It doesn’t get much better than this. A robotic orbiter snaps a photo hundreds of miles above the surface of an alien planet, capturing a geological event as it happens. Yes, we’ve seen Io’s immense volcanoes erupt, and we’ve seen huge storms rage on Jupiter, but often these large-scale planetary events are too massive for us to put into context and so we file them under “astronomy”. But, when we see an event like an avalanche on Mars, we can relate it with events on Earth, we have a “feel” for what this means. Suddenly an avalanche on Mars holds a special meaning to us; we instantly have a connection with other planets in our Solar System.
And now for the question… what caused the four near-simultaneous avalanches recently observed by the Mars Reconnaissance Orbiter?
NASA’s Mars Reconnaissance Orbiter has been taking pictures of the Red Planets surface since March 10th 2006. The High Resolution Imaging Science Experiment (HiRISE) is currently the most advanced imaging system in orbit and continues to return the most detailed view of Mars. Most recently the HiRISE instrument has been used by University of Arizona researchers to understand the mechanisms behind gully features observed on crater edges. The data from HiRISE supports simulations of liquid water and dry debris flowing down a slope. Thought to be created by rapid flows of liquid water, it now seems that these gullies are in fact generated by dry debris, or landslides.
Now HiRISE has another addition to its record breaking collection of images… it has spotted four avalanches occurring near-simultaneously, along an escarpment region in the North Pole. What makes this special is that the avalanches are occurring at the moment the picture was taken. The dusty plumes were noticed after the event when HiRISE scientists analysed the campaign images. The avalanches are labelled in the images below:
So we know they happened, but what caused this series of avalanches (and yes, they are avalanches and not landslides, as the debris is expected to contain large quantities of ice, mixed with rock and dust)?
According to HiRISE scientists, the avalanches may have been caused by one or more of the following events:
- Disappearance of carbon dioxide frost, dislodging rocks.
- Thermal expansion and contraction of water ice due to temperature.
- Small Mars-quakes.
- A nearby meteorite impact.
- Vibrations from other avalanches causing other avalanches along the scarp.
The first two explanations are possibly the most likely, and may feed into reason #5. Currently, the northern hemisphere of Mars is warming up as it comes out of Mars spring and moves into Mars summer (Martian summer solstice occurs on June 25th 2008). Although the surface temperatures or pressure will never allow water ice to melt into liquid water, significant thermal contraction may occur, opening cracks in the cliff sides of the escarpment, weakening the rock. A terrestrial example of water ice eroding rocks is known as frost action where repeated freezing and thawing erodes cliffs and valley sides.
Apart from water ice, there may also be significant quantities of carbon dioxide stored in pockets between rock layers and deposits. Should a layer of carbon dioxide frost or pockets of liquid be exposed to in increase in temperature, it will very quickly boil off, sometimes explosively (if the conditions are right) . The property of thermal expansion in water ice and fast vaporization of carbon dioxide may both have had a part to play in the initiation of these Mars avalanches.
In the case of reason #3, this poses an interesting question. Whilst water and carbon dioxide erosion on the rock could be possible, the near-simultaneous nature of the four avalanches, along the length of the escarpment (some 12 km of it, pictured below), suggests they are connected in some way.
Seismic activity – or “Mars-quakes” – may be a possible explanation. Seismic waves travel very fast through the crust of the Earth (a few thousand km/hr), and the same can be expected on Mars. Could a Mars-quake have shaken the cliffs of the escarpment?
The biggest problem with this theory is that Mars is largely inactive, it has very low tectonic activity as the planets core is possibly solid – the reactions required to keep the core in a hot, liquid state have ceased. This is also why Mars does not have a planetary magnetic field. Unless there is some other driver, it seems unlikely a Mars-quake could have caused the series of avalanches.
An external driver, such as a chance meteorite strike, may have generated seismic waves, triggering the avalanches. Although entirely possible, NASA reported no significant meteorite impact in the area, and the likelihood of an impact occurring at the time of observation seems highly unlikely. Unless there is evidence to the contrary, explanation #4 can be discounted.
So we are left with the possibility of one avalanche causing a chain reaction, triggering other avalanches along the escarpment. If a large enough mass of ice and rock fell down the cliff face (falling 700 meters to the gentle slope at the bottom), it may be possible that vibrations from the collapse may be felt a significant distance through the rock, perhaps triggering other collapses along weaknesses in the cliff. Therefore, possibility #5 probably has a part to play, but it does not explain how the avalanches occurred in the first place.
It seems most likely that the avalanches observed by HiRISE can be attributed to seasonal changes, changing the properties of water ice and possibly carbon dioxide stored in pockets of rock layers. A Martian form of “frost action” most likely triggered one or more of the avalanches, due to seasonal temperature change, and there may be additional instability provided by vibrations passing through the crust, possibly causing further avalanches.
It will be exciting to find out what comes of the analyses of these images in the hope of understanding the triggering mechanism. It is interesting, however, to know that Mars is far from being a geologically “dead” planet and still shows us some dynamic events as we watch over the planet…