I have a new weather station for my home, and it included a UV radiation monitor. But I then thought, “What’s the UV radiation levels on Mars? How much higher is it than Earth?” I tried searching the website, but to no avail.

7 for the radiation monitor is pretty high. So maybe I’m predicting around 25-30 UV radiation on Mars, but I don’t know.

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    $\begingroup$ What units is your raditation monitor in? It seems like milliwatts per square meter $(\text{mW}/\text{m}^2)$ is the default, but it would be good to know. $\endgroup$
    – Freddie R
    Jun 3 at 20:42
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    $\begingroup$ Does it matter? Nobody on Mars will be able to step outside unless they are wearing a full space suit. Their suit will protect them from UV rays. $\endgroup$ Jun 3 at 21:41
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    $\begingroup$ @FreddieR I have a hunch that it's UV index where a 7 would be "pretty high" for a mid-latitude location (e.g. noon in NY City on a sunny day in September). $\endgroup$
    – uhoh
    Jun 3 at 23:11
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    $\begingroup$ The title mentions only "radiation", though the question is about specifically UV radiation. In space-related literature, though, "radiation" with no qualifiers is more likely to mean highly-energetic, possibly massive ionizing radiation from things like the solar wind or galactic cosmic rays. I would consider changing the title to better group the question with other UV-specific questions. $\endgroup$
    – Bear
    Jun 4 at 13:38

tl;dr: The UV index is primarily concerned with the level of UV-A that can cause sunburn and, potentially, skin cancer. The actual UV-A flux at Mars is not dramatically different from that at Earth (less because of distance, more because of no attenuation). *But the big difference is the unattenuated UV-C and UV-B!

The total solar constant at the Earth is 1366 watts/square meter with about 8% in the UV. Much of the UV portion of the radiation is absorbed by ozone in Earth's upper atmosphere. (1) Importantly, almost all of the shortest wavelengths (UV-C 200-290 nm) and most of the mid wavelengths (UV-B 290-320 nm) are absorbed. (2) So the roughly half of the UV that reaches Earth's surface is mostly the less harmful UV-A (320-400 nm) radiation. (1,2). Since the solar flux per unit area decreases as the square of the distance and the distances from the Sun of the Earth and Mars are 149.6 (10)6 km and 229.9 (10)6 km respectively (3), the solar constant at Mars is about 588 watts per square meter. So the solar UV portion is reduced from 110 watts/square meter to about 47 watts/square meter. However, the mean atmospheric pressure on the Martian surface is less than 0.007 Bar. (4) Also, there is no ozone in the Martian atmosphere. So there is virtually no atmospheric absorption of UV at Mars. This means that the most harmful UV-C and UV-B radiation is unattenuated, so much greater than that at the Earth's surface.

Bottom Line: a human will need serious UV protection to survive on Mars.

The increased UV radiation is due to the fact that Mars does not have a thick atmosphere with UV blocking gases, especially ozone. However, there are other sources of radiation harmful to humans where the lack of a significant Martian global magnetic field also plays a role. A NASA site on Space Radiation includes a note that "Research studies of exposure in various doses and strengths of radiation provide strong evidence that cancer and degenerative diseases are to be expected from exposures to galactic cosmic rays (GCR) or solar particle events (SPE)". (5)

Although the flux of galactic cosmic rays are relatively constant, the most important SPEs are the rather rare SEP (Solar Energetic Particle) events. These SEP events can be harmful to humans, especially the particles with energies of 40 MeV or greater. (6) They can be produced by energetic solar flares or shock waves associated with coronal mass ejections. Although they can travel at a significant fraction of the speed of light, there would be a few minutes warning. In a modular Martian colony, each module could be designed so, in the event of an SEP warning, the inhabitants could huddle under the module's water supply, which would provide some protection. If the roofs of the modules were transparent silica glass domes, incorporation of a little iron oxide (the material that makes Martian sand red) in the glass would provide protection from the UV, especially the most dangerous UV-C and UV-B. Incorporating some high Z (high atomic number) metal salts would add some protection from the galactic cosmic radiation.

Although the protections discussed above may be helpful, they should not be interpreted as meaning that protection, especially from the constant bombardment of GCR, is straightforward--it is not. See the answers and comments to the question in the link below for more information. (7)

1 https://www.ecologycenter.us/population-dynamics-2/solar-constant-and-solar-spectrum.html

2 https://www.fondriest.com/environmental-measurements/parameters/weather/photosynthetically-active-radiation/

3 https://www.google.com/search?client=firefox-b-1-d&q=distance+of+the+Earth+and+Mars+from+the+Sun

4 https://www.google.com/search?client=firefox-b-1-d&q=atmospheric+pressure+at+Martian+surface%3F

5 https://www.nasa.gov/analogs/nsrl/why-space-radiation-matters

6 https://en.wikipedia.org/wiki/Solar_energetic_particles

7 Why can't we use the same radiation shielding in Mars that we used when going to the moon?

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    $\begingroup$ Nice answer! btw since the question asks "How much...?" and predicts a (probably) UV index of 25 to 30, is it possible add an approximate value to "much greater than"? Thanks! $\endgroup$
    – uhoh
    Jun 3 at 23:14
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    $\begingroup$ @uhoh, the UV index is primarily concerned with the level of UV-A that can cause sunburn and, potentially, skin cancer. The actual UV-A flux at Mars is not dramatically different from that at Earth (less because of distance, more because of no attenuation). The big difference is the unattenuated UV-C and UV-B. $\endgroup$
    – Vince 49
    Jun 3 at 23:28
  • $\begingroup$ I hope you don't mind, but I've moved your insightful comment into your answer as a tl;dr I think it really summarizes the situation for the OP. Please feel free to roll back or edit further. $\endgroup$
    – uhoh
    Jun 3 at 23:33
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    $\begingroup$ I think the addition is a good idea. $\endgroup$
    – Vince 49
    Jun 4 at 0:00
  • $\begingroup$ The GCR hazard may be the greater. As you point out, people can take shelter against SEP events, but the GCR is always present. The GCR includes energetic heavy nuclei, a type of radiation that terrestrial inhabitants don't encounter. The biological effects of this "high LET" radiation are poorly understood, and may be very serious. $\endgroup$
    – John Doty
    Jun 4 at 14:45

Basically none, because you're not going outside without a pressure suit.

Mars's atmosphere is completely unbreathable. Not only is it completely lacking in oxygen, but it's so thin that humans wouldn't be able to survive breathing it even if it was 100% oxygen. If you go outside without a pressure suit and an oxygen mask, you'll promptly suffocate.

As a result, it's safe to say that a Martian colonist will be receiving basically no UV radiation at all; at all times, they'll either be inside their habitat (whatever form that habitat might take - even something like a glass dome could probably be expected to be engineered to block dangerous UV radiation), or wearing a suit that completely covers their body.

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    $\begingroup$ UV could be quite an issue for the pressure suit itself. UV degrades a lot of materials pretty quick. While we (sort of) have the same problem in spacewalks, spacewalks are pretty short compared to whatever tasks one will have outdoors on Mars. $\endgroup$
    – fraxinus
    Jun 4 at 13:19
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    $\begingroup$ UV's still a concern: with no atmospheric attenuation, you'll need some pretty good UV absorption on your visor. $\endgroup$
    – Mark
    Jun 4 at 21:34
  • $\begingroup$ This was my first thought. $\endgroup$ Jun 4 at 22:11
  • $\begingroup$ Besides what is mentioned in the comments, it is known that airline crews are exposed to higher doses of radiation than people on the surface; and they are always "inside". I don't think it's that obvious that good protection can be achieved by a spacesuit. $\endgroup$ Jun 5 at 6:39
  • $\begingroup$ @MartinArgerami Martian colonists probably would receive a higher dose of ionizing radiation, but UV can be blocked by things like clothing. $\endgroup$
    – nick012000
    Jun 5 at 6:46

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