Sky and Telescope - November 2016 - 8

Letters
Fran Bagenal responds: Jupiter's atmospheric gas is primarily hydrogen, along
with small amounts of water, ammonia,
methane, and other compounds. Deep down
these gases are pretty well mixed. However,
as you go up in an atmosphere the temperature decreases, and at some point the
temperature drops below the condensation
temperature of a component. (That's why
water clouds form over Earth.)
At Jupiter, the water condenses out at
lower levels where it's warmer. Above this
cloud deck of water, as the atmosphere
continues to cool down, the ammonia
hydrosulphate vapor condenses out. Then
higher up (and colder) the gaseous ammonia condenses out. These are all tiny aerosol
droplets or ice crystals kept aloft by atmospheric motion.
Were they to collect into very large masses,
then, yes, they would precipitate downward
- just as, again on Earth, rain falls once
the water droplets get too big to stay buoyed
by air currents.

in every photograph. It was a remarkable
view, and one that I shared with (and had
conﬁrmed by) my fellow observers.
Aaron Dittrich
Cochrane, Alberta

Why Does Ammonia Float?
In reading Fran Bagenal's beautifully
written account of Juno's imminent
arrival at Jupiter (S&T: July 2016, p. 18),
I noticed something that prompts a
question. How is it that clouds of such
heavy molecules as NH3 (ammonia) and
NH4SH (ammonium hydrosulﬁde) can
ﬂoat above an atmosphere composed of
extremely light H2 molecules?
Our planet's air is composed mainly
of O2 and N2, so it makes sense that its
clouds of lower-mass water vapor (H2O)
should ﬂoat in it, as they do. Yet the
opposite seems to be the case at Jupiter.
So what's the story?
Jim Baughman
West Hollywood, California

75, 50 & 25 Years Ago

Peter Tyson's reﬂective "To the Unknowable" (S&T: July 2016, p. 4) reminds us
that, without eschewing rigor or even a
temperate pride in humanity's achievement, intellectual humility nonetheless
remains a requisite for full contemplation
upon the sublime and ultimate unfathomability of our cosmos.
James Tracy
East Greenwich, Rhode Island

Visiting Mars: No Rush Needed
Here's one more reason why we should
go slowly before sending humans to Mars
(S&T: June 2016, p. 84). Any human presence there will inevitably leave behind
some biological footprints, defying the
very reason for our fascination with
the Red Planet: the prospect - slim as
it might be - of ﬁnding evidence of
ancient life there. So I say, let's go slow!
Eli Maor
Morton Grove, Illinois

Roger W. Sinnott

November 1941
Vol. 1, No. 1 "Once
again we start on a new
venture. This time, however, it is really a synthesis of many existing
projects and agencies. It
is expected that Sky and
Telescope will endure
for many years to come,
and play an important part in the development
of the layman's interest in astronomy.
"We wish to call attention to the pictures
and letters on page 13. Scientists are still active
abroad, in spite of the hardships which they
must endure - they merit all the support we
can give them."
Charles and Helen Federer mustered little
fanfare for their ﬁrst editorial in the new magazine. Europe had been in chaos more than two
years, and America's entry into World War II was
just a month away.
November 1966
On the Moon "The successful soft landing on
the moon by the Soviet Union's Luna-9 spacecraft on February 3, 1966, enabled earthmen for
the ﬁrst time to see millimeter-sized details of
the lunar surface. [The soft landing itself was]

Probing the Unknowns

November 2016 sky & telescope

carried out for the ﬁrst time in the history of
cosmonautics. . . .
"That the top layer of lunar soil must be very
porous has been known for some time [because]
brightnesses of lunar features increase to sharp
maxima near full moon, indicating considerable small-scale shadowing at other times.
Calculations and laboratory experiments had
shown that this shadow eﬀect was well simulated by a structure of randomly oriented ﬁbers
. . . and also by a 'fairy castle' structure.
"It had been conjectured that very ﬁne dust
from meteoric impacts on the moon could
produce fairy castles, since in the lunar vacuum
the particles could fuse together at their points
of contact to form ﬂuﬀ y conﬁgurations of very
low density. . . . In the transmitted pictures,
there are no ﬁne-ﬁbered structures, no fairy
castles, no heaps of angular fragments, sand
or loose dust [that] many
authors had supposed to
form the moon's microrelief. . . ."
Cold War rivalries and
the space race didn't stop
Yuri N. Lipsky (Sternberg State Astronomical
Institute, Moscow) from

excitedly sharing with S&T readers mankind's
ﬁrst close-up glimpses of another world.
November 1991
Starry Vista "Astronauts always return to
Earth gushing superlatives about their views
of our planet from space. But they have said
comparatively little about the appearance of
the night sky from orbit. So Sky & Telescope
queried astronaut Jay Apt about the stargazing
he did last April during his ﬁrst trip aboard the
Space Shuttle. . . . 'I did not need to be as well
dark adapted in space as on the ground to see
the Magellanic Clouds'. . . .
"As he picked out familiar constellations,
using the Milky Way itself as a guidepost, Apt
noticed one thing right away: 'I was able to see
more stars with color.' He attributes this to
the complete absence of atmospheric blurring,
which allowed starlight to concentrate very
eﬀectively onto his eyes'
color receptors. . . .
"When asked for
the most interesting
astronomical phenomenon he saw from space,
Apt answered: 'To see a
meteor burn up below
you is pretty neat!'"

Sky and Telescope - November 2016

Table of Contents for the Digital Edition of Sky and Telescope - November 2016