QUOTE: "The short answer to 'why is Mars red' is that the planet is covered in rust. Iron oxide to be exact... The short answer does not explain where all of the iron oxide comes from, though. There is a larger percentage of iron on the Martian surface than there is on other planets. The exact source is unknown, but many scientists believe that it came from the volcanoes that used to erupt all over the planet."
According to the answer above, volcanoes could have been the source of rust. But volcanoes did not produce the same result on other planets, not even close! Needless to say that there is no evidence to show that volcanoes existed all over Mars, let alone they were behind this rust. The total number of volcanoes on Mars that once were active is about 20 only. Now all of them are dead.
On the other hand, Venus had more volcanoes than Mars:
more volcanoes than any other planet in the solar system. Over 1600 major
volcanoes or volcanic features are known (see map), and there are many, many
more smaller volcanoes. (No one has yet counted them all, but the total
number may be over
100,000 or even
is known to be active at present, but our data is very limited. Thus, while
most of these volcanoes are probably long dead, a few may still be active."
Even though Venus had much more volcanoes than Mars, the planet does not have any noticeable rust, or we better say it cannot be seen from long distances like Mars.
Venus, as well as all of the other inner planets, must have some amount of iron oxide on their surfaces, came from the dying second sun.
Iron oxide covering the planet Mars
By looking at the surface of Mars, we see a layer of very fine rust covering the whole planet. It is like someone held a blower and blew fine rust, in equal quantities, on the surface of Mars. And that is exactly what happened, but the blower was very, very big!
Triassic sandstone mountains on earth contain high level of iron oxide (rust).
This rust could have been occupying a portion of the the debris disk, or the disk itself kept changing color from time to time
depending on the released material from the dying second sun. And then it has
fallen on Earth and mixed with the Earth's soil.
Desert sands on Earth
"Apartment buildings made of sandstone from the late Triassic and early Jurassic periods... This sandstone is the result of a 200-million-year journey that began when dinosaurs first started to evolve."
If we take now the radiometric dating of the
Triassic sandstone as a reference, we can tell that these desert sands
have got the iron oxide at the same time as the mountains, because the sandstone
was originally sand: "Triassic sandstone was formed in a sandy desert
environment. It is made up of quartz crystals held together in an iron oxide
rich cement; this cement gives the rock its characteristic red colour."
The Earth's moon also has iron oxide in some areas: "The mare [lowland]
regions have low reflectance because they contain relatively high
amounts of iron oxide (FeO). Some mare basalts contain unusually high amounts of
titanium oxide (TiO2) in addition to iron oxide, making for even lower
reflectance. TiO2 also shifts the color of the mare from
red to blue."
As you can see in the quote above, the lowlands on the moon should be red, but the addition of titanium oxide turned them to blue. Titanium oxide (TiO2) is also a product of dying stars. Because the moon is a very dry object, no wind is there, these chemical elements remained where they have been "deposited" initially. It is very likely the highlands also got some amounts of iron oxide, but much less. The moon's lowlands are on the facing side only; the dark side has almost no lowlands.
The question now is: if iron oxide is (iron + oxygen), and there is no oxygen on the moon, what caused the iron to rust?
According to the quote above, some of the moon's lowlands contain unusually higher amounts of titanium oxide than other lowlands. Unlike Earth, the moon does not have any atmospheric or environmental changes, so what makes one basalt area different from another basalt area? This is an indication that these chemical elements have been deposited on the moon at a later time.
The crater counting technique gives the lowlands of the moon an age of 200
million years only: "Lunar maria (lowlands) have
only 1/20 the crater density of the lunar highlands, therefore, they should be
1/20 the age, right? So, by this reasoning, if the highlands were 4.5 billion
years old, as old as the Earth, then the maria would be just 200 million years old."
This age estimate of 200 million years is very reasonable, because it matches the age of the sea floor and rocky mountains on earth, plus the rusty Triassic sandstone mountains. So if the moon's lowlands have an age of 200 million years, then the iron oxide and titanium oxide must have been deposited on these areas after they were created.
It doesn't seem there is any data about iron oxide on the "surface" of Venus published on the web in the time being, or it hasn't been known yet.
According to the link below, Mercury has about 3% of iron oxide on its surface:
"Direct observations from Earth indicate that it is 3
percent iron oxide by mass, compared to Earth's 8 percent."
The same logical question arises again: if Mercury does not have any oxygen at all, what caused the iron to rust on Mercury?
QUOTE: "Mars has twice as
much iron oxide in its outer layer as Earth does."
So if the percentage of iron oxide on Earth is 8 percent, Mars, according to the Wikipedia link above, should be 16 percent.
If we try now to arrange the inner planets based on the percentage of iron oxide they got on their "surfaces," we should have them in the following order: Mars, Earth, Venus, Mercury.
The closer the inner planet is to the asteroid belt, the higher percentage of iron oxide it has on its surface. Anything below the surface is just a supposition.