[meteorite-list] Mars Express Images - March 29, 2005

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Mar 31 12:01:09 2005
Message-ID: <200503311700.j2VH0iL05323_at_zagami.jpl.nasa.gov>

http://www.esa.int/export/SPECIALS/Mars_Express/SEMSSZRMD6E_0.html

The Medusa Fossae formation on Mars
European Space Agency
Mars Express
29 March 2005

These images, taken by the High Resolution Stereo Camera (HRSC) on board
ESA's Mars Express spacecraft, show part of the Medusa Fossae formation
and adjacent areas at the highland-lowland boundary on Mars.
 
The HRSC obtained these images during orbit 917 with a resolution of
approximately 13 metres per pixel. The scenes show an area located at
about 5? South and 213? East.
 

Medusa Fossae in black & white

The Medusa Fossae formation is an extensive unit of enigmatic origin
found near the Martian "highland-lowland dichotomy boundary" between the
Tharsis and Elysium centres of volcanic activity. This dichotomy
boundary is a narrow region separating the cratered highlands, located
mostly in the southern hemisphere of Mars, from the northern
hemisphere's lowland plains.
 
The cratered highlands stand two to five kilometres higher than the
lowland plains, so the boundary is a relatively steep slope. The
processes that created and modified the dichotomy boundary remain among
the major unanswered issues in Mars science.
 
 

Medusa Fossae - perspective view looking southwest
 
The boundary between the old volcanic plateau region and part of the
widespread deposits of the Medusa Fossae formation, called Amazonis
Sulci, is shown in this image. In general, the formation appears as a
smooth and gently undulating surface, but is partially wind-sculpted
into ridges and grooves, as shown in the mosaic of nadir images.
 
 

Medusa Fossae in colour

It is commonly agreed that the materials forming Medusa Fossae were
deposited by pyroclastic flows or similar volcanic ash falls. The
plateau walls of the volcanic massif are partly covered by lava flows
and crossed in places by valleys which were most likely carved by
fluvial activity.

The remains of water-bearing inner channels are visible in the centre of
the valleys and at the bottom of the massif. Superposition of the
lobe-fronted pyroclastic flows indicates that the water erosion ended
before their deposition. Later, a "bolide" impacted near the massif and
the ejecta blanket was spread as a flow over parts of the plateau,
implying water or ice was present in the subsurface at the time of impact.

A bolide is any extraterrestrial body in the 1-10 kilometre size range,
which impacts on a planetary surface, explodes on impact and creates a
large crater. This is a generic term, used when we do not know the
precise nature of the impacting body, whether it is a rocky or metallic
asteroid, or an icy comet, for example.

 
 

Anaglyph (3D) image of Medusa Fossae

The colour images have been derived from the three HRSC colour channels
and nadir channel. The perspective views have been calculated from the
digital terrain model derived from the stereo channels. The anaglyph
image was calculated from the nadir and one stereo channel. Image
resolution has been decreased for use on the internet.
 
 

Detail 1 - Senus Vallis
 
The highland-lowland boundary. Here the volcanic plateau fed by the
southernmost Tharsis Montes volcano Arsia Mons, is dissected by several
valleys which were most likely carved by running water. Here the Senus
Vallis is shown in detail as an example where the latest-stage inner
channel is still visible.
 
 

Detail 2 - mouth of Abus Vallis
 
This is the mouth of Abus Vallis, a channel of the same type as in
Detail 1. Here the walls are too steep or the valley too narrow to look
down to the bottom of the channel due to insolation and shading of the
image, but the remains of the last stage of water activity can be traced
as a small channel at the floor of the lowland plain. Additionally the
emplacement of pyroclastic flows is visible in this detail. It shows
that the action of water erosion ended before the emplacement of the
pyroclastic flow.
 
 

Detail 3 - an impact crater
 
An impact crater is the youngest feature of the "stratigraphic" sequence
(layers of rocks produced over time) which can be observed in this
image. This crater has a well preserved ejecta blanket with a "lobate"
(lobe-like) appearance, which is believed to indicate the presence of
water or water ice in the impacted target. As a crater forms on a flat
surface, it expands in a circular fashion. Due to the topography of the
impact site the shape of the crater during expansion was disturbed by
the walls of the plateau and resulted in an asymmetrical shape. The
distribution of ejecta, resembling the wings of a butterfly, is due to a
non-vertical impact (less than 45 degrees).
 
 

Detail 4 - Amazonis Sulci
 
The Amazonis Sulci, with its ridges and grooves, appears to be
wind-sculpted. The lack of craters superposing this surface indicates
that wind erosion has been the latest stage of eroding processes acting
here.
 
Received on Thu 31 Mar 2005 12:00:43 PM PST