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 surface.
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 water!
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
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
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 to
http://www.skymaps.com. You will find extensive descriptions of what's worth
looking for, and you can download and print a single copy for your personal use.