October 2007 Sky from the Keeble Observatory
Mars fascinates! And, the rovers are alive and well. Last month we discussed some
of the reasons for concern about the extended mission of the robotic explorers dubbed
Spirit and Opportunity. Due largely to a global dust storm that reduces sunlight
to dangerously low levels, both rovers had been on severely restricted activity.
The dust is settling, and the rovers have resumed normal activity, now 3 years into
a planned 3 month mission! Spirit is now exploring a low plateau dubbed “Home Plate,”
while Opportunity has begun its descent into Victoria Crater.
Several months ago we reported evidence interpreted as flowing water on the Martian
surface within recent years. Comparison of images taken at a five year interval
by the Mars Global Surveyor (MGS) showed bright streaks in several craters which
appeared to be new gullies carved by a sudden rush of water. Now, higher resolution
images from the Mars Reconnaissance Orbiter (MRO) are casting some doubt on the
earlier conclusions. The streaks are still there, but infrared analysis reveals
that they are not salt deposits or frozen water (remember, Mars is very cold!).
Researchers cannot prove that they were not formed by water, but some now suggest
that the crater walls are steep enough, that we could simply be looking at landslides
of loose surface material.
Last May the same probe (MRO) returned an image which was interpreted as possible
cave skylight. Imaged from directly overhead, it appears as a dark circle about
150 meters in diameter A more recent image of the same pit taken from a slight angle
shows that it must be at least 78 meters deep, and it appears similar to deep pits
found on the slopes of some Hawaiian volcanoes.
MGS also returned images showing dark spots on the slopes of the Martian volcano
Arsia Mons. Ranging in size from 100 to 300 meters, these were dubbed the “Seven
Sisters.” Using a newer probe, known as Mars Odyssey, to examine the Seven Sisters
in infrared, they were found to be cooler than the surrounding terrain during daylight,
and warmer during night. This thermal behavior, similar to what is found in terrestrial
caves, suggests that they are indeed connected to larger caverns. For more information
you can visit the Mars Odyssey
Lunar phases for October: Last Quarter on the 3rd, at 6:06 am; New Moon on the 11th,
at 1:01 am; First Quarter on the 19th, at 4:33 am; Full Moon on the 26th, at 12:52
Early risers will get a good show this month, with lots of planet watching possible
in predawn hours. We begin the month with both Venus and Saturn bright to the east,
Venus is much brighter and higher. Between the two planets you’ll see the bright
star Regulus, in Leo. As the month progresses, both will drift downward relative
to Regulus, with Venus appearing to move faster – this because it’s closer to us.
In the predawn twilight on the 13th, you’ll see these three objects forming an equilateral
triangle, about 4 degrees on a side, with Regulus at the top. By month’s end, Venus
is below Saturn, with Regulus above by a similar angle. Mercury also joins the predawn
show by the end of the month. It will be easier to see than its evening apparition
early in September, since the plane of the ecliptic makes a nearly right-angle with
the horizon in the morning at this time of year, but a very shallow angle in the
Mars is due south at sunrise, above Orion.
Evening planet watchers have few opportunities this month. Jupiter is low to the
south-southwest and drifting lower and towards the western horizon as the month
goes on. That red star below Jupiter is Antares, not Mars! Mercury begins the month
low to the west at sunset, probably not out of the horizon clutter and haze. It
will disappear into the Sun’s glare by mid-month.
Our overhead view a few hours after sunset at mid-month finds the constellation
Cygnus just west of zenith, marked by the bright star Deneb. It’s shape is evocative
of its namesake, the Swan, with Deneb marking the tail. This constellation lies
in the direction toward which the Sun moves as it orbits in a near circle about
the center of our Galaxy. The plane of the Galaxy bisects the sky at this time from
northeast to southwest. This luminous glowing band is what we call the Milky Way
– indeed, the word galaxy comes from the Greek, and it means “milky way!” Get away
from city lights on a moonless night to see it with your own eyes.
Low on the southwest horizon is the familiar “teapot” asterism of the constellation
Sagittarius, which marks the direction toward the center of the Galaxy. The actual
center is obscured by vast clouds of dust, which prevent its observation at visible
wavelengths. To the northeast lies the direction directly away from the center in
the plane of the Milky Way. At this time, then, the direction towards the “galactic
north pole” is just below the northwest horizon, with galactic south to the southeast.
Note that these are directions in space – there is no object marking these poles,
which are actually imaginary points on the sky.
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.