WWW.ASTRONOMY.COM 35raining down from above or
being dredged up from below,
reacting with the Sun’s ultra-
violet radiation to produce a
host of more complex com-
pounds. The Juno and Cassini
spacecraft can track the atmo-
spheric temperatures, the soup
of chemical species, and the
properties of the giant planet
aerosols to understand how
they all inf luence one another,
and how theoretical concepts
from Earth’s complex meteo-
rology and clouds can be
applied to these environments.
Leigh Fletcher
Senior Research Fellow in
Planetary Science, University of
Leicester, United Kingdom
Q: IF OUR SOLAR SYSTEM
IS COMPOSED OF THE
REMAINS OF STARS THAT
USED UP THEIR HYDROGEN
AND WENT SUPERNOVA,
WHERE DID OUR SUN
GET THE HYDROGEN
IT BEGAN WITH?
Alfred D’Amario
Hudson, Florida
A: It’s true that our solar sys-
tem is built from the remains
of earlier generations of stars,
which over their lifetimes con-
verted hydrogen into heavier
elements. However, plenty of
hydrogen was left over to form
subsequent generations of stars
because the overall efficiency
of stars — as the universe’s
primary mechanism for con-
verting hydrogen into heavier
elements — is quite low.
The inefficiency crops up in
a number of places. First, when
an interstellar cloud of (mainly
hydrogen) gas collapses under
its own gravity to create a new
group of stars, only a small
fraction of the material in the
cloud actually ends up incor-
porated into the stars. The
remainder is swept away by
strong winds from circumstel-
lar accretion disks.
Second, winds can also
develop during a star’s life-
time, either driven by the
pressure of hot gas in the
corona above the stellar
surface, or — in the most
luminous stars — by the
pressure of light itself.
These winds mean that stars
reach the end of their lives
having lost a significant frac-
tion of their outer layers. The
material in these layers rejoins
the interstellar medium from
whence it originally came.
Finally, the conversion of
hydrogen into heavier elements
occurs only in the core region
of a star, where the tempera-
ture and density are suffi-
ciently high for nuclear fusion
reactions to take place.
Hydrogen farther out in the
star’s envelope generally
remains unburned (although
some exceptions can arise). As
a consequence, when a massive
star (with an initial mass
around nine or more times the
mass of the Sun) reaches the
end of its life and explodes as a
core-collapse supernova, there
is still plenty of hydrogen in
the star’s envelope; and this
hydrogen is driven out by the
explosion and again rejoins the
interstellar medium.
Rich Townsend
Assistant Professor,
Department of Astronomy,
University of Wisconsin–MadisonQ: SEASONAL CHANGES IN
COLOR OR CONTRAST ON
MARS WERE ONCE THOUGHT
TO INDICATE THE PRESENCE
OF VEGETATION OR WATER.
ARE THERE STILL SEASONAL
CHANGES, AND IF SO, WHY?
Paul W. H. Tung
Freedom, New HampshireA: In the days before spacecraft
observed Mars in detail, we
had to rely on what we could
see with telescopes on Earth
and the f lyby Mariner missionsof the 1960s. These fuzzy
images only gave the barest
indication of geologic features
on the surface, indicated by
light regions and dark regions,
as well as the white polar caps.
Because there was no real
understanding of the planet’s
surface, observed changes in
color and brightness through-
out the year were ascribed to
any number of explanations,
from water to vegetation. Upon
closer inspection, starting with
the Viking orbiters in the 1970s,
it became clear that Mars is a
dry desert world devoid of veg-
etation or water, which is not
stable under current environ-
mental conditions. However,
huge canyons and channels
reveal that, several billion years
ago, Mars was home to f lowing
water. How it transitioned from
a warm, wet planet to a cold,
dry one is still up for debate, but
the question of seasonal
changes is one we can explain.
Like Earth, Mars has an axial
tilt (25° for Mars, 23.5° for Earth)
that gives the planet seasons. Its
polar caps are composed of both
water ice and carbon dioxide ice.
During the northern or southern
summer, the carbon dioxide ice
in that polar cap vaporizes (sub-
limates) into the atmosphere and
the polar cap shrinks, revealing
the surface beneath. In the win-
ter, the ice is redeposited and the
caps grow again. Some of this
sublimated carbon dioxide isalso redistributed in the atmo-
sphere toward the equator, and
then deposited on the surface
when the atmosphere cools or
the pressure decreases, leaving
a covering of frost in the mid-
to high latitudes, and at high
elevations at low latitudes. This
frost can lead to changes in
color and brightness, and will
sublimate again as spring and
summer return.
Sublimation of carbon diox-
ide frost can also trigger small-
scale events such as dust
avalanches and debris flows on
steep slopes, which have now
been observed in real time.
Another cause of these changes
is global dust storms, which
occur every few years and are
triggered by seasonal changes
in temperature and pressure of
the atmosphere.
Dan Berman
Research Scientist, Planetary
Science Institute, Tucson, ArizonaSend us your
questions
Send your astronomy
questions via email to
[email protected],
or write to Ask Astro,
P. O. Box 1612, Waukesha,
WI 53187. Be sure to tell us
your full name and where
you live. Unfortunately, we
cannot answer all questions
submitted.A polar ice cap and
variations in the
color of Mars’ mid-
latitude terrain
are readily visible
in this Hubble
Space Telescope
image of the Red
Planet, taken
March 21, 1995.
NASA, ESA, AND THE HUBBLE
HERITAGE TEAM (STSCI/AUR A)