Astronomy

billion light-years

11.25

Boötes Supercluster
936 million light-years

26 ASTRONOMY • DECEMBER 2015

and composition. Yet with an average sur-

face temperature of 864° F (462° C) day and

night, Venus stays hotter than Mercury gets

on its worst day despite its greater distance

of 0.72 AU. The reason is the planet’s dense

atmosphere, which is composed almost

entirely of heat-trapping carbon dioxide

gas. Compounding the lack of hospitality,

the surface pressure of the atmosphere is

about 92 times sea-level pressure on Earth,

the same we would experience at a depth of

3,000 feet (1,000 meters) under the ocean.

Unsurprisingly, no spacecraft

landing on our inner

neighbor has continued

to transmit for much

more than two hours.

Radar mapping of

Venus from Earth

and from orbiting

spacecraft shows a world of fascinating

geography. It is the only other planet in

the solar system known to host active vol-

canoes. In 2015, scientists studying ther-

mal imaging by the European Space

Agency’s (ESA) Venus Express orbiter

reported 1,530° F (830° C) hot spots along

the planet’s Ganiki Chasma rift zone, a

type of feature associated with terrestrial

volcanism. Researchers have observed epi-

sodes where the temperatures of these

spots abruptly increase and then cool

down, suggesting ongoing eruptions.

Goldilocks zone

Next out is Earth, home sweet home and the

only planet in the solar system where liquid

water freely exists on the surface. As far as

we know, the presence of water is a necessity

for life. Astronomers extend this concept to

define a star’s “habitable zone” — a range

of orbital distances where liquid water

potentially could exist — as a way to iden-

tify exoplanets that may be capable of sup-

porting life as we understand it. While we

can quibble with the definition — perhaps

there’s a biology that uses solvents other

than water or maybe life can develop entire-

ly beneath the surface — it’s a place to start.

For the solar system, conservative values

place the habitable zone between 0.99 and

1.69 AU. More optimistic values extend it in

both directions, from 0.75 to 1.84 AU. Either

way, the zone excludes desiccated Venus but

includes Mars, thought to be warmer and

wetter in the distant past.

Located about 1.5 AU out, Mars long has

been viewed as the best bet for finding life in

the solar system. But with half Earth’s size

and only 38 percent of its surface gravity, the

Red Planet was crippled in its ability to hold

onto the thick atmosphere needed to main-

tain surface water. In its first billion years,

the upper layers of the atmosphere slowly

bled into space, and occasional asteroid

impacts drove away great masses of martian

SOLAR SYSTEM YARDSTICK

After Copernicus placed the

Sun at its center, astronomers

had a solid sense of the rela-

tive dimensions of the solar

system. They knew, for exam-

ple, that Venus orbited 30

percent closer to the Sun

than Earth. But they had no

way of putting everyday

measurements to these dis-

tances. In effect, the astro-

nomical unit (AU; average

Earth-Sun distance) was a

yardstick with no markings.

In 1663, the Scottish math-

ematician James Gregory

described a way to use transits

— the apparent passage of

Mercury or Venus across the

Sun’s face — to determine the

AU’s value. But efforts to use

Mercury transits and other

techniques, such as observing

Mars at opposition, produced

inconsistent results.

Edmond Halley, famous

today as the first to predict

the return of a comet,

showed in 1716 that Mercury

was too far from Earth for

transit measurements to be

effective, but Venus was

ideal. He proposed a plan for

observations of the 1761

Venus transit and suggested

that astronomers be dis-

patched across the globe to

observe and time the event.

He believed the technique

could establish the length of

the astronomical yardstick to

an accuracy of 0.2 percent.

It proved far more difficult

in practice. To reach even 4

percent of the current value,

astronomers had to combine

the results of Venus transits in

1761 and 1769 — though this

was still a huge improvement

in accuracy. By the time the

next transit rolled around, in

1874, astronomers had begun

exploring more promising

techniques, such as photo-

graphic observations of Mars.

We now can measure the

distances to Venus, Mars,

asteroids, and many other

solar system objects by ping-

ing them with radar. By send-

ing a pulse of energy from a

radio telescope and knowing

the speed of light, the time

taken for the signal to return

provides the distance. Today

astronomers define the AU

as equal to 92,955,807.273

miles (149,597,870.700 kilo-

meters). — F. R.

The Soviet probe Venera 13 holds the record at 2 hours, 7 minutes for the longest spacecraft to survive on the surface of Venus. It sent back pictures of its

basalt-like surroundings, with the spacecraft itself partially visible at the bottom of the image. NASA HISTORY OFFICE

Mercury holds highly reflective material in its

permanently shadowed craters that could be

water ice, as revealed here by NASA’s MESSENGER

spacecraft and Earth-based radar mapping.

NASA/JHUAPL/CIW

NASA (BOÖTES SUPERCLUSTER)

Kandinsky

Prokofiev