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54 March 2014 sky & telescope

OBSERVING

Exploring the Solar System

Ever since early 2013, Earth has been steadily closing in

on Mars. The approach culminates April 8th in opposi-

tion. As the closest rocky planet to Earth whose surface is

visible in modest telescopes, Mars easily garners the most

attention of all the inner planets. The Red Planet often

reveals subtle changes from year to year that stand out to

the patient observer. But occasionally, big changes can

occur from one apparition to the next.

Part of the great attraction Mars has for observers is

that the planet once looked more like Earth. Even today,

with white cirrus clouds, dust storms, and ice caps that

grow and shrink with the seasons, Mars is the most

Earthlike of any planet in our solar system. Although the

planet’s thin atmosphere and dry environment make it

look desolate, Mars is far from an unchanging, dead world.

As the planet approaches opposition, keep an eye out

for some of these diff erences. A guide to observing Mars,

as well as a map, can be found on page 50.

The Shrinking North Polar Cap

Among the fi rst features even a beginning planetary

observer will notice are the Red Planet’s bright polar caps.

This year, Mars near opposition presents its northern

hemisphere to Earth, giving us a nightly view of the

rapidly receding North Polar Cap (NPC). At opposition in

April, the planet will be well into its long summer season,

so the cap should be at its smallest as seen from Earth.

That said, the NPC is substantially larger than the SPC

and presents many interesting rifts and other features.

As the NPC shrinks, it will unveil what looks like a dark

ring encircling the pole. In addition, shortly after New

Year’s a large, dark rift known as Chasma Boreale should

begin to form around 300°°W. And at roughly the same

time on the NPC’s opposite side, a dark swath known as

Rima Tenuis will seem to cut off a large, crescent-shaped

section of the bright ice cap from the rest of the pole. This

Changes on Mars

The nearby Red Planet displays remarkable changes every apparition.

Though a predominately dry, desert planet, Mars presents a

changing appearance to patient observers. The Hubble Space

Telescope captured this series of images in late 2007 show-

ing the entire surface of the planet, including the North Polar

Hood enshrouding the polar cap underneath. South is up.

NASA / ESA / THE HUBBLE HERITAGE TEAM (STSCI / AURA) / J. BELL / M. WOLFF

LEFT:

E. C. SLIPHER / LOWELL OBSERVATORY;

R IGH T: S &T:

SEAN WALKER

One vivid example of albedo changes occurred sometime

in the mid-20th century, when the well-known feature

Thoth-Nepenthes vanished. Left: This 1956 image shows

this curved feature (the middle of the dark, thick C at bot-

tom). Right: The author’s 2012 image shows only the dark

remnant known today as Alcyonius Nodus.

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