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From MESSENGER Finds New Evidence for Water Ice at Mercury's Poles :
Mercury's polar region
Credits: NASA/John Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory

Shown in red (in the image above) are areas of Mercury's north polar region that are in shadow in all images acquired by MESSENGER to date. The polar deposits imaged by Earth-based radar are in yellow.

New observations by the MESSENGER spacecraft provide compelling support for the long-held hypothesis that Mercury harbors abundant water ice and other frozen volatile materials in its permanently shadowed craters.

Three independent lines of evidence support this conclusion: the first measurements of excess hydrogen at Mercury's north pole with MESSENGER's Neutron Spectrometer, the first measurements of the reflectance of Mercury's polar deposits at near-infrared wavelengths with the Mercury Laser Altimeter (MLA), and the first detailed models of the surface and near-surface temperatures of Mercury's north polar regions that utilize the actual topography of Mercury's surface measured by the MLA. These findings are presented in three papers published online today in Science Express.

(Emphases by me)

For obvious reasons these findings of abundant water ice and other frozen volatile materials make the poles of Mercury much more suitable to compare with Mars in general with regard to colonization.

The most striking advantage for Mercury is of course that it gets much more energy from the Sun than Mars does:

about 12 times as much energy on average

Because the axial tilt of Mercury is only about 2 arc min., there is an elevated region near the north pole where that energy could be harvested all day and year around !

Furthermore Mercury has about the same surface gravity as Mars does, and it has a magnetic field that is strong enough to deflect the solar wind around the planet.

There's only one severe disadvantage that I could think of: the delta-v required for Mercury to enter Hohmann orbit is 7.5 km/s against 2.9 km/s for Mars, but that could be reduced with many flyby's like BepiColombo will attempt to do, so that will need much scheduling ahead !

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    $\begingroup$ this isn't exactly related to your question but it's interesting: earthsky.org/space/… $\endgroup$
    – uhoh
    Jul 12 '20 at 17:26
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    $\begingroup$ @uhoh Interesting indeed ! And the other proposal, DAVINCI+ also for Venus , too ! "...NASA will continue development of up to two missions towards flight" . Wouldn't it be nice if htey could combine the two for Venus ! $\endgroup$
    – Cornelis
    Jul 12 '20 at 17:53
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    $\begingroup$ It should be noted that the Hohmann transfer orbit delta-V's provided from the Wikipedia article also make the assumption that the orbit of Mercury is circular, and in the plane of the ecliptic. As anyone who's tried a flight to Moho (the Mercury analog) in Kerbal Space Program knows, those numbers are a cruel, cruel lie when your target's orbit is close the the Sun, inclined, and ellpitical. :) $\endgroup$
    – notovny
    Jul 12 '20 at 20:09
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    $\begingroup$ related (and currently unanswered) How did Arecibo make radar images of ice on Mercury's poles? $\endgroup$
    – uhoh
    Jul 13 '20 at 10:45
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    $\begingroup$ About that "delta-v required for Mercury to enter Hohmann orbit is 7.5 km/s against 2.9 km/s for Mars".. That's the delta-v to pass near your destination. You need to add the delta-v needed to capture (mars: 0.67km/s, Mercury 6.31km/s), and then to circularize to low orbit (Mars 2.1km/s, Mercury 1.22km/s). the totals are from LEO to Low planetary orbit: Mars=5.7km/s, Mercury=15.0km/s) $\endgroup$ Mar 29 at 7:52
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Your delta-v analysis doesn't account for the landing delta-v. On Mars, only a fraction of a km/s has to be done propulsively, on Mercury the entire landing will be propulsive.

You also don't account for transit time. BepiColombo launched in 2018 and won't be able to enter orbit until the end of 2025. MESSENGER similarly launched in 2004 and didn't enter orbit until 2011. You're looking at a 7 year trip in a solar radiation environment that peaks at 6 times as bad as that at Earth, 12 times as bad as that at Mars, and that's just one way. Worse, your Mercury spacecraft will spend much of its overall time at the high-radiation end of that range as it does its final Mercury flyby maneuvers.

Mars, for comparison, can be reached with transits of just a few months. In Musk's 2016 IAC talk, he proposed high-energy transits lasting 80-150 days. These start out in the near-Earth radiation environment and move rapidly outward, spending the bulk of the transit closer to the near-Mars radiation environment, with about half the solar radiation experienced at the distance of Earth. And once you land, the atmosphere provides radiation protection similar to that provided in LEO by Earth's magnetosphere: https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA03480

You need a major advantage to compensate for the delta-v issue, and there really isn't one. Volatile ices are available on Mercury, but they really don't compare to the large bodies of pure water ice and the CO2/N2/Ar atmosphere of Mars. Launch windows are more frequent, but given a certain size of vehicle, you're going to need a lot more launches to move the same amount of material due to the lower payload fractions. And Mercury does have 12 times the power for a solar panel facing the sun...but you'll be at the poles, where you'll have to space panels widely apart or erect them on towers so they don't shade each other, or have them at such a steep angle that they don't actually catch much sunlight. On Mars, you can land at mid-latitudes and just unroll/unfold solar panels in fields.

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    $\begingroup$ @Cornelisinspace Because even the minimum energy transit with nothing left for entering orbit or landing takes 7.5 km/s. That's more delta-v than a fully loaded Starship can manage. $\endgroup$ Jul 12 '20 at 16:57
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    $\begingroup$ Completely emptying the Starship wouldn't provide enough extra delta-v to enter orbit around Mercury, let alone land there...only around 8 km/s total, depending on how heavy Starship turns out to be. That's a vehicle that easily has enough for a high energy transit to Mars with a full load. The delta-v difference is not a minor issue, in practice it means you'd need something exotic like nuclear or solar thermal propulsion just to get humans there, while Mars is easily reachable with chemical propulsion...not even the highest performance chemical propulsion, at that. $\endgroup$ Jul 12 '20 at 17:59
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    $\begingroup$ @JohnDvorak Not for quickly getting people there and back. $\endgroup$ Jul 13 '20 at 12:02
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    $\begingroup$ Plus the whole point of starship was to be big enough to be able to quickly bring a lot of stuff (things and people) to the Mars landing site , not to be stripped down to near nothing and slowly limp to mercury. And colonization requires literal tons and tons of stuff, so how much you can bring in one go becomes incredibly important. $\endgroup$
    – eps
    Jul 13 '20 at 15:20
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    $\begingroup$ @eps Exactly. SpaceX's Mars colonization plan has them landing at a site with several hundred tons of supplies already there waiting for them in other Starships. It also means the advantage of the more frequent launch windows is greatly reduced: the higher delta-v requirement means a lower payload fraction, so a vehicle of the same size will need more trips to deliver the same payload. $\endgroup$ Jul 13 '20 at 23:39
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You can't land on the day side of Mercury nor on a peak of perpetual light because it's far too hot (800 deg F / 430 deg C), even if not as hot as on Venus. A crewed mission must land either on the night side or in a crater of long darkness. This crater can and probably will be on the north or south pole if there's perpetual darkness.

Also, a crewed flight can't afford multiple planet flybys like BepiColombo. Their goal would be to get to Mercury (and back to Earth if they wanna) as fast as possible unless they have a really large spacecraft and one that can provide centripetal gravity.

Permanent colonization of the entire planet is impossible with current and near-future technology because Mercury is not in synchronous rotation with the Sun, therefore all of Mercury (except craters of perpetual darkness) will eventually be enlightened by the Sun.

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    $\begingroup$ The day side very close to the poles would not be so hot. The angle between the Sun and the horizon of Mercury should be small, 5.8 ° would reduce the solar intensity to a horizontal plane to 10 % of the intensity at the equator. $\endgroup$
    – Uwe
    Jul 12 '20 at 15:52
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    $\begingroup$ The question is explicitly about the poles of Mercury versus Mars ! $\endgroup$
    – Cornelis
    Jul 12 '20 at 16:44
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    $\begingroup$ @Cornelisinspace Peaks of perpetual light and craters of perpetual darkness can only be found near/on the poles of Mercury. Mars is tilted and doesn't have such. You're also invited to look onto my question: space.stackexchange.com/questions/44566/… $\endgroup$ Jul 12 '20 at 17:14
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    $\begingroup$ How large is the largest such known polar crater of perpetual darkness? Is it Prokofiev at 85.77°N 62.92°E, with 112km diameter? How much would that limit a hypothetical Mercury colony? (population/raw materials). Also, how much distance separation would there need to be between the colony and the return launch site? $\endgroup$
    – smci
    Jul 14 '20 at 8:12
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    $\begingroup$ @Uwe: since Mercury has no atmosphere, I suspect the unshielded UV and gamma radiation levels would be lethal, even at 86°N. (Anyone got any rough numbers?) So, the colonists would have to hide below their crater rim almost all the time. $\endgroup$
    – smci
    Jul 14 '20 at 9:28
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There are several arguments which favour Mars over Mercury as a candidate for human missions.

Getting there and coming home

As the question and other answers have highlighted, the delta-v for transferring to Mercury is significantly higher than required for transferring to Mars. This will translate to monumentally higher propellant and launch costs for the outbound transfer.

Coming home is also a problem. Using in-situ resources to manufacture propellant can in theory greatly reduce overall mission launch costs. The greatest advantage in terms of launch mass saving will typically come if you are able to produce all the return propellant in-situ. Returning from Mercury will require more propellant than to return from Mars. This means the scale of any ISRU propellant production from indigenous resources will also have to be scaled up in size. This means MORE hardware that needs to be transported to the surface of Mercury, and hence more propellant for the outbound trip still etc.

There are always solutions to reduce transit delta-v with alternative transfer strategies, but this almost always comes at the cost of increasing transfer time. This is not currently a viable solution for human missions as minimizing time in the micro-gravity is a primary concern. BepiColombo will take around 9 years just to reach Mercury. This is an about an order of magnitude of the combined outbound and return transfer time needed for a Mars mission if short (type-I/II) transfers are used.

Surviving the local environment

The surface environment of Mercury is a lot more hazardous to life than that of Mars.

Mercury is much closer to the Sun compared to Mars and will receive approximately 14x greater solar flux. This would be good for powering operations with solar arrays, but this might be outweighed by the complexity of the thermal control systems needed to keep all hardware in acceptable temperature ranges. Thermal control on EVA suits would similarly be challenging, which might limit what crew can do on the surface of Mercury. If this is not a showstopper it would certainly be a large issue to address.

The proximity to the Sun and the lack of any discernible atmosphere on Mercury would also make the radiation environment significantly more hazardous than that of Mars. Again, there would be ways to mitigate this, but this would come at the added cost of additional hardware, systems and design complexity.

Using local resources

Accessing and using water-ice on either Mercury and Mars will be challenging due to the location and complexity of accessing the resource (i.e. in polar shadowed craters or buried beneath the surface). These resources are not necessarily ubiquitous, so the location of the resources may dictate where a colonization site is established. The jury is out whether the first human missions to Mars will need to be reliant on water-ice ISRU operations, colonization would likely need to take this into account.

However, Mars has another indigenous resource which is much more accessible than water-ice, simpler to acquire, and would not dictate where on the planet you need to land – the atmosphere, composed primarily of Carbon Dioxide. This can be used in a variety of different ISRU processes including the production of oxygen for life support systems and oxidizer for return propellant. This is an advantage for Mars over Mercury if considering the scope of local resources for first crewed missions.

Communicating with home

Mars goes into superior conjunction with Earth every two years. During this time, Mars and the Earth are on opposite sides of the Sun and direct communication is not possible. This period of conjunction can last 1-2 months, depending on the frequency band of communications used.

This presents a challenge for human Mars mission design. It is operationally hazardous to go for such a long period of time without crew on Mars being able to communicate with ground support on the Earth. It could be possible however to design missions such that the surface stay occurs outside of periods of conjunction.

Mercury is much closer to the Sun and therefore completes and orbit in much less time, about 88 days. These conjunction events between the Earth and Mercury will therefore happen much more frequently (though, likely for less duration). This means there will be frequent periods of time when communication will be lost.

In both cases this could in theory be remedied by establishing communications relay spacecraft in suitable Lagrange points. This might be essential for the first human missions to the surface of Mercury, but not necessarily for the first human Missions to Mars.

What will humans do on the surface?

Mars is scientifically compelling. We know Mars once hosted a much more habitable environment. It was once much more like Earth than it is today, and it is one of the important destinations for searching for life in our solar system.

There is a compelling argument for sending humans to Mars; they would be able to advance our exploration and scientific inquires at a much greater rate than if we rely on robotic spacecraft alone. Simply put, humans can do a lot more than robots.

I am unsure if the same argument for sending humans to Mercury is as compelling. The extreme temperatures and radiation environment imply that Mercury is an unlikely home to life. This may deter some stakeholders from investing in human missions to Mercury when Mars is already on the table.

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    $\begingroup$ FWIW, the mean synodic period of Mercury is ~115.88 days, but there are two Sun-Mercury conjunctions in that period (and the actual time between conjunctions is fairly variable due to Mercury's large eccentricity). $\endgroup$
    – PM 2Ring
    Nov 28 at 6:11

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