“You’ve not seen half of it, Mr Feder.  Mars has the Solar System’s tallest volcano, most massive volcano, biggest planetary meteor strike, deepest and longest  canyon…”

“Wait, kid, I’ve been to the Grand Canyon.  Thing is … BIG!  What’d they say?  A mile deep, 18 miles wide, 250 miles long.  No way Mars can beat that.”

“Valles Marineris is 4½ miles deep, 120 miles wide and 2500 miles long.  The Grand Canyon meanders, packing its length into only 150 miles of bee-line distance.  Marineris stretches straight as a string.  No river carved that formation, but the planetologists can’t agree on what did.”

“They got evidence, don’t they?”

“Not enough.  Different facts point in different directions and no overall theory has won yet.  Most of it has to do with the landforms.  Start with the Tharsis Bulge, big as a continent and rising kilometers above Mars’ average altitude.  Near the Bulge’s highest point, except for the volcanoes, is a fractured-looking region called Noctis Labyrinthus.  Starting just west of  the Labyrinth a whole range of wrinkly highlands and mountains arcs around south and then east to point towards the eastern end of Marineris.  Marineris completes the arc by meeting the Labyrinth to its west.  Everything inside that arc is higher than everything else around it.  Except for the volcanoes, of course.”

“Looks like something came up from underneath to push all that stuff up.”

“Mm-hm, but we don’t know what, or what drove it, or even how fast everything happened.  There are theories all over the place”

“Like what?”

“Well, maybe it’s upwellng from a magma hotspot, like the one under the Pacific that’s been creating Hawaiian Islands one at a time for the past 80 million years.  Some people think the upwelling mostly lifted the existing crust like expanding gas bubbles push up the crust of baking bread.  Other people think that the upwelling’s magma broke through the crust to form enormous lava flows that covered up whatever had been there before.”

“You said ‘maybe.'”

“Yeah.  Another group of theories sees a connection between Tharsis and Hellas Basin, which is almost exactly on the other side of the planet.  Hellas is the rock-record of a mega-sized meteorite strike, the third largest confirmed one in the Solar System.  Before you ask, the other two are on the Moon.  Like I said, it’s a group of theories.  The gentlest one, if you can call it that, is that energy from the impact rippled all around the planet to focus on the point opposite the impact.  That would have disrupted the local equilibrium between crustal weight and magma’s upward pressure.  An imbalance like that would encourage uplift, crustal cracking and, ultimately, Valles Marineris.”

“Doesn’t sound very gentle.”

“It wouldn’t have been but it might even have been nastier.  Another possibility is that the meteorite may not have stopped at the crust.  It could have hit hard enough, and maybe with enough spin, to drill who knows how far through the fluid-ish body of the planet, raising the Bulge just by momentum and internal slosh.  Worst case, some of Tharsis’ rock might even have come from the intruder.”

“Wow, that would have been a sight to see!”

“Yeah, from a distance.  Any spacecraft flying a Mars orbit would be in jeopardy from rock splatter.  We’ve found meteors on Earth that we know originated on Mars because they have bubbles holding trapped gas that matches the isotope signature of Martian atmosphere.  A collision as violent as the one I just described could certainly have driven rocky material past escape velocity and on its way to us.  Oh, by the way and speaking of sights — you’d be disappointed if you actually visited Valles Marineris.”

“How could anything that ginormous be a disappointment?”

“You could look down into it but you probably couldn’t see the far side.  Mars is smaller than Earth and its surface curves downward more rapidly.  Suppose you stood on one side of the valley’s floor where it’s 4 miles deep.  The opposite wall, maybe 100 miles away, would be beyond your 92-mile horizon limit for an object that tall.”

“Aw, phooey!”

~~ Rich Olcott