Astronomy

Hercules Cluster

501 million light-years

Shapley Supercluster

586 million light-years

500 600 700 800 900

WWW.ASTRONOMY.COM 25

called perihelion, it’s just 0.308 AU away.
Standing on Mercury at this time, the Sun
would appear 3.2 times larger than it ever
appears from Earth. Surface temperatures
reach 800° F (400° C), hot enough to melt
lead. Baked by sunlight, pummeled by
charged particles streaming from the Sun,
and with just 38 percent of Earth’s diameter,
Mercury maintains only a wisp of an atmo-
sphere. But a growing body of evidence
suggests that despite its front-row seat in
the solar system’s hot zone, Mercury has ice.
In 1992, astronomers at Arecibo
Observatory in Puerto Rico bounced radar
off the planet’s polar regions, revealing
radar-reflective deposits on the floors of
a few craters where daylight never shines.
Decades earlier, astronomers had theo-
rized that ice from comets and asteroids
crashing on Mercury could find their way
to “cold traps” in permanently shadowed
polar craters. Data collected by NASA’s
Mercury-orbiting MESSENGER spacecraft
in 2012 confirmed that these craters con-
tain hydrogen-rich layers consistent with
the presence of water ice. A similar process
appears to have preserved subsurface ice
in the Moon’s polar areas.
Next out is Venus, a planet 95 percent
of Earth’s size and likewise a good match
in properties like mass, density, gravity,

Spacecraft have now

visited all the planets,

plus members of the

asteroid and Kuiper

belts, as well as sev-

eral comets. The solar

system family portrait

grows ever more

complete.

NASA (EARTH, JUPITER, NEPTUNE); NASA/SDO (SUN); NASA/

JHUAPL/CIW (MERCURY); NASA/JPL (VENUS); NASA/USGS: JODY

SWANN, TAMMY BECKER, & ALFRED MCEWEN (MARS); NASA/JPL-

CALTECH/UCLA/MPS/DLR/IDA (CERES); NASA/JPL/SSI (SATURN);

UNIV. OF WI-MADISON: LAWRENCE SROMOVSKY/W.M. KECK

OBSERVATORY (URANUS); NASA/JHUAPL/SWRI (PLUTO); ESA/

ROSET TA/MPS FOR OSIRIS TEAM MPS/UPD/LAM/IAA/SSO/INTA/

UPM/DASP/IDA (COMET 67P)

E SO (G R E AT AT T R AC TO R); NA SA /E SA / T HE HUBBLE HER ITAG E

TEAM (STSCI/AURA) (COMA CLUSTER); ESO/INAF-VST/OMEGACAM

(HERCULES CLUSTER); NASA/ESA/J. BLAKESLEE (WASHINGTON

STATE UNIVERSITY) (SHAPLEY SUPERCLUSTER)