Warning: Creationist rant incoming. If such things annoy you, stop reading.
This is a pet theory of mine, and there's absolutely no way of proving it, so it's entirely conjecture, and will remain so until we get to Heaven and ask God about it.
First off, this is from the Astronomy blog on accuweather.com.
According to a new study conducted by scientists at the University of Colorado at Boulder, around 3.5 billion years ago, one-third of the surface of Mars could have been covered by a huge ocean. This particular study was the first to combine analyses of water-related features and river valleys to test for an ocean that would have been supported by a hydrosphere. The idea of an ocean on Mars has been defended and challenged for the past two decades; this new study provides support for those defending the idea.
Per the study findings, over half of the fifty-two river delta deposits, which are fed by river valleys, could have marked the boundaries of the ocean (all were at the same elevation). Twenty-nine of these deltas were likely connected directly to the ocean or the groundwater table and adjacent lakes. The data sets of deltas, valley networks and topography comes from various NASA/ESA Mars orbiting missions since 2001, and use of these data in the study produced results suggesting that ancient Mars could have had a similar hydrological cycle to Earth that would have included precipitation, run off, cloud formation, etc.
A Geographic Information System (GIS) was also used to map the Martian terrain, and it was determined that the ocean may have taken up around thirty-six percent of the planet at depths up to about 1,800 feet.
I've heard about the potential for oceans on Mars for years, but I'd never heard the speculation about Venus.
When we think of Venus, we're probably not picturing a habitable world with beautiful flowing oceans and a breathable atmosphere, or perfect atmospheric pressure. ESA's Venus Express spacecraft is on a mission to find out whether our "sister" planet was actually more of a "twin" at one point, perhaps having oceans.
So far, Venus Express is finding several similarities between Earth and Venus, including the composition of both planets. One of the largest differences thus far, however is the lack of water on Venus. It has been proven however that Venus has lost a vast amount of water to space. This is because ultraviolet radiation breaks up water molecules in Venus' atmosphere into two hydrogen atoms and one oxygen atom, which then escape into space. Venus Express confirmed that twice the amount of hydrogen was escaping than oxygen.
There was another article somewhere about how they've identified water ice on certain asteroids of the asteroid belt. I can't find the article for the life of me, though.
Anyway, all that put together along with a Creationist slant, and here comes the fantasy.
Imagine that as God was building the universe and Earth, He knew that since there was no death, yet He was going to bless people and animals with reproduction, eventually there would come a point when Earth would become overpopulated.
So Mars had oceans, and an atmosphere. So did Venus, and possibly Earth's Moon. (It has frozen water and the remains of a magnetic field.)
There was probably a fifth inner planet, one that became the asteroid belt, that was also inhabitable, seeing as it apparently had water on it.
Then there's the various moons of Jupiter and Saturn that conceivably could have supported life at one point, like Europa, which is thought to still have liquid oceans under its ice.
Anyway. Whether it was when man sinned, or whether it came later, during the Flood, disaster struck the solar system.
Imagine the Angel of Death flying inward, toward the sun, smiting moons and planets as he goes. The moons of the outer planets are frozen, the fifth inner planet explodes into asteroids.
Mars is so blasted that its oceans and atmosphere boil away, leaving a ruined desert.
Earth is so damaged that its vapor canopy falls as 40 days of rain, its magnetic field reverses polarity a couple of times, true north shifts, and the planet's axis is tilted, bringing about seasons. Earth's moon is horribly scarred, oddly enough, on the side of it that faces Earth.
The Death Angel moved on and poisons the atmosphere of Venus so that it can no longer retain any liquid water.
Who knows if Mercury was ever supposed to be inhabitable, but it is slowly burning up from proximity to the sun anyway.
The entire solar system is so maimed that mankind is confined to Earth, which he will now have a much harder time overpopulating, since mankind's lifespan is so shortened by the harsher climates and increased radiation.
Man could visit the other planets and moons, but living there would be extremely difficult and dangerous. Not like it used to be.
So there's my pet theory. No way of ever proving it, but it's fun to ponder.