July 2008 Sky from the Keeble Observatory
Not only has Phoenix landed, but it seems to have found frozen water exactly where
it was supposed to find it. Recall that Phoenix is in the far northern hemisphere
of Mars, near but not on the north polar cap. Its latitude is similar to northern
Alaska on Earth. Early images from under the probe show that the landing rockets
blew away the surface material and revealed a hard, light colored layer beneath.
It looks like ice. Also, the robotic digging arm found a hard layer beneath the
surface. The arm is equipped with tools which will make it possible to chip off
some of that material and bring it into the spacecraft’s analyzing instruments to
melt and determine its exact composition. Even before those samples are taken, however,
we’ve seen visual evidence that this material is really ice and not salt (we’ve
found salts in the Martian soil at the southern sites being explored by the rovers
Spirit and Opportunity). At the bottom of one of the early trenches excavated by
Phoenix, we see a handful of dice-size light chunks of material. A few days later,
they were gone. At the temperature and atmospheric pressure found at the landing
site (pressure is about 1% what you’re experiencing, temperatures range from minus
115 to minus 4 degrees Fahrenheit) water ice doesn’t melt. Rather it sublimates
– changing directly from solid to gas. Calculations show that the time for the chunks
to disappear was what would be expected of water ice.
Lunar phases for July: New Moon on the 2nd, at 10:19 pm; First
Quarter on the 10th, at 12:35 am; Full Moon on the 18th, at
3:59 am; Last Quarter on the 25th, at 2:42 pm.
Predawn skies still provide little opportunity for planet watchers. Jupiter sets
just about at sunrise early in the month, and buy the end of July it will set nearly
an hour and a half before sunrise. Mercury will be rising to the east about an hour
and a half before the Sun on the 1st, reaching its maximum western elongation
of 22 degrees. However, the tilt of the ecliptic leaves it only 15 degrees off the
horizon at sunrise. By month’s end it will disappear back into the solar glare.
Evening viewers will see Mars and Saturn to the west near Regulus, in Leo. Early
in the month they will make a nice binocular target, all within about 5 degrees,
with Mars only about a degree from Regulus. Jupiter rises about 30 minutes before
sunset, and travels low to the south as the evening advances. Later in the month
you will see Mars, Saturn, Regulus, and Venus aligned like beads on a string to
the west shortly after sunset, with Venus closest to the horizon. The alignment
of the planets is because they all orbit in essentially the same plane around the
Sun as Earth does – the so-called ecliptic plane. Regulus lies on the ecliptic,
but that alignment is accidental.
About two hours after sunset, our overhead view at mid-month find the constellation
Hercules at zenith. On a clear, moonless night you should be able to see even without
binoculars the fuzzy patch of M13, the “great globular cluster in Hercules.” This
swarm of perhaps a million stars is one of many orbiting the Milky Way Galaxy. The
stars in this cluster are among the oldest stars in the Galaxy.
As Mars sets to the west, you can also see the red giant star Antares (“Mars’ rival”)
to the south, in the constellation Scorpio. To the left of Mars you’ll find Jupiter
in the constellation Sagittarius, which makes a familiar “teapot” asterism. Well
beyond the stars we can see in Sagittarius lies the center of our own Galaxy, some
28,000 light years away. Following the plane of the Milky Way from south to north,
bowed to the east we find Cygnus the Swan and its brightest star Deneb to the east-northeast.
This constellation lies in the direction of the Sun’s motion about the center of
Above Deneb, high to the east we find the bright blue star Vega in the constellation
Lyra. Turning to the northeast we see the familiar crooked W shape of Cassiopeia.
To the northwest we find the even more familiar Big Dipper of the constellation
Ursa Major. If you follow the arc of the dipper’s handle towards the west, you’ll
find Arcturus in Bootes.