April 2007 Sky from the Keeble Observatory
Water, water everywhere. Well, not exactly. Earth’s surface
is 70% covered by water. Some of the moons in the outer solar system
are thought to be mostly water – though most are either solid
ice, or with a thick icy surface layer over a vast liquid interior.
We wrote recently of the discovery that water has flowed in liquid
form on the Martian surface within the last five years.
But, most of Mars is very cold and very dry. Evidence that huge
amounts once flowed there, carving sinuous channels and leaving
signs of ancient floods, has been noted since the Viking missions
in the 1970s. There are even signs of ancient lakes, and perhaps
even the remnant of a shallow ocean. So, where is the water?
Mars has polar caps, observed since the 19th Century, which appear
to expand and recede with the seasons. Once we knew how cold the
planet’s surface actually is, and that the atmosphere is very
thin and mostly made of carbon dioxide, many researchers surmised
that the seasonal variations were actually the result of frozen
carbon dioxide (familiarly known as “dry ice” –
you can buy it at Ukrop’s) freezing and sublimating. Others
suggested that the underlying polar caps might contain a mixture
of water ice and the ubiquitous fine dust that coats much of the
One of several active Mars probes is a satellite called Mars Express,
launched and controlled by the European Space Agency (ESA). One
of its instruments is a powerful ground-penetrating radar (known
as MARSIS, which stands for Mars Advanced Radar for Subsurface and
Ionospheric Sounding – everything launched into space seems
to require a tortured acronym!), which has now been used to map
the southern polar regions of Mars. They’ve found a lot of
The dry ice theory appears correct, but incomplete. Under that
transient layer, and layered dust, lies enough ice to cover the
entire planet to a depth of 30 feet with liquid water, if the polar
cap were ever to melt. More than 90% of this ice gives a radar echo
which suggests strongly that it’s almost purely ice, with
only a small admixture of dust. If confirmed, these results suggest
that the south pole holds about 10% of all the water that once flowed
so freely across the surface. The next target for these radars is
the north polar cap. We’ll update you in a future column.
Lunar phases for April: Full Moon at 1:15 pm, on the 2nd; Last
Quarter on the 10th, at 2:04 pm; New Moon on the 17th, at 7:36 am,
and First Quarter on the 24th, at 2:36 am.
Mars rises to the east, about 2 hours before the Sun reaches the
horizon, moving slowly up and toward the southeast, where it will
be seen about 20 degrees off the horizon at sunrise. Mercury is
to the east at about 10 degrees elevation. It’s rounding the
Sun ahead of us, so it will disappear into solar glare by month’s
end. Mars will appear higher as the month advances, about 25 degrees
at the end of April, but still to the southeast at sunrise. It’s
actually getting farther from the Sun’s position, but the
Sun is moving toward the north as we advance through spring toward
summer. Jupiter is bright to the south-southwest at dawn. Watch
for the waning moon to pass below April 7th through 9th.
Venus continues brilliant, high at sunset to the west all month.
Saturn emerges from evening twilight about 50 degrees above the
east-southeast horizon early in the month, 70 degrees and due south
by month’s end. The waxing crescent moon will pass Saturn
early evening on the 19th.
Evening skies begin with Saturn to the east, about 20 degrees above
the horizon at sunset, and very close to the Moon on the 1st. Venus
will be brilliant to the west, the first “star” to emerge
from twilight, and setting about 2 hours after sunset. Both planets
will be higher as the month goes on.
Two hours after sunset at midmonth, look for the familiar sickle
of Leo, high to the south. The bright star below the sickle is Regulus,
which marks the heart of the Lion. Saturn is above and to the right.
Saturn lies midway between Regulus and the Beehive Cluster in Cancer,
a good binocular target. To the west, Castor and Pollux are high
above the horizon, while Orion is almost gone for the season. Turning
to the east, the bright star about 10 degrees above the horizon
is Arcturus, in the constellation Bootes. Below and to the southeast
is Spica, the brightest star in Virgo. The Big Dipper (Ursa major)
is high to the north, with the familiar “pointer stars”
almost directly above the pole star, Polaris.
For your own monthly star chart, you can direct your web browser
You will find extensive descriptions of what's worth looking for,
and you can download and print a single copy for your personal use.