-products of methane molecules destroyed in the sun’s ultraviolet light react with other molecules in Titan’s atmosphere, forming organic droplets and particulates that fall onto the moon’s surface, blanketing the icy bedrock and forming lakes and oceans.

Four other large moons can be found around another of our neighbours, Jupiter. These are Io, Europa,Ganymede, and Callisto. Europa has captured attention because beneath its icy surface, scientists think that an ocean covers the entire moon. Some scientists have even speculated that microscopic life might be found in that ocean. Streaks of reddish-brown color highlight cracks in Europa’s outer layer of ice. Some scientists have speculated that microorganisms suspended in Europa’s ice may be the cause of these colorations. To test this theory, planetary geologist Brad Dalton of the NASA Ames Research Center compared the infrared (IR) signature of Europa’s ice with the IR signature of microorganisms living at hot water vents in Yellowstone National Park. Dalton discovered that the IR signatures are very similar. "Just on a lark, I asked a colleague of mine who did a lot of work at Yellowstone if he had any IR spectra of extremophile bacteria," Dalton says. He was shocked by how well they matched Europa’s spectrum. But none of the known terrestrial extremophile bacteria could survive the harsh conditions of Europa’s surface. They possibly could live in the supposed liquid ocean under Europa’s ice crust, however. Europa’s average surface temperature is minus 162 C (minus 260 F), and it has an almost non-existent atmospheric pressure of 10^-7 (1/10,000,000) bar. (In comparison, the average atmospheric pressure at the surface of the Earth is approximately ten million times more, or 1 bar.)

Most critical to astrobiologists studying Jupiter’s moons, the eccentricity or oval shaped orbits of Jupiter’s moons are pumped or oscillated by tidal forces as they orbit. This input of Jupiter’s gravitational energy heats up the inner moons particularly without relying only on the Sun’s radiant heat, and thus gives an interesting way to provide one of the three ingredients for life–an energy source–even if far from the Sun.

What remains to be found among the Giant Planets like Jupiter and Saturn are some candidates that combine all three ingredients for primitive life: energy, liquid water and some atmosphere. Only Saturn’s moon, Titan, has an appreciable atmosphere, and only Jupiter’s Europa or Ganymede have any indications of water ice. But uniquely powerful tidal forces around the Giant Planets do offer some promising, non-radiant and non-volcanic heat sources.

The farthest known planet ever discovered is a strange world indeed. Whizzing around its star every 29 hours, it is shrouded in clouds made not of water droplets but of iron atoms. This is a world of iron rain. Credit: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics

Habitable moons?

By detecting the drop in light seen when a world passes in front of its parent star, discovering planets the size of Jupiter, and also those smaller than Mars might be possible. That means, if our own Solar System is anything to go by, it will be capable of detecting moons similar in size to Titan and the four large moons of Jupiter.

It would be particularly exciting if such combinations of planets and moons were found orbiting a star at Earth’s distance from the Sun. Perhaps then the surfaces of the moons would be warmed to habitable levels.

Orbital dancing

What about moons similar to our own? An equivalent of Earth’s moon would be too small to be detected directly, but such a body would affect the way its planet moves and it is that movement which a space telescope might detect.

The Earth and the Moon orbit the Sun like ballroom dancers who move around the floor, simultaneously twirling about one another. This means the Earth does not follow a strictly circular path through space, sometimes it will be leading the Moon and sometimes trailing.

This causes variations of up to five minutes from where the Earth would be if it did not possess a moon. By precisely timing when a rocky planet passes in front of its star, a space telescope might be able to show if a moon is pulling its planet out of a strictly circular path around the star.

So, how many moons can a space telescope expect to find circling planets around other stars? If one makes an estimate based on our own Solar System, several thousands will be found.

What’s Next

The discovery of planets around other stars is extraordinarily difficult, because a planet is about one billion times fainter than its host star, so the planets get lost in the glare of the host star. One reason for optimism exists for finding planets (or moons) now, is the unique technical combination of big telescopes, fast computers and most importantly, exquisite optics.

During the next 15 years, American and European scientists hope to launch more than half a dozen missions to search our corner of the Milky Way galaxy for terrestrial planets.

The Kepler mission is scheduled for launch into solar orbit in October 2006. Kepler will simultaneously observe 100,000 stars in our galactic "neighborhood," looking for Earth-sized or larger planets within the "habitable zone" around each star – the not-too-hot, not-too-cold zone where liquid water might exist on a planet. To highlight the difficulty of detecting an Earth-sized planet orbiting a distant star, Borucki, Kepler’s principal investigator, points out it would take 10,000 Earths to cover the Sun’s disk.

One NASA estimate says Kepler should discover 50 terrestrial planets if most of those found are about Earth’s size, 185 planets if most are 30 percent larger than Earth and 640 if most are 2.2 times Earth’s size. In addition, Kepler is expected to find almost 900 giant planets close to their stars and about 30 giants orbiting at Jupiter-like distances from their parent stars. Because most of the gas giant planets found so far orbit much closer to their stars than Jupiter does to the Sun, Borucki believes that during the four- to six-year mission, Kepler will find a large proportion of planets quite close to stars. If that proves true, he says, "We expect to find thousands of planets."

ESA’s Herschel mission will take detailed pictures of stars that might harbor dusty remnants of entire solar systems. As these images become available, astronomers will be able to predict the sizes and orbits of giant planets within a distant solar system.

Also due for launch in 2009 is the almost $1 billion NASA-ESA Next Generation Space Telescope, or NGST [James Webb Space Telescope], a near-infrared telescope that will succeed the Hubble Space Telescope. Planet hunting will be a minor part of its job. Like Hubble, NGST will be a general-purpose telescope with an emphasis on cosmology. But it will investigate stars with dusty disks – the early stage of planet formation – and may also be able to study Jupiter-size planets.