JuanCastillo29/space-mission-intelligence-data huggingface dataset:
As mentioned above, and reported in previous studies (Jaquin et al., 1986; Forget et al., 1999; Montmessin et al., 2006; Heavens et al., 2011; Smith et al., 2013) the vertical structure of Martian aerosols depends on the latitude. To isolate this latitudinal dependence from the seasonal variation discussed in section 3.1, Figure 3 presents the vertical profiles of water ice crystals size within the clouds. We adjust several Ls bins to minimize the impact of the seasonal variations while providing enough coverage in terms of latitude. If the entire range of latitudes is not covered in each panel, we can observe on panels a, c & d that the altitude of the water ice clouds (and more generally of the Martian aerosols) increases when getting closer to the equator. This also produces a bell-shaped distribution of the altitude of the clouds as a function of latitude. Indeed, we observe on Figures 3 & 4 that if the water ice crystals with radius reff ≤1 µm are located between 10 and 40 km in the polar regions (> 60◦N or < 60◦S), they are found typically between 30 and 80 km around the equator, and occasionally up to 85 km.

140° Ls < 230° (MY 35)
230° Ls < 325° (MY 35)
2.0
a
b
1.5
Altitude [km]
reff [µm]
1.0
0.5
0.1
325° Ls < 60° (MY 35-36)
60° Ls < 182° (MY 36)
2.0
c
d
1.5
Altitude [km]
reff [µm]
1.0
0.5
0.1
90°S 60°S 30°S 0° 30°N 60°N 90°N Latitude
90°S 60°S 30°S 0° 30°N 60°N 90°N Latitude
pose the clouds detected by ACS-MIR: we observe that for a fixed size of particle,
the altitude of the detections still follows the bell-shaped distribution with latitude
already identified for the general distribution of the clouds in the previous paragraph.
Thus, there is no strict correlation between the size of a crystal and its altitude in
the atmosphere. Other parameters have to be considered, such as the latitude of the
cloud. This is consistent with the decrease of the scale height of the atmosphere in the
high latitudes, where the temperatures are lower. As the temperatures decrease with
increasing latitude, one finds a decrease in the cloud altitudes. However, we observe
in Figure 4 that we also do not detect crystals with radius ≥ 1.5 µm above 55 km
(outside the MY 34 GDS), there is no crystal with size ≤ 0.2 µm at altitudes lower
80 km. Thus, we can identify with Figure 4 the range of altitudes that are populated
by water ice crystals for a given radius.
unlike small-grained high altitude hazes that are observed at the top of almost every
profile.
The localized aspect together with the observed altitudes of these clouds Midlatitudes (45°S lat 45°N)

a
195° Ls < 250° (MY 34 GDS) 140° Ls < 230° (MY 35) 230° Ls < 325° (MY 35) 325° Ls < 60° (MY 35-36) 60° Ls < 182° (MY 36)
Altitude [km]
0.0 0.5 1.0 1.5 2.0 2.5 3.0 reff [µm]
Polar latitudes
b
North (lat 60°N) South (lat 60°N)
Altitude [km]
0.0 0.5 1.0 1.5 2.0 2.5 3.0 reff [µm]
from non-GDS years (MY 35 & 36).

composed of crystals with reff ∼2 µm is consistent with the results obtained using CRISM limb observations presented in figures 10 to 17 of Smith et al. (2013).
Concerning the local dust storm occurring in the Northern hemisphere during MY 35 (cf. section 3.1), panel b of Figure 3 presents profiles of clouds acquired during this period. This event allows us to study its specific impact on the clouds in the Northern latitudes in relation to the rest of the planet. Although TGO’s orbit over this period did not allow ACS to probe latitudes above 65◦S and 70◦N, we observe that the maximum altitude of the clouds observed near 70◦N extends to 70 km, which is higher than clouds observed at these latitudes outside of the storm (cf. panels a, c & d of Figure 3), but lower than the altitudes of the mesospheric clouds between 40◦S and 20◦N (Clancy et al., 2019).
Stcherbinine et al. (2020) used the early observations provided by ACS-MIR to study the planetary-scale dust storm that encircled Mars during MY 34. With the current extended dataset, it is now possible to compare observations acquired during the MY 34 GDS with the ones obtained for the same range of Ls in MY 35, when there was no GDS.

[Figure: Figure 5 presents a superimposition of the profiles obtained between Ls = 190◦]

and Ls = 250◦during MY 34 (panels a & e) and MY 35 (panels b & f). The latitudes and local times of the observations are indicated in panels c, d, g & h respectively. This information should be taken into account when comparing the two years, as the latitude of the clouds has a significant impact on its profile characteristics (cf. section 3.2). Even when the two datasets cover the same latitude range, we note in Figures 4 & 5 that the altitude of the clouds for MY 35 does not exceed 85 km (consistent with the results presented in Clancy et al. (2019)), while small-particle clouds (reff ≤0.1 µm) are observed up to 100 km at the beginning of the GDS. In particular, we observe at the onset of the GDS (Ls = 195.5◦, MY 34) a profile in the Southern hemisphere (62◦S) with a cloud that extends up to 92 km along with large water ice crystals (reff ∼2 µm) around 58 km, which contrasts with the observations acquired under the same conditions during MY 35 with clouds that do not extend over 40 to 50 km.
The period around Ls ∼240◦in the Northern hemisphere is also interesting, It not only corresponds to the end of the MY 34 GDS, but also to the maximum of dust activity for a regular (i.e., without GDS) year (Lemmon et al., 2015). We also note the presence of a regional dust storm during this period in MY 35 (cf. section 3.1). We observe that the maximum altitudes of the clouds are quite similar in both cases, i.e., between 70 and 85 km. However, we do not observe the large water ice crystals (reff > 1 µm) during the MY 35 dust storm that were seen between 50 and 60 km in MY 34 during the GDS.
Regarding the larger water ice crystals (reff ∼1.5 −2 µm), whose detection between 55 and 65 km during the GDS was surprising (Stcherbinine et al., 2020), similar sizes can be observed up to 55 km in the equatorial regions in MY 35 (i.e., no GDS). However, these larger crystals generally remain confined to altitudes below 50 km. For smaller particle sizes (reff ∼1 −1.5 µm), we observe that while the detections during the GDS are mostly concentrated around 60 km of altitude, they are typically found near 50 km during the same period in MY 35 (cf. Figure 4). Similarly, we also observe in Figure 5 that the detections of water ice crystals with sizes 0.13 µm ≤reff ≤0.5 µm that are detected between 75 and 85 km during the GDS (panel a) are not present in the following year (panel b), where this size of crystals is not observed above 75 km. Thus, the increase of the average altitude of the clouds during the GDS does not affect all the ice particles in the same way, rather it depends on their size:

2.0
stronger at high latitudes than in the equatorial region. Indeed, it has been reported
that the altitude of clouds does not vary as a function of latitude during the GDS,
which tends to homogenize the vertical structure of the atmosphere across the planet
(Stcherbinine et al., 2020; Luginin et al., 2020; Liuzzi et al., 2020).
This effect is
particularly visible in the Southern hemisphere (panels e to h of Figure 5). Thus, while
the typical maximum altitude of equatorial clouds varies from ∼ 80 km to ∼ 90 km
during the GDS in the Southern latitudes are significantly higher than all the other
detections in MY 35 that usually do not exceed 20 km of altitude for this particle size.
altitude where such clouds were detected during the GDS.
Along with particle size, the extinction coefficient is an important property. It will influence the amount of incoming solar energy that will be thermally transferred to the atmosphere (Gierasch & Goody, 1972; Montmessin et al., 2002; Madeleine et al., 2011, 2012). The SO geometry used by ACS-MIR gives us direct access to the vertical profiles of the atmospheric extinction at each observed wavelength through the vertical inversion technique. In fact, as described in Section 2 and Stcherbinine et al. (2020), this step has already been performed as part of our derivation of the properties of the water ice clouds.

[Figure: Figure 6 shows 8 extinction profiles at λ = 3.4 µm acquired by ACS-MIR at various latitudes during MY 35, showing altitudes where water ice particles have been detected in each profile. Because 3.4 µm is located on the edge of the 3 µm band, it is less sensitive to the composition of the atmospheric layer (water ice or dust) and to the size of the water ice crystals (Vincendon et al., 2011). As a result, it is more indicative of the general aerosol extinction of the atmosphere (compared to the extinction at 3.2 µm that highlight the presence of small-grained water ice crystals). We observe a difference between polar latitudes (c, d, g & h) and more tropical or equatorial observations (a, b, e & f). Indeed, while the main cloud layers are typically observed above 50 km in the equatorial profiles, we also note the presence of additional layers at lower altitudes (around 10 km). This is particularly clear in profile b, where the extinction goes from 7 · 10−3 km−1 at 3 km to 1.3 · 10−2 km−1 at 7 km before progressively decreasing to 10−4 km−1 at 40 km. Then, the extinction increases again up to 4 · 10−3 km−1 between 40 km and 52 km, which corresponds to the bottom of the water ice layer that extends from 47 km to 82 km. The presence of multiple high-extinction layers has been previously observed by SPICAM (Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars) onboard Mars Express (Fedorova et al., 2009, 2014).]

We also note in profiles b & e that we only detect water ice crystals in the upper part of the detached layers and above, which means that the lower altitudes of these layers are primarily composed of either dust or large water ice crystals. Similarly, we observe in profile a some detections of large (reff = 1.5 −3 µm) water ice crystals between 23 km and 36 km of altitude, which suggests the presence of water ice and not only dust in these lower layers. This vertical structure of ice layers capping the dust layers has already been reported with limb observations (Smith et al., 2013). In addition, clouds are detected at the top of the profiles associated with extinction down to a few 10−5 km−1. The detection of these very tenuous water ice hazes is made possible by the high sensitivity provided by ACS-MIR and the SO geometry technique.
Figures 7 & 8 present vertical profiles of the extinction coefficient kext at 3.4 µm acquired by ACS-MIR between Ls = 140◦(MY 35) and Ls = 90◦(MY 36), filtered by latitude and Ls to highlight the seasonal and latitudinal variations of the atmospheric vertical structure (similarly to Figures 2 & 3). We observe in Figure 7, similarly to what has been noted above for the global behavior of the clouds, that there is a strong latitudinal dependence in the atmospheric extinction profiles: a given kext value is observed at higher altitude close to the equator than in the polar regions. This is in agreement with previous retrievals of the atmospheric dust extinction from Mars Climate Sounder observations (Kleinb¨ohl et al., 2015). This variation is only about 10 km for the lower layers of the atmosphere, but between ∼30◦S and 55◦N we observe in most of our profiles the presence of a second detached atmospheric layer (i.e., atmospheric extinction decreases with altitude then increases again by at least one order of magnitude) around ∼30–55 km (as previously noted in individual profiles 140° Ls < 230° (MY 35)

230° Ls < 325° (MY 35)
a
b
kext(  = 3.4 µm) [km 1]
Altitude [km]
325° Ls < 60° (MY 35-36)
60° Ls < 182° (MY 36)
c
d
kext(  = 3.4 µm) [km 1]
Altitude [km]
90°S 60°S 30°S 0° 30°N 60°N 90°N Latitude
90°S 60°S 30°S 0° 30°N 60°N 90°N Latitude
of altitude for a given extinction value are of lower amplitude than the variations of
the average water ice clouds altitude for the same profiles (cf. section 3.1). Between
We
also note that when the haze top altitude increases, for a fixed range of latitudes, it
is largely due to the expansion of low-extinction layers to the higher altitudes rather
than a shift of the vertical structure of the extinction profile, i.e., a given extinction
value remains approximately associated with the same altitude, but layers with lower 45°N lat 90°N a kext(  = 3.4 µm)

[km 1]
Altitude [km]
45°S lat 45°N
b
kext(  = 3.4 µm) [km 1]
Altitude [km]
90°S lat 45°S
c
kext(  = 3.4 µm) [km 1]
Altitude [km]
150° 180° 210° 240° 270° 300° 330° 0° 30° 60° 90° 120° 150° 180° Ls
from MY 27 to MY 31 (M¨a¨att¨anen et al., 2013).

Midlatitudes (45°S lat 45°N)
a
195° Ls < 250° (MY 34 GDS) 140° Ls < 230° (MY 35) 230° Ls < 325° (MY 35) 325° Ls < 60° (MY 35-36) 60° Ls < 182° (MY 36)
kext(  = 3.4 µm) [km 1]
0.0 0.5 1.0 1.5 2.0 2.5 3.0 reff [µm]
Polar latitudes
b
North (lat 60°N) South (lat 60°N)
kext(  = 3.4 µm) [km 1]
0.0 0.5 1.0 1.5 2.0 2.5 3.0 reff [µm]
[Figure: Figure 9 presents the extinction coefficient kext of the clouds at λ = 3.4 µm as a function of the retrieved size of the water ice crystals reff for 3 ranges of latitudes (midlatitudes, northern and southern polar latitudes) and 5 ranges of Ls, including the MY 34 GDS. We observe at all latitudes the presence of 2 regimes depending on the size of the water ice crystals: for reff ≤1 µm the extinction kext increases with reff, while for larger particles the extinction of the clouds remains in the same range of values when reff varies between 1.5 and 3 µm.]

We can still note that, if we exclude the data from the MY 34 GDS and the MY 35 dust storm (discussed further in the next paragraph), the points in the polar regions (panel b) have less scatter in terms of kext than in the midlatitudes (panel a), especially for the large particles (reff ≥1.5 µm). This may be related to the smaller variations of the cloud altitudes in these regions (cf. Figure 4), which suggests a more stable vertical distribution of the properties of the clouds (both kext and reff). Also, for fixed values of reff and λ the extinction coefficient kext is solely a function of the number of particles that have scattered the light along the line of sight. Thus, the small dispersion in the cloud extinction values in the polar regions indicates that the number density of the water ice crystals within the clouds is overall constant over the year.
However, we observe in Figure 9b that some detections of particles with reff between 0.9 and 2.2 µm are associated with lower extinction values than the ones from the typical trend: kext ∼10−4–10−3 km−1 instead of kext ∼10−2 km−1. A noteworthy point is that most of these detections correspond to observations that have been mostly acquired either during the MY 34 GDS or during the regional dust storm in the Northern hemisphere in MY 35. In addition, even for lower size of ice crystals, the GDS detections are correlated with lower kext values. These observations with lower extinction values are associated with detections at higher altitudes than those typical for these sizes of crystals (cf. Figure 4b). Thus, while water ice clouds are observed at higher altitudes during dust storms, their ice crystal number density is lower than what is typically observed outside these events.
Our dataset provides precise and systematic monitoring of the cloud altitude, effective radius, and extinction over two Martian years. Thus, these new observational constraints can be compared to predictions by numerical simulations such as GCMs. Here we will compare our results with the Mars PCM (Forget et al., 1999; Montmessin et al., 2002; Madeleine et al., 2012; Navarro et al., 2014). We will focus on the observations acquired during the second half of MY 35, a non-GDS year for which we have access to a climatology of the atmospheric dust distribution (Montabone et al., 2015, 2020).
Even though the size of the ice crystals within the clouds is among the quantities available from version 5 of the Mars PCM, the model sizes are systematically and significantly larger than the values retrieved in this study as well as in previous ones (e.g., Wolff & Clancy, 2003; Vincendon et al., 2011; Clancy et al., 2019; Stcherbinine et al., 2020; Luginin et al., 2020; Liuzzi et al., 2020). Investigations on this discrepancy are currently ongoing by the Mars PCM team. Thus, we do not discuss further the crystals’ size here, but we instead focus on the altitude of the clouds, through the volume mixing ratio of H2O ice crystals in the atmosphere.
[Figure: Figure 10 shows the comparison between the water ice clouds detected by ACSMIR (left column) and predicted by the Mars PCM (right column) from Ls = 140◦]

to Ls = 360◦(MY 35) for three ranges of latitude (North and South high latitudes, mid-latitudes) in order to delineate the latitudinal from the seasonal trends.
We observe that the seasonal trend shown in the ACS-MIR data for the midsouthern latitudes (i.e., increase of the altitude of the clouds around the perihelion) in the panels c & e is well reproduced by the PCM (panels d & f). However, the strong increase observed in the altitude of the clouds associated with the regional dust storm in the Northern hemisphere is only reproduced for a few model profiles (panel b).
45°N lat 90°N
a
b
Altitude [km]
45°S lat 45°N
c
d
Altitude [km]
90°S lat 45°S
e
f
Altitude [km]
150° 180° 210° 240° 270° 300° 330° 360° Ls
150° 180° 210° 240° 270° 300° 330° 360° Ls
MY 35
MY 35
0.1 0.5 1.0 1.5 2.0 reff [µm]
H2Oice [ppmv]
[Figure: Figure 11 presents a close-up view of the comparison between the ACS-MIR retrievals and the Mars PCM predictions for 6 individual profiles taken between Ls = 121◦and Ls = 352◦in MY 35. For each profile, the information about the altitude of the water ice cloud in both the ACS-MIR observation and the Mars PCM simulations are provided, as well as the extinction coefficient at λ = 3.2 µm.]

We observe that the modeled extinction values (kext) at λ = 3.2 µm predicted by the model match the values retrieved by ACS-MIR, at least for the altitudes where the PCM produces clouds (panels a, c, d & e). However, we still note a shift of about 10 to 20 km in the altitudes where a cloud layer is typically detected versus where they are predicted by the PCM (panels a & f). Despite the limitations of our method, it is clear that ACS observes clouds higher than predicted by the PCM. Indeed, by looking at the ACS-MIR kext profiles, we suspect that some clouds layers are missing at the bottom of the clouds (panels a, e & f) as the detached layer that is visible in the extinction profile extends to a lower altitude than the water ice cloud detection. However, we note that when a cloud layer is not predicted to extend as high as it is observed by ACS, the model atmospheric extinction also decreases faster than the measured one (panels a & f), which strengthens the validity of the differences noted between PCM predictions and ACS observations for these altitudes. Plus, we also find that the largest discrepancies are observed for the higher altitude clouds, and seem likely to be to be related to the difficulty of the model to reproduce in general the high-altitude detached layers of water ice detected by ACS (panels b & d).
Thus, clouds predicted at lower altitudes in the Mars PCM compared to the ACS-MIR observations may be the manifestation of the limits of our retrieval method, while the lack of the upper layers in the PCM is a shortcoming in the current model.
, G.

Prat159, T.

Prod’homme2, 21, F.

Raison160, 146, J.M.

Rebordao143, D.

Risquez2, B.

Rocca-Volmerange161, S.

Rosen26, 72, M.I.

Ruiz-Fuertes33, F.

Russo27, S.

Sembay72, I.

Serraller Vizcaino162, A.

Short1, A.

Siebert45, 89, H.

Silva92, D.

Sinachopoulos37, E.

Slezak14, M.

Soffel11, D.

Sosnowska8, V.

Straižys163, M. ter Linden42, 164, D.

Terrell165, S.

Theil166, C.

Tiede5, 167, L.

Troisi58, 168, P.

Tsalmantza5, D.

Tur77, M.

Vaccari169, 170, F.

Vachier38, P.

Valles10, W.

Van Hamme171, L.

Veltz89, 40, J.

Virtanen55, 56, J.-M.

Wallut16, R.

Wichmann172, M.I.

Wilkinson7, 72, H.

Ziaeepour59, and S.

Zschocke11 (Affiliations can be found after the references) Received 2016-07-08; accepted 2016-08-18
In this paper, we present the results of our study on the identification and characterization of the Martian water ice clouds properties from IR SO data acquired by the ACS-MIR instrument onboard TGO between Ls = 163◦(MY 34) and Ls = 181◦ (MY 36). Using the methodology previously described in Stcherbinine et al. (2020), we are able to simultaneously detect the presence of water ice clouds in the Martian atmosphere, and constrain the size of their crystals at each observed altitude. As TGO’s orbit allows ACS to span all the latitudes between 80◦S and 80◦N in about ∼20◦of Ls, this new dataset allows us to monitor the properties of the water ice clouds as a function of both the season and the latitude. In addition, the data acquired during MY 35 give us reference measurements at the same period to be compared to the observations acquired during the MY 34 GDS to better constrain the effects of such an event.
The main results are summarized below:
• The Solar Occultation technique used by ACS-MIR provides highly sensitive measurements that allow us to observe optically thin clouds (kext(λ = 3.2 µm) ∼ 10−4 km−1). Such clouds are typically much harder to detect through other observing geometries (on-disk or limb scattering).
• Where thick water ice clouds appear locally in the Martian atmosphere (Smith et al., 2013; Wolff et al., 2019; Szantai et al., 2021), our SO observations reveal the quasi-systematic presence of small water ice crystals within the upper layers | Ls = 273° (MY 35)

\| lat = 34 100 ACS-MIR Mars PCM 80 |  | °S a | Ls = 235° (MY 35) \| lat = 62°N 100 ACS-MIR Mars PCM b 80 |  |  |  |

| --- | --- | --- | --- | --- | --- | --- |
|  |  |  |  |  |  | b |
| m] |  |  | m] |  |  |  |
| 60 [k e |  |  | 60 [k ltitude 40 |  |  |  |
| ltitud 40 |  |  |  |  |  |  |
| A |  |  | A |  |  |  |
| 20 |  |  | 20 |  |  |  |
| 0 0 1 2 3 10 6 10 4 10 2 0 reff [µm] kext, 3.2µm [km 1] |  |  | 0 0 1 2 3 4 10 6 10 2 0 50 reff [µm] kext, 3.2µm [km 1] H2Oice [ppmv] |  |  |  |

Ls = 121° (MY 35)  |  lat = 1°S
Ls = 279° (MY 35)  |  lat = 6°N
ACS-MIR Mars PCM
ACS-MIR Mars PCM
Altitude [km]
Altitude [km]
0 1 2 3 reff [µm]
0 50 H2Oice [ppmv]
0 1 reff [µm]
0 20 H2Oice [ppmv]
kext, 3.2µm [km 1]
kext, 3.2µm [km 1]

| Ls = 352° (MY 35) \| lat = 44 100 ACS-MIR Mars PCM 80 [km] 60 titude 40 |  |  | Ls = 170° (MY 35) \| lat = 79°S 100 ACS-MIR Mars PCM f 80 [km] 60 titude 40 |  |  |  |
| --- | --- | --- | --- | --- | --- | --- |
|  |  |  |  |  |  | f |
| Al 20 |  |  | Al 20 |  |  |  |
|  |  | 20 2Oice [ppmv] | 0 0 1 10 6 10 2 0 200 reff [µm] kext, 3.2µm [km 1] H2Oice [ppmv] |  |  |  |
| 0 0 1 2 3 4 10 6 10 4 10 2 0 reff [µm] kext, 3.2µm [km 1] H |  |  |  |  |  |  |