Cfrgtkky

I need to put a and b option in item 3, more inside and with more space from the left side. This is my code:

begin{document}

   begin{enumerate}

      item The nano-indentation technique can be successfully applied to study micron-sized particles returned in sample-return missions, providing very valuable information about the physical properties of the materials forming asteroids. 

      item The three Itokawa regolith particles studied here formed part of Itokawa’s asteroid soil. Probably a significant part of the regolith has been processed, ground, powdered by impacts, and biased to high-strength materials to a certain extent. 



      item We found that the mechanical properties of Itokawa’s regolith particles are comparable with the silicates forming LL chondrite meteorites, with relatively minor differences. 



     a. The reduced Young’s modulus values obtained for Itokawa’s samples are higher than those measured for Chelyabinsk chondrite, so they seem to be more compacted than the minerals forming that particular LL chondrite. This might be a natural consequence of being particles surviving long exposure times on the surface of a NEA. 



      b.    Concerning the elastic recovery of Chelyabinsk chondrite minerals, it is significantly lower than that measured for the Itokawa samples, while hardness values are similar. This indicates that the Itokawa particles have larger ability to absorb elastic energy during an eventual impact, as compared to Chelyabinsk chondrite minerals.



    item Concerning the magnetic properties, we found a soft magnetic behavior with a coercivity in the range 5-10 mT. The weak magnetic answer of the particles is  consistent with the relative absence of noticeable (at micron scale) metal grains and other magnetic or paramagnetic components in the particles. The presence of $Fe_{x}S_{1-x}$ grains plus additional antiferromagnetic phases with surface uncompensated spins can explain the observed magnetic behavior of these particles.



   end{enumerate}

end{document}

Do you mean like this?

documentclass[12pt,a4paper]{article}

usepackage{enumerate}

begin{document}



   begin{enumerate}

      item The nano-indentation technique can be successfully applied to study micron-sized particles returned in sample-return missions, providing very valuable information about the physical properties of the materials forming asteroids. 

      item The three Itokawa regolith particles studied here formed part of Itokawa’s asteroid soil. Probably a significant part of the regolith has been processed, ground, powdered by impacts, and biased to high-strength materials to a certain extent. 



      item We found that the mechanical properties of Itokawa’s regolith particles are comparable with the silicates forming LL chondrite meteorites, with relatively minor differences. 



      begin{enumerate}[{a.}]           

      item The reduced Young’s modulus values obtained for Itokawa’s samples are higher than those measured for Chelyabinsk chondrite, so they seem to be more compacted than the minerals forming that particular LL chondrite. This might be a natural consequence of being particles surviving long exposure times on the surface of a NEA. 



      item Concerning the elastic recovery of Chelyabinsk chondrite minerals, it is significantly lower than that measured for the Itokawa samples, while hardness values are similar. This indicates that the Itokawa particles have larger ability to absorb elastic energy during an eventual impact, as compared to Chelyabinsk chondrite minerals.

      end{enumerate}



      item Concerning the magnetic properties, we found a soft magnetic behavior with a coercivity in the range 5-10 mT. The weak magnetic answer of the particles is  consistent with the relative absence of noticeable (at micron scale) metal grains and other magnetic or paramagnetic components in the particles. The presence of $Fe_{x}S_{1-x}$ grains plus additional antiferromagnetic phases with surface uncompensated spins can explain the observed magnetic behavior of these particles.



   end{enumerate}



end{document}

The shortlabels option of the enumitem package has the same syntax as the enumerate package in AboAmmar's response.

documentclass[a4paper,10pt]{article}

usepackage[shortlabels]{enumitem}

begin{document}

   begin{enumerate}

      item The nano-indentation technique can be successfully applied to study micron-sized particles returned in sample-return missions, providing very valuable information about the physical properties of the materials forming asteroids. 

      item The three Itokawa regolith particles studied here formed part of Itokawa’s asteroid soil. Probably a significant part of the regolith has been processed, ground, powdered by impacts, and biased to high-strength materials to a certain extent. 



      item We found that the mechanical properties of Itokawa’s regolith particles are comparable with the silicates forming LL chondrite meteorites, with relatively minor differences. 

        begin{enumerate}[a.]





     item The reduced Young’s modulus values obtained for Itokawa’s samples are higher than those measured for Chelyabinsk chondrite, so they seem to be more compacted than the minerals forming that particular LL chondrite. This might be a natural consequence of being particles surviving long exposure times on the surface of a NEA. 



      item    Concerning the elastic recovery of Chelyabinsk chondrite minerals, it is significantly lower than that measured for the Itokawa samples, while hardness values are similar. This indicates that the Itokawa particles have larger ability to absorb elastic energy during an eventual impact, as compared to Chelyabinsk chondrite minerals.

            end{enumerate}

    item Concerning the magnetic properties, we found a soft magnetic behavior with a coercivity in the range 5-10 mT. The weak magnetic answer of the particles is  consistent with the relative absence of noticeable (at micron scale) metal grains and other magnetic or paramagnetic components in the particles. The presence of $Fe_{x}S_{1-x}$ grains plus additional antiferromagnetic phases with surface uncompensated spins can explain the observed magnetic behavior of these particles.



   end{enumerate}

end{document}