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The prevailing theory for how liquid water existed in the distant past of Mars, is that the warmth required was caused by a greenhouse effect caused by high concentrations of CO2 in the atmosphere. This is important because it is believed the Sun was actually weaker in the past than it is today. If high concentrations of CO2 was responsible, there should be high concentrations of carbonate and the Curiosity rover should find be able to find this. So far this has not been the case.

This leads me to the question, could Mars have once had an orbit that was much closer to the Sun in the past? What kind of experiment could be done to answer that question? If it did orbit closer, what caused it to move to its current orbit? We do know Mars has received many significant impacts in its past, which may account for the thin atmosphere today. Could an impact give a sufficient nudge to gradually affect the orbit over time?

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  • $\begingroup$ You pose two interesting questions here: where's the carbonate, and could Mars have migrated from a closer orbit? What's the connection between them? Are you positing that a closer orbit would enable liquid water without an atmosphere? I don't believe that's true - the ice would simply sublimate. $\endgroup$ – Bear Feb 23 '17 at 14:41
  • $\begingroup$ I stated in the first sentence the question is how liquid water existed on Mars in the distant past. Heat is required for this. So what was the source of the heat? Greenhouse gas heat from CO2 or that the orbit was closer to the sun so the heat came from solar radiation. Mars has a thin atmosphere, but it was once thicker, with or without CO2 I did not say without an atmosphere; you did. I said it appears that high CO2 concentrations did not exist based on the lack of carbonate so far. $\endgroup$ – Thomas Boomer Feb 23 '17 at 17:31
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    $\begingroup$ For those of you unfamiliar with the subject I am referring to here is a link:lightsinthedark.com/2017/02/06/… $\endgroup$ – Thomas Boomer Feb 24 '17 at 0:04
  • $\begingroup$ Eh, if Mars had hydrocarbons! USA would liberate Mars within a decade! $\endgroup$ – SF. Feb 24 '17 at 20:33
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To add more to what Happy Koala wrote (this would be a comment but I don't have the rep. yet to post comments): if you try to model the solar system without planetary migration, you get an Earth-sized (or larger) planet where Mars is and several high-inclination / eccentricity Mars-sized objects in the asteroid belt. There's now an increasingly high degree of confidence that planetary migration did, in fact, occur.

Specifically, Jupiter gorged on the combination of having a high radius before the dropoff in density in the protoplanetary disc, boosted by volatiles being blasted away from the inner solar system, and quickly became a large, dominant body in the solar system. It slowly migrated inwards (casting icy material from the outer solar system into the inner solar system in the process). It reached all the way to where Mars is today and cleared the zone between Mars and Earth's present orbit (casting a good chunk of this material into the outer solar system). However, during this time Saturn had slowly been forming, and the action of Saturn slowed and ultimately reversed Jupiter's inward migration. Jupiter slowly migrated outward to its current position, casting material into what is now the asteroid belt in the process.

Venus, and Earth formed from material in their respective orbits, a combination of original material and material cast in by Jupiter. Mars however had to wait for scraps being cast in by Venus/Earth or Jupiter; there was little to accrete in its zone. As a consequence, it ended up much smaller. In the process of accretion, all of the inner planets moved around, although not as much as the giants. This includes not just semimajor axis but inclination and eccentricity as well.

As to how Mars retained liquid water, it used to have a more substantial atmosphere, but its low gravity and lack of a magnetic field led to the atmosphere's loss by the solar wind. This can be clearly seen in its hydrogen and nitrogen isotopic ratios, both of which are well enriched in their heavier isotopes, as lighter isotopes escape more readily. Venus, while having lost little nitrogen due to its size, is even more water-depleted than Mars; Mars has a 5-7 times higher D:H ratio than Earth, while Venus has a 150-240 times higher ratio. It's amazing and kind of sad to think about how Earthlike Venus used to be.

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If there is something the past 20 years of exo planet research has taught us, it is that solar systems, over time periods we humans cannot readily comprehend, are not the clockworks the ancients envisioned them to be, but that planets migrate, sometimes get booted out of the solar system altogether and other times they collide with the sun. So it could very well be the case that Mars's orbit changed in the early years of the solar system, but if it did, it would have been because of the migrations of Jupiter and Saturn, and not because it got knocked into a new orbit by a collision. I say that because the amount of energy it would take to alter the orbit of Mars by flinging a rock at it would be immense. While I'm pulling a rabbit out of a hat now, I don't even think flinging every asteroid in the asteroid belt at Mars would significantly alter its orbit. As for testing this hypothesis, I guess one could measure the isotopes found in Martian rocks with those found in other locations of the solar system to get an idea of where it formed (this is how we know the Moon was created through a collision, because if the object had been captured its set of isotopes would not be identical to those on Earth).

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