click to enlarge A wider contextual image showing the region around Nili Fossae. Nili Fossae is a system of deep fractures around the giant Isidis impact basin seen at lower right. It is found at 22°N / 77°E, northeast of the Syrtis Major volcanic province. Credits: NASA MGS MOLA Science Team

Newly
released images from ESA’s Mars Express show Nili Fossae, a system of
deep fractures around the giant Isidis impact basin. Some of these
incisions into the martian crust are up to 500 m deep and probably
formed at the same time as the basin.

Nili
Fossae is a ‘graben’ system on Mars, northeast of the Syrtis Major
volcanic province, on the northwestern edge of the giant Isidis impact
basin. Graben refers to the lowered terrain between two parallel faults
or fractures in the rocks that collapses when tectonic forces pull the
area apart. The Nili Fossae system contains numerous graben
concentrically oriented around the edges of the basin.

It
is thought that flooding of the basin with basaltic lava after the
impact that created it resulted in subsidence of the basin floor, adding
stress to the planet’s crust, which was released by the formation of
the fractures.

click to enlarge Nili Fossae is a graben system on Mars, northeast of the Syrtis Major volcanic province, on the northwestern edge of the giant Isidis impact basin. Some of these incisions into the martian crust are up to 500 m deep and probably formed at the same time as the basin. The image was created using a Digital Terrain Model (DTM) obtained from the High-Resolution Stereo Camera on ESA’s Mars Express spacecraft. Elevation data from the DTM is colour coded: purple indicates the lowest lying regions and grey the highest. The scale is in metres. Credits: ESA/DLR/FU Berlin (G. Neukum)

A
strongly eroded impact crater is visible to the bottom right of the
image. It measures about 12 km across and exhibits an ejecta blanket,
usually formed by material thrown out during the impact. Two landslides
have taken place to the west of the crater. Whether they were a direct
result of the impact or occurred later is unknown.

A
smaller crater, measuring only 3.5 km across, can be seen to the left
of centre in the image and this one does not exhibit any ejecta blanket
material. It has either been eroded or may have been buried.

The
surface material to the top left of the image is much darker than the
rest of the area. It is most likely formed of basaltic rock or volcanic
ash originating from the Syrtis Major region. Such lava blankets form
when large amounts of low-viscosity basaltic magma flow across long
distances before cooling and solidifying. On Earth, the same phenomenon
can be seen in the Deccan Traps in India.

Nili
Fossae interests planetary scientists because observations taken with
telescopes on the Earth and published in 2009 have shown that there is a
significant enhancement in Mars’ atmospheric methane over this area,
suggesting that methane may be being produced there. Its origin remains
mysterious, however, and could be geological or perhaps even biological.

click to enlarge The High-Resolution Stereo Camera on ESA’s Mars Express took this image. These perspective views have been calculated from the Digital Terrain Model derived from the stereo channels. Credits: ESA/DLR/FU Berlin (G. Neukum)

As
a result, understanding the origin of methane on Mars is high on the
priority list and in 2016, ESA and NASA plan to launch the ExoMars Trace
Gas Orbiter to investigate further. Nili Fossae will be observed with
great interest.

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