The Mystery of Hebes Chasma
On the western end of Valles Marineris, and just a little bit to the north, lies a canyon system, a chasma, that is unique to the entire Red Planet.
And one of the most mysterious ...
Hebes Chasma is a 320 kilometer long (from east to west), 130 kilometer wide (from north to south) chasma with massive cliff walls that rise 5 to 6 kilometers high.
And in the middle of the canyon is the mystery - a 5 kilometer high mesa that rises nearly to the top of the walls to either side called Hebes Mensa.
Now a central mesa by itself is not that surprising, but look closer at the horseshoe shaped section in the middle that appears to have collapsed and you'll see the problem.
The collapsed material is nearly all missing!
Hebes Chasma is a completely isolated and closed off depression surrounded by other nearby canyon systems like Echus Chasma to the west and Valles Marineris to the south.
But there are no outflow channels to Echus Chasma or Valles Marineris, nor even to Perrotin Crater to the southwest. There is nowhere for anything to go.
So where did it all go?
The image below, taken by ESA's Mars Express orbiter, shows incredible detail of the canyon. Some material appears to have flowed onto the valley floor, while a darker layer looks like it was halted on a downslope landing where it pooled like a dark splotch of ink.
But that's it. Where is the rest of it? Where are the rocks and boulders you would expect at the bottom of a collapsed mesa wall? The entire area looks smooth and unbroken.
One theory is that some of the lower layers of the canyon walls are made of salty rock that dissolved when melted ice flows ran down the slope. The water then drained into an underground aquifer, leaving behind the ink-splotch and smooth floors of this unique canyon.
But that's just a theory. We won't actually know until we go and take a look.
Which makes this just another great martian mystery -- and one more reason why Mars is so wickedly cool!