SKY_July2014.pdf

74 July 2014 sky & telescope

Amateur Planetary Science

highest, and because daytime seeing conditions are often

more turbulent than they are at night due to solar heating

of the surrounding terrain, the tradeoff of reduced resolu-

tion is off set by the steadier seeing in longer wavelengths.

Methane: Seeing the Altitude of Clouds

When imaging the planets, the primary information

recorded is the albedo (or refl ectivity) information of the

target’s atmosphere or rocky surface. From ultraviolet

(UV) through near-IR, the information that you glean

about a planet comes from the color contrast of its details;

you see the Great Red Spot (GRS) on Jupiter as a red-

dish color because it refl ects red light and absorbs blue

wavelengths. However, there are two notable exceptions to

this rule.

The fi rst exception is methane (CH 4 ) imaging using

a highly specialized (and often expensive) CH 4 passband

fi lter. A methane fi lter passes a very narrow range of near-

IR light centered at 890 nm. This wavelength corresponds

to the strongest absorption band in the spectrum of meth-

ane gas. The atmosphere of each gas giant contains a

small portion of methane. When imaged through a meth-

ane-band fi lter, the planets appear very dark, because

their atmospheres are absorbing (rather than refl ecting)

that wavelength of light. But not all cloud features appear

the same. Storms that reside in the highest altitudes have

less CH 4 above them, so they appear brighter than their

surroundings. Images recorded through a methane fi lter

thus help astronomers study cloud stratigraphy in the gas

giants’ atmospheres.

Jupiter responds best to a CH 4 fi lter; most anticyclonic

structures, such as the white ovals and particularly the

GRS and Oval BA (Red Jr.), appear bright in methane

images due to their positions high above the surround-

ing atmosphere. The planet’s South Polar Hood, which

is practically invisible at other wavelengths, also appears

bright in CH 4. The primary North and South Equatorial

Belts are revealed to be lower-altitude clouds and white

zones of bright, higher hazes. But most interesting of

all, you can detect temporarily bright features, which are

rapidly ascending convective clouds — a sign of the onset

of a strong disturbance. As such, the revival of faded belts

(such as the South Equatorial Belt in 2010) begins with

the sudden appearance of bright cumulonimbus clouds,

and a CH 4 fi lter will confi rm its altitude.

Due to the poor sensitivity of imaging detectors at

890 nm, combined with the nature of the information

recorded, the other gas giants respond rather poorly

to imaging through a methane fi lter. Saturn displays

a brighter Equatorial Zone but not much else; even its

storms don’t appear bright in CH 4 because the atmo-

sphere’s upper layers are quite thick. Uranus and Nep-

tune would show very interesting features in methane,

but their disks are tiny as seen from Earth, and the

absorption of methane in their atmospheres makes the

signal too faint to record with amateur equipment.

Thermal Emissions in IR

Beyond the methane band, there is one other spectral

region at the red end of the spectrum that is of interest

to enterprising amateurs. This second exception to the

albedo rule serves to reveal what is completely unseen at

visual wavelengths: the surface of Venus. Earth’s “sister”

is permanently hidden beneath a thick and opaque layer

of sulfuric-acid clouds. Due to an extreme greenhouse

eff ect, ground temperatures average a hellish 460°C (860

°F). When a body is heated to this temperature, it literally

glows at a wavelength of around 1 micron (1,000 nm) in

the near-IR, a band of light just barely visible to amateur

CCD cameras. And by lucky coincidence, the primary

Saturn captured through a 742-nm near-IR fi lter (bottom)

displays atmospheric banding in higher contrast than in

the visible wavelength image at top. Notice the planet’s

hexagonal polar vortex is easier to see in the near-IR.

Left: Uranus currently displays atmospheric banding when

imaged in near-IR wavelengths. Right: Faint markings are some-

times visible in near-IR images of Venus.

WAYNE JAESCHKE

URANUS: DAMIAN PEACH; VENUS: SEAN WALKER