He opened with a slide stating a provocative claim: "Mars is Mars-like, NOT Earth-like." (The statement was made by Nick Hoffman, he of "White Mars" fame.) Jeff commented that he could not but agree with the first half of the comment, but thought the second half worth exploring. Mars is, indeed, very different to Earth in a number of important ways:
Its gravity is 40% of Earth's.
Its geothermal flux [heat output] is about 40% of Earth's.
Its atmosphere is 1% as dense as Earth's, dominated by carbon dioxide [not nitrogen or oxygen], 80 Kelvin colder than Earth's, has very low relative humidity, etc....
It has no plate tectonics, no oceans, little or no granite [and therefore limited reprocessing of its crust, unlike what has happened on Earth], maybe no life [whereas everything we can see on, in, and above Earth has been affected by life], and has a unique geological history.
Mars has a unique volatile assemblage and petrology [meaning that it started out with a different initial elemental budget, which is reflected in the present in different rocks and gases].
With all these differences, you can't expect comparable processes and landforms on both planets. Furthermore, even when the same materials are present on both planets, they behave differently under different conditions. If Mars is so un-Earth-like, how Earth-like could Titan or any other icy satellite possibly be? Is there any hope at all for comparative planetology?
Panorama of the channels at the Huygens landing site
Jeff says there is hope for comparative planetology, and here's his reasoning.
Solid planetary bodies are made of many substances.
The substances exhibit a wide range of physical properties.