ASTRONEWS
>Annual maximum
temperature (K)
<60 110 160 230 290
Location of water ice detections,
Moon’s Southern HemisphereLocation of water ice detections,
Moon’s Northern Hemisphere80 °–90° N180 °180 °150 °150 °120 °120 °90 °60 °30 °
0 °330 °330 °300 °300 °270 ° 270 °240 °240 °210 °210 °80°–90° S0°
30°60°90°Kepler-
systemKepler-452 system
Solar system
Earthbcde
fSun
Mercury
VenusMarsb
Habitable zone16 ASTRONOMY • DECEMBER 2018
Ice discovered on the Moon’s surface
COSMIC RAY GUN. The massive binary stars that make up Eta Carinae produce cosmic rays when their colliding stellar
winds accelerate charged particles to nearly the speed of light.SHUAI LI, UNIVERSITY OF HAWAII SOEST/HIGPFAST
FAC TThe Sun’s habitable zone will move outward
as it ages, eventually leaving Earth and Mars
to warm Jupiter, Saturn, and Neptune.ASTRONOMY: ROEN KELLYSOLAR SYSTEM COMPARISON
SIZE IT UP. A star’s habitable zone is a band around
the star in which water on the surface of an orbiting
planet could remain liquid. Its size and distance
depend on the star’s brightness. In our solar system,
only Earth currently resides in the habitable zone.
But how does our solar system measure up against
others? Are there other planets circling within the
habitable zones of distant stars?The Kepler-186 system, about 500 light-years away,
would fit entirely within the orbit of Mercury. Its star is
smaller and cooler than the Sun, so its habitable zone is
much closer as well. One of its five planets, Kepler-186f,
lies within this habitable zone. It orbits every 130 days,
receiving one-third the energy that Earth gets from our
Sun. The planet is 10 percent larger than Earth, but its
mass and composition are not yet known.The Kepler-452 system 1,400 light-years away has
a habitable zone that stretches slightly farther
than our Sun’s, owing to its slightly larger, older,
and brighter star. Kepler-452b is a super-Earth 60
percent larger than our planet, though its mass
and composition are unknown. It orbits inside the
star’s habitable zone once every 385 days, at a dis-
tance 1.05 times the Earth-Sun separation. —A.K.COLD AS ICE. A recent
review of data from NASA’s
Moon Mineralogy Mapper (M^3 ),
a visible and infrared spectro-
meter that rode aboard India’s
Chandrayaan-1 orbiter from 2008
to 2009, shows the first direct
evidence of water ice in the cold,
dark craters near the Moon’s
poles. Scientists discovered
the ice by studying scattered
sunlight from permanently
shaded craters, which produced
a weak but detectable signature
that matches pure water. This
map shows exposed surface ice
as orange and blue dots, and
indicates the annual maximum
temperature of the region.
(Darker is colder, brighter is
warmer.) The amount of ice on
the surface is unknown, but
further exploration could help us
understand how it got there, as
well as its role in the Moon’s
formation and evolution. — A.J.