This is a wonderful book.  Some of it is challenging to read, like this:  "I have reported that the spiral-structured troughs [in Mars' poles] have topographic forms and associated fine-scale tectonic features consistent with plate flexure similar to that seen in Earth's oceanic lithosphere around major loads, such as the Hawaiian Islands and in the vicinity of oceanic trenches".  However, he doesn't assume the reader knows geology; he teaches it while telling you about Mars.
The book is mostly about the role of water on Mars.  Water has shaped the surface a lot, may have allowed life to evolve there, and will help humans to colonize Mars, if we do.
Mars likely had oceans, a long time ago.  But they weren't like the oceans of your experience.  Maybe a mud ocean.  Very, very salty, so it had a lower freezing point than water.  Very very cold and a lot of sulfuric acid.  He has a vivid description of what it would be like to take a dip.
Mars' atmosphere was much more dense then, with a lot of warming from carbon dioxide and other greenhouse gases.
There's no permanent water ice on the surface of Mars, only a temporary light frosting.  But there are huge reservoirs of water ice under the ground or covered with dry ice.  Mars has glaciers!  The polar ice caps flow, so they're glaciers.  And at lower latitudes, there are probably rock glaciers, meaning that ice underground lubricates the rocks so they can flow.  Mars glaciers move much MUCH more slowly than Earth glaciers, like one ten thousandth as fast, because ice is very stiff at very cold Martian temperatures.
There have been gigantic floods of *several cubic kilometers* per second, which flooded large parts of the surface.
There may never have been rain on Mars, though.
There's still a lot of water on Mars, but it's frozen underground.  There are a lot of permafrost type terrains, such as polygonal cracking.
Unlike on Earth, solid carbon dioxide is active in Mars geology.  For example, layers of  dry ice and CO2 clathrate  play a part in how the polar ice caps flow.
Mars' spin axis varies chaotically over hundreds of thousands of years.  It changes much more than Earth's spin axis, which is stabilized by Earth's large moon.  The tilt of Mars' spin axis is changed by torque applied by the Sun.  So Mars' climate was quite different, very recently.  The spin axis has been extremely tilted, like more than 80 degrees, almost in the plane of Mars' orbit around the Sun.  When it gets that tilted, some areas get a lot of sunlight in the summer and can get a lot warmer and wetter.
He thinks it's not far-fetched that microbial life would have evolved on Mars, since billions of years ago, Mars was fairly friendly to life.  And maybe microbes still hang on to life on Mars somewhere underground.  He gives a cute description of microbes living in Mars:  growing little swords of salt precipitate to jab other microbes, growing tall like trees in their competition for nutrients.
There's a detailed description of how Mars could be colonized:  how to obtain water for people and for agriculture; how to synthesize oil on Mars since it probably has no fossil fuels.  People might live in caves on Mars, created by volcanism, so they're shielded from the lethal radiation at the surface.  The entire planet might be modified to be more friendly to humans.  Maybe if the atmosphere became dense enough people could fly there.
He describes the long-term future of Earth.  Apparently life has reached a peak on Earth and the future will be a long slow decline.  The Sun will be putting out more and more heat as it gets older, and over the next hundreds of millions of years, Earth life will fade away as the Earth gets hotter and hotter, and in time there will be only microbes left, and eventually even they will die, and the Earth will turn into a ball of magma with maybe an ice cap of metal snow on the side away from the Sun, which will become a red giant expanded out to the Earth's orbit and beyond ...
He doesn't consider that animals might evolve refrigeration.  After all, people have!  Maybe a biological compressor will evolve.
Mars will be getting warmer too.  Life might begin there a billion years from now, when it's warmer and wetter and not yet geologically dead.
The image of a geologist as a naturalist of rocks, armed with a pick and hammer, is clearly only a tiny part of the reality.  He applies physics to Mars, tells how Mars works as a large-scale system where the energy of radioactive decay ultimately forms mountains; he talks about the chemistry of mineral synthesis on Mars.
The book has lots and lots of images of Mars, produced by various imaging systems, and images of analogous places on Earth.  There's a big color section in the middle of the book, which includes a map of Mars.
The author seems to have a strong artistic side.  He has a vivid imagination.  He lightens his writing with wit, sometimes delightful:  "I shall not go into the details of why the Cydonian theory falls on its Face, but rather I shall emphasize things geological that relate to The Face and its anthropomorphic friends."
It's hard reading, all right.  He casually tosses acronyms like MOC, MOLA and THEMIS at you, and maybe he defines them in some buried location in the book, but I had to look them up online.  He uses words like "diapirism" and "ultramafic" and didn't include a glossary.  Don't read this book on an airplane, because you'll likely have to look up a lot of things.
The book's index is not much help.  Many words like "sublimation" and "sulfuric acid" that should be in it, aren't.  I couldn't find passages again that I remembered reading.  Unfortunately there's no ebook edition to search.  A good index and a glossary would help the poor puzzled readers a lot.
The view he presents seems to be pretty near the consensus of Martian scientists.  This book is not in the genre of popular science writing that aims to fascinate with dramatic and very controversial theories.  Instead it fascinates - and sometimes bewilders - with a phantasmagoric intensity of information.
He flubs the English language sometimes, as in "eek out a living" :)  Maybe he just cares more about the science than about getting his words quite right.  His clumsiness with words doesn't happen often enough to interfere with the readability of the book.
It's incredible how much people know about Mars.  They must know even more now, since this book was published in 2004.  If you have been occupied with concerns other than Mars in the last couple decades, and you suddenly turn to this book, a whole new world awaits you, brilliantly illuminated by the author's fascination.

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