How about heavier hydrocarbons? Titan has the lighter ones, but are not the heavier ones only here?
And the Earth might still be doomed if they mine somewhere close enough to gravitationally affect Earth. Mars or Venus losing half their masses might be rather problematic.
Even our spacecraft do have a few components made from wood, and the best capacitors are made in part from coconut shells.
Water? Hydrocarbons?
Oxygen, carbon and hydrogen are three of the four most common elements in the Milky Galaxy.
Diatomaceous earth.
Warm probe-holes.
Aren’t fossil fuels, by definition, derived from life forms? I’m no chemist, but can you find coal in nature that wasn’t formed from some life form or another?
I would say fossil fuels by definition are derived from life forms. However, there’s no reason the combustible chemicals we refine from fossil fuels couldn’t be obtained from abiotic sources. Methane, for example, makes up 2.3% of Uranus’ atmosphere. That is unlikely to be either from cow farts or from compressed beds of 50 million year dead seaweed.
The extraterrestrial sub-surface seas elsewhere in the Solar System probably are salty–see discussions of Europa and Enceladus.
Exactly…and, from what I recall there are lakes of the stuff on some of the moons of Saturn and Jupiter. Hydrocarbons are hydrocarbons, so if they need them for some reason there are tons out there that are easier to get if you have space capabilities.
But we burn coal directly, don’t we? What if these guys need coal, not “synthetic byproducts that can be produced from coal”.
They need all the heavy metals and other noxious byproducts? 
They could make synthetic coal of course (I suppose they could add mercury and such if they really wanted those in there as well), but if they need real, actual coal that only comes from organic material then yeah, they would probably have to come here for it. But if these guys are trying not to destroy us and can live without all the nasty byproducts in real coal then they could certainly make the stuff out of the abundant materials in the outer solar system without the bother of coming all the way here to mine the stuff and then transship it to orbit or whatever.
Steampunk interstellar travelers?
If they need actual coal, I suspect Earth is the only game in town. There’s no way anyone’s moving across interstellar distances on any sort of combustion, however. Tyranny of the rocket equation combined with the Isp of propulsion based on chemical combustion makes it impossible unless you’re willing to live with transit times in the hundreds of thousands or even millions of years.
So, if you’ve got a hydrogen fusion power plant of some sort, what do you need coal for?
“… to sap and impurify all of our precious bodily fluids.”
Uranus. Farts. <Beavis and Butthead snorts and giggles.>
ETA: do they like diamonds? Is coal really the squeezably soft precursor to alien pressure? (I think there was a thead on that, but I can’t remember…)
AIUI, the hydrocarbons on Titan are mainly short chains, 2-, 3-, maybe 4-carbon molecules. Fossil fuels contain lots of longer chain hydrocarbons. But if they really need the longer chains, they could probably make them from the short ones.
I’m no chemist nor do I even pretend to be one (I got C’s and D’s in chemistry in college and that was probably my prof being generous), but I think this is something we already do ourselves. Presumably aliens with interstellar capabilities could do even better if they really wanted too. I found a white paper on the process, but frankly it’s too esoteric for me so I’m not even going to post it since I actually have no idea what they are talking about. If you are interested and actually know anything about this just Google short chain to long chain hydrocarbon transformation and you’ll probably find it.
Hu-mons on Earth are confused about combustion.
Combustion is fuel plus oxidizer plus heat. Yes, there are lakes of methane on Titan. Drop a match into one and what happens? Nothing, because there’s no free oxygen on Titan.
Stuff like hydrocarbons that hu-mons think will “burn” is just stuff that combines with oxygen in our atmosphere. The oxygen is provided for free from all the plants. It doesn’t exist in this form any where else in the solar system. But there is plenty of oxygen, it’s just already oxidized whatever fuels are nearby. So there is water (oxidized hydrogen), carbon dioxide (oxidized carbon), silica (oxidized silicon), iron oxide, and on and on.
So hydrocarbons are only half of the fuel equation, they can’t scoop up kerosene from the lakes of Titan and burn it in their space steam engines. They need to scoop up oxygen as well to complete the equation (or fluorine, or potassium nitrate, or whatever).
And Earth is the only place to do that. Except are the going to drop down into the gravity well of Earth to try to suck up our oxygen, liquify it, and then boost it back up into orbit? With what? If they need our oxygen and hydrocarbons for chemical energy the energy required to lift the oxygen and hydrocarbons out of the gravity well are greater than the chemical energy contained in the oxygen and hydrocarbons. If they have some other sort of method for lifting stuff out of Earth’s gravity well, why exactly do they need our free oxygen?
If they have magic that can boost oxygen out of Earth’s gravity well for free, they have enough magic that they don’t need our oxygen to burn the kerosene they found on Titan. Why not just use the magic directly?
The most common elements in the universe are hydrogen, helium, carbon, oxygen, and nitrogen. Nobody’s coming to earth to steal stuff made out of these elements.
The only unique items on Earth are chemicals created through biological processes, and of course these chemicals are only unique due to the peculiar arrangement of the ordinary atoms they are made of. There are no organic chemicals that are impossible to synthesize, just ones that are difficult and expensive.
If aliens have enough magic to bring a portable industrial civilization from Tau Ceti to our Solar System, they surely have enough magic to synthesize whatever arbitrary organic molecule they wish from whatever arbitrary feedstock they have on hand. The challenges of simply living in space boggle the imagination. The challenges of transporting a suitable habitat and accompanying industrial infrastructure from one solar system to another square the amount of boggling needed.
Porn.
sure lets assume they don’t need our oxygen because they have plenty of energy from fusion and can just electrolyse water.
What about Uranium? is it available in large quantities anywhere else in the solar system?
Uranium isn’t available in large quantities on Earth either. It’s a pretty rare mineral in the Earth’s crust.