1.5 1.75
Sculptor Supercluster
1.37 billion light-years
WWW.ASTRONOMY.COM 27air. Once the atmosphere was thin enough,
Mars cooled, and its water froze into the
glaciers and ice caps we see there now.
Today, the average atmospheric pressure is
0.6 percent that at Earth’s mean sea level.
Near the winter pole, temperatures reach
–195° F (–126° C) — so cold that roughly
30 percent of the carbon dioxide atmosphere
snows out there, covering the surface with a
dry ice veneer. While only
the south pole containsa permanent dry ice component, water ice
resides in both poles in the form of polar
layered deposits, which, thanks to extensive
spacecraft mapping, provide a minimum
inventory of the planet’s water ice content.
If melted, the water in these deposits would
cover an idealized smooth Mars in a liquid
layer 69 feet (21 meters) deep. According to
recent studies, the planet may have hosted a
sea containing more water than the Arctic
Ocean about 4.3 billion years ago. Thiswould have formed a global water layer at
least 6.5 times deeper than today’s polar
deposits could provide.
Mars may yet have its day in the Sun.
Our star’s luminosity is gradually increas-
ing, which pushes the habitable zone out-
ward. In about a billion years, the habitable
zone will leave Earth roasting outside its
inner edge. But the Red Planet will experi-
ence a more temperate period lasting a few
billion years as the Sun transitions into its
red giant phase. For Mars, summer is com-
ing — though since much of its water has
been lost to space through the years, it may
not grow much more hospitable.The rocks
Next comes the asteroid belt, which, along
with the Kuiper Belt beyond Neptune, is a
remnant of the disk of rocky and icy debris
that gave rise to our planetary system.
Contrary to Hollywood imaginings, the
asteroid belt is mainly space. The main
belt totals less than 5 percent the mass of
Earth’s Moon, and about a third of this is
contained in Ceres, the largest object and
the only dwarf planet in the asteroid belt.
Add in Vesta, Pallas, and Hygiea, the next
three largest asteroids, and half the belt’s
mass is accounted for.
The main belt starts at 2.06 AU, where
objects make four orbits around the
Sun in the time it takes Jupiter to make
one. Astronomers call this a 4:1 orbital
resonance. Whenever an asteroid’s orbital
period is a whole number fraction of
Jupiter’s orbital period, the giant planet
easily destabilizes the tiny rock, quickly —
on astronomical timescales — clearing it
out of its original orbit. The 2:1 resonanceAstronomers recently spotted regions on
Venus that grow rapidly hotter and then cool
again over short time periods. The most likely
explanation is volcanic eruptions on the
planet’s surface. E. SHALYGIN, ET AL. (2015)The bases of mesas in Deuteronilus Mensae on
Mars exhibit strange textures. Astronomers think
they could have formed from the rapid melting of
ice that lurks under the dust and rock currently
overlaying the features. ESA/DLR/FU BERLIN (G. NEUKUM)Our planet Earth is now under observation by DSCOVR, a joint project by NASA, the U.S. Air Force, and
the National Oceanic and Atmospheric Administration, designed to monitor the solar wind in real
time. Visually stunning images like this one, taken from a million miles away, are a bonus. NASAVe nu s i a n h o t s p o t
June 22, 2008 June 24, 2008 Oct. 13, 2008Relative brightness0.75 0.85 0.95 1.05 1.15 1.2510°20°185° 195° 185° 195° 185° 195°Object Aabc