<blockquote>
  <p>How can we detect water  ... without actually going to the surface and heating up the local dirt?</p>
</blockquote>

<p>The Hubble Space Telescope can detect water from 111 light-years away on the planet <a href="https://en.wikipedia.org/wiki/K2-18b#Physical_characteristics" rel="nofollow noreferrer">K2-18 b</a>. The spectrum is detected when the planet passes in front of its sun, the results are analyzed to determine the composition.</p>

<p>NASA Goddard: "<a href="https://svs.gsfc.nasa.gov/13312" rel="nofollow noreferrer">Hubble Finds Water Vapor On Distant Exoplanet</a>", video released on September 11, 2019.</p>

<p>See also this paper: "<a href="https://www.nature.com/articles/s41550-019-0878-9" rel="nofollow noreferrer">Water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18 b</a>" (Sept 11 2019), by Angelos Tsiaras, Ingo P. Waldmann, Giovanna Tinetti, Jonathan Tennyson &amp; Sergey N. Yurchenko:</p>

<blockquote>
  <p>"... Here, we report the detection of a spectroscopic signature of water in the atmosphere of K2-18 b — a planet of eight Earth masses in the habitable zone of an M dwarf<span class="math-container">$[7]$</span> — with high statistical confidence (Atmospheric Detectability Index<span class="math-container">$[5]$</span> = 5.0, ~3.6σ (refs. <span class="math-container">$[8,9]$</span>)). In addition, the derived mean molecular weight suggests an atmosphere still containing some hydrogen. The observations were recorded with the Hubble Space Telescope/Wide Field Camera 3 and analysed with our dedicated, publicly available, algorithms<span class="math-container">$[5,9]$</span>.</p>
  
  <p>... <span class="math-container">$\require{\mhchem}$</span></p>
  
  <p>This marks the first atmosphere detected around a habitable-zone super-Earth with such a high level of confidence. Although the <span class="math-container">$\ce{H2O}\ce{+H2-He}$</span> case appears to be the most favourable, this preference is not statistically significant. Concerning the composition, retrieval models confirm the presence of water vapour in the atmosphere of K2-18 b in all of the cases studied with high statistical significance. However, it is not possible to constrain either its abundance or the mean molecular weight of the atmosphere. For the <span class="math-container">$\ce{H2O}\ce{+H2-He}$</span> case, we found the abundance of <span class="math-container">$\ce{H2O}$</span> to be between 50% and 20%, while for the other two cases, it was between 0.01% and 12.5%. The atmospheric mean molecular weight can be between 5.8 AMU and 11.5 AMU in the <span class="math-container">$\ce{H2O}\ce{+H2-He}$</span> case, and between 2.3 AMU and 7.8 AMU for the other cases. These results indicate that a non-negligible fraction of the atmosphere is still made of <span class="math-container">$\ce{H2-He}$</span>.".</p>
</blockquote>

<p>Graphic from page 2:</p>

<blockquote>
  <p><a href="https://i.sstatic.net/Xegvl.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Xegvl.png" alt="Figure 2"></a></p>
  
  <p><sub><strong>Fig. 2 | Best-fit models for the three different scenarios tested.</strong> A cloud-free atmosphere containing only H<span class="math-container">$_2$</span>O and H<span class="math-container">$_2$</span>-He (blue), a cloud-free atmosphere containing H<span class="math-container">$_2$</span>0, H<span class="math-container">$_2$</span>-He and N<span class="math-container">$_2$</span> (orange) and a cloudy atmosphere containing only H2O and H2-He (green). Top: best-fit models only. Bottom: 1<span class="math-container">$\sigma$</span> and 2<span class="math-container">$\sigma$</span> uncertainty ranges.</sub></p>
  
  <p><code>[5]</code> Tsiaras, A. et al. A population study of gaseous exoplanets. Astron. J. 155, 156 (2018).</p>
  
  <p><code>[7]</code> Montet, B. T. et al. Stellar and planetary properties of K2 Campaign 1 candidates and validation of 17 planets, including a planet receiving Earth-like insolation. Astrophys. J. 809, 25 (2015). </p>
  
  <p><code>[8]</code> Benneke, B. &amp; Seager, S. How to distinguish between cloudy mini-Neptunes and water/volatile-dominated super-Earths. Astrophys. J. 778, 153 (2013). </p>
  
  <p><code>[9]</code> Waldmann, I. P. et al. Tau-REx I: a next generation retrieval code for exoplanetary atmospheres. Astrophys. J. 802, 107 (2015). </p>
</blockquote>

CC BY-SA 4.0
2019-09-12T03:53:53.367Z
2019-09-12T03:53:53.367Z
Rob
<p>According to <a href="http://www.bbc.co.uk/news/science-environment-24287207">data from Curiosity, Mars' dust holds about 2% water by weight</a></p>

<p>This wasn't previously detected, so the impression we have had of Mars being incredibly dry may need to be altered. Okay it is still very dry, but there is potentially extractable water in amounts usable by colonists.</p>

<p>But this sort of thing would be good to discover <strong>before</strong> going to another planet, so how can we detect this sort of thing without actually going to the surface and heating up the local dirt?</p>

<water>
How can we detect water on Mars-like exoplanets?
Rory Alsop
# How can we detect water on Mars-like exoplanets?

(Asked by: username_0 on 2013-09-26)

According to [data from Curiosity, Mars' dust holds about 2% water by weight](http://www.bbc.co.uk/news/science-environment-24287207)

This wasn't previously detected, so the impression we have had of Mars being incredibly dry may need to be altered. Okay it is still very dry, but there is potentially extractable water in amounts usable by colonists.

But this sort of thing would be good to discover **before** going to another planet, so how can we detect this sort of thing without actually going to the surface and heating up the local dirt?

**Tags:** water


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## 1 answers

### Answer by username_1 on 2019-09-12. Score: 1

> How can we detect water ... without actually going to the surface and heating up the local dirt?

The Hubble Space Telescope can detect water from 111 light-years away on the planet [K2-18 b](https://en.wikipedia.org/wiki/K2-18b#Physical_characteristics). The spectrum is detected when the planet passes in front of its sun, the results are analyzed to determine the composition.

NASA Goddard: "[Hubble Finds Water Vapor On Distant Exoplanet](https://svs.gsfc.nasa.gov/13312)", video released on September 11, 2019.

See also this paper: "[Water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18 b](https://www.nature.com/articles/s41550-019-0878-9)" (Sept 11 2019), by Angelos Tsiaras, Ingo P. Waldmann, Giovanna Tinetti, Jonathan Tennyson & Sergey N. Yurchenko:

> "... Here, we report the detection of a spectroscopic signature of water in the atmosphere of K2-18 b — a planet of eight Earth masses in the habitable zone of an M dwarf$[7]$ — with high statistical confidence (Atmospheric Detectability Index$[5]$ = 5.0, ~3.6σ (refs. $[8,9]$)). In addition, the derived mean molecular weight suggests an atmosphere still containing some hydrogen. The observations were recorded with the Hubble Space Telescope/Wide Field Camera 3 and analysed with our dedicated, publicly available, algorithms$[5,9]$.
>
> ... $\require{\mhchem}$
>
> This marks the first atmosphere detected around a habitable-zone super-Earth with such a high level of confidence. Although the $\ce{H2O}\ce{+H2-He}$ case appears to be the most favourable, this preference is not statistically significant. Concerning the composition, retrieval models confirm the presence of water vapour in the atmosphere of K2-18 b in all of the cases studied with high statistical significance. However, it is not possible to constrain either its abundance or the mean molecular weight of the atmosphere. For the $\ce{H2O}\ce{+H2-He}$ case, we found the abundance of $\ce{H2O}$ to be between 50% and 20%, while for the other two cases, it was between 0.01% and 12.5%. The atmospheric mean molecular weight can be between 5.8 AMU and 11.5 AMU in the $\ce{H2O}\ce{+H2-He}$ case, and between 2.3 AMU and 7.8 AMU for the other cases. These results indicate that a non-negligible fraction of the atmosphere is still made of $\ce{H2-He}$.".

Graphic from page 2:

> [![Figure 2](https://i.sstatic.net/Xegvl.png)](https://i.sstatic.net/Xegvl.png)
>
> **Fig. 2 | Best-fit models for the three different scenarios tested.** A cloud-free atmosphere containing only H$\_2$O and H$\_2$-He (blue), a cloud-free atmosphere containing H$\_2$0, H$\_2$-He and N$\_2$ (orange) and a cloudy atmosphere containing only H2O and H2-He (green). Top: best-fit models only. Bottom: 1$\sigma$ and 2$\sigma$ uncertainty ranges.
>
> `[5]` Tsiaras, A. et al. A population study of gaseous exoplanets. Astron. J. 155, 156 (2018).
>
> `[7]` Montet, B. T. et al. Stellar and planetary properties of K2 Campaign 1 candidates and validation of 17 planets, including a planet receiving Earth-like insolation. Astrophys. J. 809, 25 (2015).
>
> `[8]` Benneke, B. & Seager, S. How to distinguish between cloudy mini-Neptunes and water/volatile-dominated super-Earths. Astrophys. J. 778, 153 (2013).
>
> `[9]` Waldmann, I. P. et al. Tau-REx I: a next generation retrieval code for exoplanetary atmospheres. Astrophys. J. 802, 107 (2015).


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