Unlike the moon, Mars has winds. Mars has exposed ice seen glinting with sunlight. Even when viewed in a 1950's telescope, the frozen polar ice caps could distinguish Mars from the moon. These icy poles sometimes contain frozen water but have a core of frozen carbon dioxide, or dry ice. The matian surface generally is rust-colored though, because it is literrallly covered in rust. Iron oxide is a prevalent soil component.

When first observed during the 1960's Mariner orbital missions to Mars, the red planet has craters and fault rifts like the moon but unlike our lunar neighbor, shows signs of ancient water flowing across its surface.

"River valleys with extensive tributaries," said Chaikin, show that Mars was "once a place where water flowed in vast quantities." Even in more recent geological times, "tiny channels on the walls of craters have been observed from the orbiting Mars Global Surveyor, and this water may trickle down embankments below a frost layer."

Attempts to resolve this watery past with Mars' apparent dryness today rely on understanding the planet's climate history, said Chaikin. "The atmosphere is seven parts in a thousand compared to Earth's pressure at sea level, which is too thin for liquid water" to be exposed. The current round of rover missions have taken up the mantle of tracing this watery past, and uncovered the ground-truth to compare to what previously could only be speculated upon.

On the question of life on Mars or one of Jupiter's moons like Europa, Chaikin noted that "life is found in hostile environments ranging from mid-ocean ridges to dry and salty places. Even inside rocks. Life is proving more clever and resilient than previously imagined."

Comparing the Moon and Mars visually on the surface is difficult, said Chaikin, because almost all the Apollo astronauts have noted that images of the lunar surface cannot do justice to seeing the brilliant Sun in complete blackness for yourself.

Referring to the computer renderings of the "brilliant Dan Maas, who created wonderful animations before the Mars Exploration Rovers landed", Chaikin thought the human eye looking on the martian surface would be struck foremost by the red, rusty surface compared to the grey lunar landscape.

But both are "desolate places. On the Moon, the brilliant sunlight and black sky make a striking scene. On Mars, the sky is peach-colored, while the light is diffuse with dust, kind of like Los Angeles. The dust floating in the thin martian air scatters enough light to make the sky blue at sunset. The martian sunset is the reverse of what we have here." While the daylight sky on Earth is blue and its sunset pink, the martian daylight is pink and the sunsets are blue.

Catching up Chaikin's unique historical perspective on exploration to the present rover missions, he agreed with the incredible luck that JPL scientists and engineers felt when the first images from the Opportunity rover were viewed.

"The rovers bounce haphazardly, the lander unfolds, and its camera takes a look around. What are the chances of bouncing into a crater, one with an outcrop thought to be martian bedrock? This outcrop on the crater rim includes rocks in the same place they were formed, not blasted from some other impact. And like sedimentary rocks, these are layered with their geological history."

The luck of landing in a crater exposing bedrock however was dwarfed by finding water evidence only ten meters from the lander's base station. "The spherical grains, called concretions, are the geological equivalent of hail stones. They speak of being formed in water. Meridiani Planum was once drenched with water, maybe billions of years ago. But we don't have to speculate anymore, Mars has given us ground-truth as evidence of water"--the first such finding on the surface of another world.

While Chaikin marvelled at the luck and resourceful of the robotic rovers, he shared an Apollo anecdote to illustrate what a human explorer might bring to our understanding of Mars. "Nothing can replace the power of the human mind and hand in exploration. These robots are incredible and currently the only way to do exploration. But even the builders of the robots agree that eventually we will have to have humans for the next level of discovery."

To illustrate historically, Chaikin selected the mission of the only trained geologist to the moon. During his packed surface itinerary, Apollo 17's Harrison Schmidt made one happenstance discovery on the moon.

The lunar soil is almost exclusively grey and tan, but as Schmidt walked across the dusty surface, he saw that his boot was kicking up alot of the grey soil. Underneath his own bootprint, he saw a layer that was not muted in hues of grey, but instead struck his geologist's eye as unlike anything thought to exist on the moon. What Schmidt's boot revealed was the first and only lunar region that differed from its surrounding strata.

Chaikin described Schmidt's find as not unlike what a human might find on Mars--"the moon revealed a layer of brilliantly orange soil. Where Schmidt's boot had kicked up grey to reveal orange were these microscopic glassy beads, not entirely unlike the spherules or concretions found by the Opportunity rover on Mars."

If beads on Mars meant water, beads on the moon meant active volcanoes. "On the moon, the orange soil meant that lunar geology had a time when lava or a fiery fountain sprayed molten material high into the air and formed these orange, glassy beads. Those beads then had been exhumed by an impact or crater."

Could a robot transmit enough high-resolution imagery, kick over enough rocks, or turn over mounds of lunar soil to have mimicked Schmidt and found the first orange soil on the moon? Was a geologist's trained eye a better means of discovery than a roomful of the best terrestrial experts pouring over mission photos?

Chaikin did not complete any judgements on whether a robot or human could best map Mars. In either case, a good camera lens is almost always going to filter the actual objects observed. A glass layer will inevitably come between a raw planetary landscape and our best trained eyes. But Chaikin did offer a philosophical perspective: "There will be no substitute for the sound of a human voice from another planet, one to which we have never been to before."

The next installment of this series of interviews by Astrobiology Magazine with Dr. Andrew Chaikin addresses the question of exploration with humans or machines.

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