* Astronomy

Lying east of the enormous volcanic region of Tharsis, Kasei Valles forms perhaps the largest outflow channel on Mars. Like other such channels, it carved by liquid water - and in Kasei's case, probably during gigantic floods that originated in volcanic and tectonic activity in Tharsis.

Kasei runs for 2,400 kilometres across the lava plains of Lunae Planum. It begins in Echus Chasma, near Valles Marineris, and empties into Chryse Planitia, not far from where the robotic probe Viking 1 landed in 1976. Studying Kasei and its surroundings, scientists see evidence for several episodes of flooding and possibly also glacial activity.
The Thermal Emission Imaging System (THEMIS), an instrument aboard the Mars Odyssey orbiter, took this false-colour photo. It shows a portion of Kasei lying north of a large tableland, Sacra Mensa, which divides Kasei into a northern and southern channel. In places, Kasei Valles spreads as much as 300 kilometres across, although this stretch of the northern channel averages only 11 kilometres wide.
The THEMIS image combines a visible-light daytime view with an infrared night time one. The result shows landscape details as small as 18 metres in size, while the false colours provide clues to the nature of the ground surface.
Looking down from orbit late at night, THEMIS senses residual warmth left from the previous day's solar heating. Coarse-grained material - ranging from gravel, cobbles, and boulders, to rock outcrops - holds heat better than fine-grain particles such as dust and sand. The false colours map the temperatures detected by THEMIS, with warmer surfaces showing in reds and yellows, while cooler and dustier areas appear green and blue-green.


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The greenish colour of the mesa top indicates that it is covered with fine material that cools quickly after dark. But large grooves, as well as a sprinkling of tiny craters - the smallest visible being 100 metres wide - indicate that dust does not blanket the mesa top too deeply.
The grooves mark places where the rock that caps the mesa has cracked and eroded, probably by the escape of groundwater. Elsewhere on Sacra Mensa, joints and fissures opened as groundwater burst out, pressurised by stresses from Tharsis as it grew, roughly 3 billion years ago.
The ungrooved 'shelf' lies about 900 metres lower than the mesa surface. It may have collapsed and become covered by debris hides its original surface. Its outer edge shows a characteristic spur-and-gully erosion pattern.

According to scientists, Kasei Valles has seen floods of different sizes. Some may have been deep enough to cover even the mesa top, while lesser flows remained in the channels, deepening them. The water flowed from left to right through the image.
The channel floor lies 2,500 metres below the south mesa top, and 1,600 metres below the shelf. In contrast, the northern rim of the channel rises 800 metres above the valley floor.
Unlike much of Kasei, the floor in this part of the channel is flat and almost featureless. Nonetheless, a close look reveals that where the floor lies near the southern wall, it is both smoother and has fewer exposed rocks (as its colour indicates) than the other side.
It's no surprise the channel floor is rockier on the northern side, given its curving shape. Water flows faster on the outside of river bends and erodes more deeply there. Conversely, a slower flow on the inside of a bend allows particles to drop out of suspension and accumulate on the bottom.
Another kind of flow, however, created the tiny white flecks along the foot of the northern shore. Driven by the wind, these are small drifts of sand dunes, which stand out by their cooler temperatures compared to the rockier banks.



This unnamed crater spans a diameter of 11 kilometres, and has a floor that descends 250 metres below the channel. Its channel-side rim, however, rises only about 60 metres above the valley floor. While the crater formed after the main channel was carved, later flows through the valley appear to have removed or buried the apron of debris from the crater's impact.
Many portions of the south-facing walls of both the channel and the impact crater show as redder, hence warmer, than other parts of the scene. Two effects are probably combining to produce this result.
The first is solar heating. This part of Kasei Valles lies in mid-northern latitudes. Compared to the south side of the channel, the slopes on the northern side face the Sun more directly and become warmer each day.
The second effect is that the south-facing walls appear rockier as well. Many areas show a layer of red along the cliffs, and at the foot of the crater's channel-side rim lies a thin red line marking rocky debris. Away the edge of the channel lie numerous slopes that also face generally south, but they are not as warm, showing yellows and greens. Subjected to the same solar heating, these are probably blanketed more thickly with finer-grain materials.
Finally, if any water or ice still lingers in the ground throughout the region, the extra warmth found on the slopes may have driven it away, leaving dry, bare rock exposed.

These images, taken by the High Resolution Stereo Camera (HRSC) on board ESA's Mars Express, show Pavonis Mons, the central volcano of the three 'shield' volcanoes that comprise Tharsis Montes.


Credit: ESA

ESA's Mars Express spacecraft obtained these images using the HRSC during orbit 902 with a ground resolution of approximately 14.3 metres per pixel. The images were acquired in the region of Pavonis Mons, at approximately 0.6° South and 246.4° East.
Pavonis Mons, rising roughly 12 km above the surrounding plains, is the central volcano of the three 'shield' volcanoes that comprise Tharsis Montes. Gently sloping shield volcanoes are shaped like a flattened dome and are built almost exclusively of lava flows.
The dramatic features visible in the colour image are located on the south-west flank of Pavonis Mons. Researchers believe these are lava tubes, channels originally formed by hot, flowing lava that forms a crust as the surface cools. Lava continues to flow beneath this hardened surface, but when the lava production ends and the tunnels empty, the surface collapses, forming elongated depressions. Similar tubes are well known on Earth and the Moon.

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Scientists have discovered additional evidence that Mars once underwent plate tectonics, slow movement of the planet’s crust, like the present-day Earth.
A new map of Mars’ magnetic field made by the Mars Global Surveyor spacecraft reveals a world whose history was shaped by great crustal plates being pulled apart or smashed together.
Scientists first found evidence of plate tectonics on Mars in 1999. Those initial observations, also done with the Mars Global Surveyor’s magnetometer, covered only one region in the Southern Hemisphere. The data was taken while the spacecraft performed an aerobraking manoeuvre, and so came from differing heights above the crust.



This is a map of the magnetic field of Mars observed by the Mars Global Surveyor satellite at a nominal 400 km altitude. Red and blue stripes represent magnetic fields with opposite directions. Darker hues represent more intense magnetic fields. To show the location of the magnetic stripes on Mars, the map is superimposed on a topography relief map from the Mars Observer Laser Altimeter instrument.
Credit: NASA

Scientists see similar stripes in the crustal magnetic field on Earth. Stripes form whenever two plates are being pushed apart by molten rock coming up from the mantle, such as along the Mid-Atlantic Ridge. As the plate spreads and cools, it becomes magnetized in the direction of the Earth’s strong global field. Since Earth’s global field changes direction a few times every million years, on average, a flow that cools in one period will be magnetized in a different direction than a later flow. As the new crust is pushed out and away from the ridge, stripes of alternating magnetic fields aligned with the ridge axis develop.
Transform faults, identified by “shifts” in the magnetic pattern, occur only in association with spreading centres.

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Fields of volcanic cones discovered at the North Pole of Mars suggest the Red Planet could still be geologically active.
The cones, seen in images from Europe's Mars Express probe, have no blemishes from impact craters.
This suggests the volcanoes erupted very recently and that the site could have ongoing volcanism.
Mars Express scientist Gerhard Neukum presented the results at a conference in Cambridge.

"Mars is a planet that was very recently active - maybe one, or two, or three million years ago. And in some areas, I have the impression it is really ongoing" - Dr Gerhard Neukum, the Free University in Berlin, Germany.

But what cannot be determined is when, if at all, some of these volcanoes might erupt again: "It could be a million years from now, it could be tomorrow".

Dr Neukum acts as the principal investigator for the High Resolution Stereo Camera (HRSC) on Mars Express, which took the images in which the cones were discovered.
There may be 50-100 of the volcanic cones covering a flank of the North Pole about one million square kilometres in area. They are between 300m and 600m tall.
In addition to the North Pole, other regions with recent - and possibly ongoing - activity on Mars include parts of Tharsis - home to the volcano Olympus Mons - parts of Elysium and the so-called highland-lowland boundary.

By counting the number of craters on the surfaces of Solar System objects, scientists can estimate the age of those surfaces.
If they are heavily cratered, they are deemed older, while smoother surfaces are considered younger. This assumes a constant cratering rate since the heavy bombardment that terrestrial planets underwent about four billion years ago.

"You can see baby cones. I think they're still growing" Professor Gerhard Neukum.

The cones appear to be fresh with no discernible evidence of cratering. Dr Neukum admitted it was possible the cones could be ancient features that have been eroded by wind, but added that this was unlikely.

"I don't see any wind-related features in the region. We should see it and we should see the remains of craters somewhere. But we don't" - Gerhard Neukum.
Volcanic activity appears to have peaked on Mars at around 1.5 billion years ago.

"Mars is still active within certain limits, but it's still not dead."

Dr Neukum thinks that volcanic activity strongly influences glacial activity on Mars. This is because on the Red Planet, eruptions also mobilise water.
In some cases, this water freezes and forms glaciers. But other scientists believe glacial activity on the planet is more strongly influenced by the inclination of Mars in its orbit around the Sun.

The results were presented at the American Astronomical Society Division of Planetary Sciences meeting in Cambridge, UK.

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