[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

New Mars Global Surveyor Image Available


NEW MARS GLOBAL SURVEYOR IMAGE

A new image taken by the Mars Global Surveyor spacecraft is now available
on the MGS home page:

http://mars.jpl.nasa.gov/mgs/msss/camera/images/5_15_98_ascraeus_release/index.h
tml

The image caption is appended below.

Ron Baalke
------------------------------------------------------------------------------

                            Mars Global Surveyor
                             Mars Orbiter Camera

              Mars Orbiter Camera (MOC) High Resolution Images:
                     Lava Flows On Ascraeus Mons Volcano

Mars Global Surveyor Mars Orbiter Camera Release:          MOC2-47a, -47b
Mars Global Surveyor Mars Orbiter Camera Image ID:         578204600.26705
                                                           P267-05

                                (A) [Image]

(A) Portion of U.S. Geological Survey Mars Digital Mosaic. Volcano is about
400 km (250 miles) across at its center. Sun illumination over most of
mosaic is from upper right. Arrow points to location of MOC image shown in
(B).

                                (B) [Image]

(B) MOC image 26705 (subframe) reproduced at full resolution, about 4.1
meters/pixel (13.4 feet/pixel). Picture shows an area approximately 3.3 x
3.3 km (2.1 x 2.1 miles) in size. Sun illumination is from the lower right.

                                   CAPTION

Ascraeus Mons Volcano:
Like Earth, Mars has many volcanoes and volcanic features. This
high-resolution view shows some of the lava flows near the summit of
Ascraeus Mons, one of the three giant shield volcanoes known as the "Tharsis
Montes". Volcanoes form when magma (molten rock) erupts out onto the surface
of a planet. Based on Viking-era observations, Ascraeus Mons is considered
to be one of the tallest volcanoes on Mars... its summit is more than 11 km
(6.8 miles) above the surrounding plain. The summit is more than 23 km (14
miles) higher in elevation than the place where Mars Pathfinder landed in
July 1997.

Description of MOC Image:
This picture shows an area that is about 20 km (12 miles) higher in
elevation than the Mars Pathfinder landing site. The picture shows three
main features: (1) a crater at the center-right, (2) a sinuous,
discontinuous channel across the upper half, and (3) a rough and pitted,
elevated surface across the lower half of the image.

(1) Crater at center right. Distinguishing meteor craters from volcanic
craters can sometimes be a challenge on Mars. This particular crater was
most likely formed by meteor impact because it has a raised rim and a faint
radial ejecta pattern around the outside of it. This crater is 600 m (2000
feet) across, about 3/4 the size of the famous "Meteor Crater" near Winslow,
Arizona.

(2) Sinuous channel. The type of discontinuous channel running across the
upper half of the image is sometimes refered to as a "sinuous rille". These
are common on the volcanic plains of the Moon and among volcanoes and
volcanic plains on Earth. Such a channel was once a lava tube. It is running
down the middle of an old lava flow. The "tube" looks like a "channel"
because its roof has collapsed. The discontinuous nature of this channel is
the result of the collapse, or "cave-in" of what was once the roof of the
lava tube. It is common for certain types of relatively fluid lavas to form
lava tubes. As it is being emplaced, the outer margins of the lava flow cool
and harden, but the interior remains hot and and continues to flow down-
hill. Eventually, the eruption stops and the lava inside the tube cools,
contracts, and hardens, leaving behind a tube (basically, a long narrow
cave).

(3) Rough elevated surface. The rough, pitted, and elevated surface across
the bottom half of the image is a lava flow. The margins of this feature are
somewhat lobate in form, and the entire feature is elevated above its
surroundings, indicating that it was the last lava flow to pour through this
region.

Putting it All Together: Aa and Pahoehoe Lava Flows:
Shield volcanoes such as Ascraeus Mons form from relatively fluid lavas.
Shield volcanoes on Earth include the well-known Islands of Hawai'i. The
kind of lava that is fluid enough to make shield volcanoes is called basalt.
This is an iron- and magnesium- rich silicate lava that, when cooled, is
usually black or very dark brown.

Basalt lava flows come in two main varieties: Aa and Pahoehoe. These are
Hawai'ian names. "Aa" (pronounced "ah-ah") lava flows have very rough,
jumbly surfaces, and they usually lack lava tubes. "Aa" lava flow surfaces
are very rough to walk on-- thus the term "aa" probably refers to the sound
a person might make when walking on a cooled/solidified aa flow in bare
feet!

"Pahoehoe" (pronounced "pa-hoy-hoy") is a term that means "ropey". The
surfaces of pahoehoe lava flows are generally very smooth and billowy.
Sometimes they have a ropy texture like melted taffy or caramel. Pahoehoe
flows very commonly contain lava tubes.

The rough-surfaced flow across the lower half of the MOC image is
interpreted to be an "aa" lava flow, and the smoother surface with a sinuous
channel running down its center is interpreted to be a "pahoehoe" lava flow.
Both would indicate that the lavas on Ascraeus Mons, at least at this
location, are probably composed of basalt.

More Picture Information:
The MOC picture (B) is a subframe of image #26705, centered approximately at
11.5°N latitude, 103.5°W longitude. It was taken on April 28, 1998, at 4:23
AM Universal Time, on Mars Global Surveyor's 267th orbit around Mars. Orbit
267 was the second-to-last orbit on which observations were obtained before
Mars and the spacecraft passed behind the Sun for several weeks known as
"Solar Conjunction".

The context frame (A) is from a U.S. Geological Survey photomosaic of Viking
Orbiter images obtained in the late 1970s. The bright, wispy feature on the
west (left) side of the volcano is a plume of dust being lofted by wind.
This plume casts a shadow that can be seen below it.

----------------------------------------------------------------------------
----------------------------------------------------------------------------

Malin Space Science Systems and the California Institute of Technology built
the MOC using spare hardware from the Mars Observer mission. MSSS operates
the camera from its facilities in San Diego, CA. The Jet Propulsion
Laboratory's Mars Surveyor Operations Project operates the Mars Global
Surveyor spacecraft with its industrial partner, Lockheed Martin
Astronautics, from facilities in Pasadena, CA and Denver, CO.
Follow-Ups: